Targeting the Gut-Brain Connection to Treat Addiction

According to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM−5), substance use disorder (SUD) refers to the harmful use of psychoactive substances, which include alcohol, caffeine, tobacco, cannabis, opioids, hallucinogens, inhalants, sedatives, hypnotics, and anxiolytics. SUD is linked to a range of adverse health effects, notably impairments in brain function and the development of maladaptive behaviors. These behaviors often involve the escalation of tolerance, withdrawal symptoms, compulsive increases in substance intake, and intense cravings. Such patterns contribute to the progressive disruption of personal and social functioning.1

SUD has detrimental effects on various bodily systems, including the immune, digestive, and nervous systems. In recent years, increasing attention has been focused on the gut-brain axis (GBA), which explores the bidirectional communication between the gastrointestinal (GI) tract and the brain, particularly regarding the reward circuits involved in SUD. Research indicates that many addictive substances share a common reward pathway, known as the dopamine system. This pathway involves dopamine neurons in the ventral tegmental area (VTA) and their projections to target regions in the limbic forebrain, particularly the nucleus accumbens (NAc).2 While activation of the reward system through this pathway induces pleasure or euphoria across various substances, each psychoactive substance exerts its effects through distinct pharmacological mechanisms.

The human gut microbiota is a complex ecosystem composed of bacteria, archaea, viruses, fungi, protozoa, and yeasts,3 which play a vital role in maintaining the integrity of the epithelial barrier, intestinal metabolism, and immune homeostasis. It is well-established that the microbiota contributes to various physiological processes, including pathogen defense,4 immune system modulation,5 fermentation of indigestible dietary fibers,6 and the production of bioactive peptides and proteins that regulate host energy metabolism.7−9 Disruption of intestinal homeostasis can profoundly impact both the gut and brain, as the GBA interacts through several biological systems, including immune, endocrine, neural, and metabolic signaling pathways, influencing overall health and disease.

Accumulating evidence in recent years indicates that addictive substances and the gut microbiota engage in a bidirectional interaction network through GBA, driving the pathological process of SUD. On one hand, addictive substances such as alcohol, tobacco, and opioids can directly or indirectly disrupt gut microbial homeostasis, leading to dysbiosis characterized by reduced beneficial bacteria and increased pathobionts.10 This compromises intestinal barrier function, resulting in leaky gut and translocation of bacterial endotoxins into the bloodstream, which triggers systemic and neuroinflammation. On the other hand, the dysregulated microbiota in turn modulates brain function via the GBA.11 Microbial metabolites, such as short-chain fatty acids (SCFAs) and tryptophan derivatives, regulate the synthesis and signaling of neurotransmitters, including GABA, serotonin (5-HT), and dopamine, thereby affecting reward and emotional circuits.12 Metabolites like SCFAs also exert epigenetic effects, such as inhibiting histone deacetylases (HDAC) or modulating DNA methylation, thereby altering the expression of addiction-related genes (e.g., dopamine receptors and µ-opioid receptors) and persistently reshaping reward circuit plasticity.13−15 Additionally, gut bacteria can produce neurotransmitters and interact with the vagus nerve (VN) and immune signaling pathways, collectively modulating hypothalamic-pituitary-adrenal (HPA) axis activity and reward circuit function, ultimately promoting drug craving and relapse.16^,17

Currently, there is no universally effective treatment for SUD, therefore, elucidating the role of the GBA in SUD is crucial for developing targeted therapeutic strategies. This review provides an overview of recent research progress on the involvement of gut microbiota and the most common substances, including alcohol, cocaine, tobacco, opioids, and methamphetamine. It further explores the potential pathogenesis underlying the relationship and intrinsic connections linking gut microbiota and SUD. By analyzing the complex and diverse interactions, we aim to reveal the potential regulatory role of microbiota within addiction pathways and emphasize its importance as a future therapeutic target.

Alcohol is one of the most abused substances, affecting multiple pathways within the central nervous system (CNS). Alcohol causes physical dependence by interacting with Gamma-Aminobutyric Acidergic (GABAergic) synapses, glutamatergic signaling, and dopamine release in the midbrain’s limbic system.18 In addition to directly interacting with neurotransmitters, alcohol is associated with significant gut damage, characterized by disruptions in the gut microbiota, alterations of intestinal metabolism, elevated intestinal permeability, impaired nutrient absorption, and altered function of mucosal immune cells.19 Alcohol addiction significantly alters the composition and function of the gut microbiota, leading to intestinal dysbiosis.1^,20

The reduction of SCFAs-related bacteria is most pronounced in alcohol use disorder (AUD) individuals, such as Akkermansia, Alistipes, Bacteroides, Clostridium, Faecalibacterium, Ruminococcus, and Parabacteroides.20−26 SCFAs primarily consist of acetate, propionate, and butyrate. The phylum Bacteroidetes mainly produces acetate and propionate, whereas the phylum Firmicutes is primarily responsible for butyrate production.23 SCFAs play an important role in maintaining intestinal barrier function, regulating immune responses, and influencing host metabolism.27 Butyrate alleviates intestinal inflammation and promotes homeostasis by activating GPR109a, which enhances the tolerogenic responses of colonic macrophages and dendritic cells.27^,28 SCFAs can drive the differentiation of naïve T cells into regulatory T cells, which secrete Interleukin (IL)-10 and help maintain intestinal immune homeostasis, preventing excessive inflammatory responses.29 Moreover, SCFAs contribute to the maintenance of intestinal barrier function by promoting the expression of tight junction proteins, thus enhancing epithelial barrier integrity and preventing the translocation of harmful substances and pathogens into the bloodstream.30^,31 SCFAs also modulate the gut microbiota by creating a favorable anaerobic environment that supports the growth of beneficial bacteria while inhibiting the proliferation of potential pathogenic microbes.32 Additionally, alterations in mucosa-associated microbiota may compromise intestinal function in individuals with AUD. The mucus layer acts as a physical barrier that prevents harmful microorganisms and their products from coming into contact with the epithelial cell surface.33In vitro studies have shown that SCFAs regulate mucus layer thickness and participate in the repair process during mucosal damage.34^,35 SCFAs can regulate the release of hormones such as glucagon-like peptide−1 (GLP−1) and peptide YY (PYY) from intestinal endocrine cells via the VN, indirectly influencing the brain's reward system and stress response.36^,37

Different microbial species exhibit varying degrees of tolerance to bile acids. Compared to Gram-positive bacteria, Gram-negative bacteria generally demonstrate greater resistance to bile.38Salmonella species show notable bile tolerance, for example, ox bile has minimum inhibitory concentrations of 18% for Salmonella typhi and 12% for Salmonella typhimurium, with minimum bactericidal concentrations of 18% and >60%, respectively. Escherichia coli also exhibits strong bile resistance and can grow even in high-concentration bile environments.38Lactobacillus and Bifidobacterium can better survive in the gut by producing bile salt hydrolases that reduce the toxicity of bile acids.39 In contrast, Helicobacter pylori is bile-sensitive, which may explain why it has never been isolated from fecal or gallbladder specimens.40^,41 Alcohol significantly affects bile acid synthesis, conjugation, and microbial transformation by directly regulating host gene expression (e.g., CYP7A1) and indirectly altering gut microbiota composition related to bile-tolerant.20^,42^,43 These changes lead to bile acid metabolism disorders, impaired intestinal barrier function, and systemic inflammation.44 Individuals with AUD display a reduced abundance of Bacteroides, Clostridium, and Ruminococcus45^,46 related to bile salt metabolism. Bile acids metabolites (e.g., taurine and glycine) can serve as nutrient sources for microorganisms, influencing the growth and metabolism of the gut microbiota.47^,48 Bile acids are also ligands for the pregnane X receptor (PXR), which plays a key role in maintaining intestinal homeostasis. Downregulation of PXR increases intestinal permeability, reduces tight junction protein expression, and elevates levels of pro-inflammatory cytokines such as Tumor Necrosis Factor-ɑ (TNF-α) and IL−1.49^,50 In addition, bile acids can activate G protein-coupled receptors (GPCRs) such as G protein bile acid receptor (TGR5), promoting an anti-inflammatory shift in cellular phenotype, enhancing IL−10 production, and influencing intestinal endocrine function and inflammatory responses.50^,51 More importantly, the gut microbiota can indirectly modulate farnesoid X receptor signaling by altering the composition and levels of bile acids, thereby participating in the regulation of neuroinflammation and CNS function.51−53

Alcohol consumption directly damages the tight junctions of intestinal epithelial cells, as evidenced by the dysregulated expression and/or altered localization of key tight junction proteins, particularly Zonula occludens−1 (ZO−1) and Claudin−1, in intestinal epithelial cell lines exposed to ethanol or acetaldehyde. This disruption ultimately leads to increased intestinal permeability (IP).54 Ethanol-induced activation of inducible nitric oxide synthase (iNOS) leads to microRNA−212 overexpression and reduced ZO−1 levels in Caco−2 cells and mice, ultimately resulting in increased IP.55^,56 Acute ethanol exposure has been found to reduce IL−22 levels in the intestine, suppressing the expression of antimicrobial peptides (AMPs), Regenerating family 3β (Reg3β) and Reg3γ, thereby increasing IP.57 IP is closely associated with the gut microbiota.58 Alcohol-induced increase in IP can activate the VN, and subsequently affects brain regions associated with emotion, cognition, and behavioral regulation, including the thalamus, hippocampus, amygdala, and prefrontal cortex. As a central pathway of the GBA, the VN transmits signals related to gut inflammation and oxidative stress to the nucleus tractus solitarius (NTS) in the CNS.59 Alcohol increases IP, allowing bacterial products such as lipopolysaccharide (LPS) and peptidoglycan (PGN) to translocate from the gut into the bloodstream.60^,61 This triggers the release of pro-inflammatory cytokines, such as TNF-α and IL−6, which activate microglia either by signaling through the VN or by directly crossing the blood-brain barrier (BBB), leading to neuroinflammation.62 Neuroinflammation further damages the BBB, creating a vicious cycle that exacerbates neuronal dysfunction and neurotransmitter imbalance, such as reduced dopamine and brain-derived neurotrophic factor (BDNF), manifesting as anxiety, cognitive impairment, and alcohol craving.63^,64 Furthermore, elevated IL−8 levels are positively correlated with heightened IP,64 whereas IL−10 levels are inversely associated with escalated alcohol intake, likely through modulation of the Gamma-Aminobutyric Acid (GABA) signaling in the amygdala.65 These findings collectively suggest a pathway whereby alcohol-induced gut dysfunction and inflammation may influence neurobehavioral mechanisms of addiction, emphasizing the GBA as a potential therapeutic target.

Gut dysbiosis related to alcohol abuse has been linked to reduced levels of reward-related neurotransmitters such as 5-HT and dopamine. Microbial and metabolic changes, especially under compulsive alcohol-seeking conditions, have been associated with alterations in striatal dopamine receptor expression, potentially driving addiction behaviors.66 The gut microbiota plays a crucial role in the synthesis and metabolism of certain neurotransmitters and their precursors.67−69 In individuals with AUD, the abundance of gut microbes such as Akkermansia, Alistipes, Bacteroides, and Clostridium is reduced.54^,69 Consequently, this leads to a shift in Tryptophan (Trp) metabolism toward the kynurenine pathway, decreasing 5-HT production, which affects mood and reward circuits, thereby exacerbating depressive and anxious behaviors in individuals with alcohol dependence.67

The intervention strategies targeting the GBA in the treatment of alcohol addiction are receiving increasing attention. For example, supplementing chronic ethanol-fed mice with Lactobacillus rhamnosus GG (LGG) significantly increases the mRNA levels of intestinal tight junction proteins (ZO−1, Claudin−1) and their adaptor proteins, thereby promoting intestinal barrier integrity.70 LGG effectively reduces alcohol-induced increases in IP and significantly alleviates alcohol-triggered oxidative stress and inflammation in intestinal tissues of male Sprague-Dawley rats.71 Prebiotics play an important role in the treatment of alcohol addiction and related diseases. In mouse models, a high-fiber diet reduces alcohol-induced IP by increasing SCFA-producing bacteria, such as Bacteroides.58 Numerous studies have explored the potential of fecal microbiota transplantation (FMT) from healthy donors as a therapeutic approach for AUD. Research has shown that transferring gut microbiota from individuals with alcohol dependence into germ-free mice leads to increased alcohol preference and anxiety-like behaviors in the mice, whereas transplantation of microbiota from healthy donors reduces alcohol consumption and alleviates depressive-like behaviors.72 In a phase I double-blind clinical trial involving patients with alcohol-related cirrhosis, FMT is a safe and effective intervention for reducing alcohol cravings and decreasing alcohol intake in the short term.73 Recent research suggests that gut microbiota-derived bacterial extracellular vesicles (bEVs) hold promise as a novel therapeutic target for alcohol addiction. Studies found that bEVs isolated from the gut microbiota of rats with high alcohol intake, when administered intraperitoneally or orally to alcohol-avoidant Wistar rats, increased their voluntary alcohol consumption by up to 10-fold (p < 0.0001). This effect is independent of bacterial colonization, indicating that bEVs alone are sufficient to directly induce behavioral changes, and do not cause systemic or brain inflammation. Crucially, bilateral vagotomy completely blocked the bEV-induced increase in alcohol intake, confirming the VN as a key signaling pathway.74 Chronic ethanol exposure (CEE) disrupts the gut microbiota in mice and induces anxiety-like behavior alongside hippocampal neuroinflammation; however, probiotic intervention effectively alleviates these symptoms. Further research revealed that bEVs are key carriers within the microbiota-gut-brain axis: bEVs from CEE mice enter the brain and trigger neuroinflammation and anxiety-like behavior, whereas probiotic-modulated bEVs significantly mitigate ethanol-induced neuroinflammation and anxiety-like phenotypes. This dual regulatory role likely stems from the ability of these vesicles to modulate inflammatory pathway activity and neurotransmitter levels within the hippocampus.75 The mechanisms between GBA and alcohol are illustrated in Figure 1.

In summary, targeting the gut microbiota through interventions such as probiotics, prebiotics, FMT and bEVs holds promising potential for alleviating alcohol addiction and its associated physical and psychological complications. However, further research is needed to elucidate the underlying mechanisms and validate the long-term efficacy and safety of these approaches in clinical settings.

Cocaine is a potent neuroactive psychostimulant that induces neural activation, resulting in heightened sympathetic and behavioral responses and significant addictive potential cocaine’s primary action on the limbic system, particularly in areas dense with dopamine-responsive cells, produces its characteristic effects of intense euphoria and reward, leading to compulsive drug-seeking behavior and addiction.76−78

Cocaine use has been associated with GI symptoms such as anorexia, nausea, vomiting, diarrhea, and reduced GI blood flow, leading to gut microbiota imbalance in both in vivo and in vitro studies.79 Cocaine users exhibit alterations in gut microbiota composition, with an increased abundance of Bacteroidetes and decreased Firmicutes.80 Exposure to cocaine in mouse models induces significant alterations in the gut microbiota, characterized by the specific depletion of core SCFA-producing bacterial taxa/genera/families such as Mucispirillum, Ruminococcaceae, Pseudoflavonifractor, Butyricicoccus, and Lachnospiraceae.81 Butyrate serves as the primary energy source for colonic epithelial cells; its deficiency leads to impaired colonic epithelial cell function. Moreover, the lack of SCFAs may impair the regulation of histone acetylation, thereby regulating the expression of addiction-related genes.82−84 In male Sprague-Dawley rats subjected to gut microbiota depletion, both significantly enhanced motivation for low-dose cocaine and increased cue-induced drug-seeking behavior following prolonged abstinence were observed, concomitantly accompanied by altered transcriptional profiles in the NAc; crucially, supplementation with bacterially-derived SCFAs metabolites completely reversed these behavioral alterations induced by microbiota depletion, as well as the changes in NAc transcription.85 This suggests that the SCFA-epigenetic regulatory axis plays a modulatory role in the mechanisms underlying cocaine addiction.

Cocaine exposure induces intestinal microbiota dysbiosis, which is closely associated with the upregulation of proinflammatory mediators and cytokines. Cocaine modulates multiple cytokines, chemokines, and Toll-like receptor (TLR) signaling pathways, particularly activating TLR−2 and increasing levels of proinflammatory cytokines such as Monocyte Chemoattractant Protein−1 (MCP−1) and Eotaxin−1 (CCL−11), thereby fostering an inflammatory environment.86 This inflammatory state is further reinforced by elevated levels of Nuclear Factor kappa B (NF-κB) and IL-1β, both of which play critical roles in gut permeability, epithelial dysfunction, and neuroimmune activation. Increased IP allows translocation of LPS, which further stimulates systemic and CNS inflammation.81 Interestingly, cocaine self-administration behavior in rats is amplified following the intraperitoneal injection of the anti-inflammatory granulocyte colony-stimulating factor (G-CSF). Similarly, cocaine-induced locomotor activity in mice is similarly heightened.87

Alterations in the gut microbiota significantly impact cocaine-induced behavioral responses, which exhibit dose-dependent characteristics while remaining independent of systemic cocaine metabolism. Studies show no significant difference in benzoylecgonine levels—the primary cocaine metabolite—between cocaine-treated and antibiotic-treated mice. However, at a low cocaine dose (5 mg/kg), antibiotic-treated animals exhibit a robust place preference that is four times greater than controls, indicating heightened sensitivity. Both control and antibiotic-treated mice develop locomotor sensitization after five days of high-dose cocaine (10 mg/kg). Interestingly, oral supplementation of bacterial fermentation products, such as SCFAs (acetate, propionate, and butyrate), reverses antibiotic-induced behavioral phenotypes, suggesting that the gut microbiota influences cocaine-mediated behaviors potentially via glutamatergic and dopaminergic systems.88 Colonization with the probiotic L. rhamnosus effectively mitigated cocaine-induced intestinal oxidative stress and inflammation, concurrently suppressing glial cell activation in the CNS alongside a reduction in cocaine-associated locomotor activity.81 These findings confirm that the gut microbiota regulates cocaine’s behavioral effects. Recent studies have further elucidated the bidirectional modulatory effects of the gut microbiota on cocaine-induced neurobehavioral responses. Intervention studies substantiate that administering L. rhamnosus suppressed cocaine-associated gut inflammation, CNS glial activation, and hyperlocomotion.81 Cocaine exposure elevates intestinal norepinephrine, which activates the QseC receptor, a histidine kinase receptor, in γ-Proteobacteria, such as Citrobacter rodentium and E. coli HS, thereby promoting their colonization by these bacteria. These colonizing bacteria consume glycine as a nitrogen source, leading to significant depletion of glycine within the intestinal lumen and cerebrospinal fluid. This systemic reduction in glycine levels modulates neuroplasticity in the nucleus accumbens, ultimately potentiating cocaine-induced behavioral responses. Critically, these behavioral enhancements are reversed by either exogenous glycine supplementation or genetic ablation of the bacterial glycine transporter (cycA), establishing glycine metabolism as a mechanistically central regulator of microbiota-dependent modulation of cocaine-associated addictive behaviors.89 Cannabidiol (CBD) intervention significantly reduces cocaine withdrawal-induced conditioned place preference (CPP) while concomitantly restoring gut microbiota β-diversity to baseline levels in mouse models. CBD specifically expands anti-inflammatory bacterial populations (e.g., Firmicutes phylum), attenuating the neural consolidation processes of reward memory via the gut microbiota-immune axis. CBD effectively disrupts the core pathological cycle of cocaine addiction through synergistic modulation of intestinal microecological homeostasis and neuroplasticity pathways.90 Notably, gut-targeted therapies (e.g., probiotics, pathogen manipulation, metabolite supplementation) not only modulate addiction-relevant behaviors but also concurrently improve key biomarkers, including neuroinflammatory markers (cytokines), neurotransmitter pathway activity (striatal dopamine/glutamate signaling), and microbial metabolite profiles (SCFAs/GABA).81^,91 Human studies corroborate that cocaine use leads to reduced gut microbial diversity and diminished beneficial metabolites, while interventions like repetitive transcranial magnetic stimulation (rTMS) demonstrate potential to partially restore microbial balance.92 In summary, intestinal intervention strategies, such as probiotics, prebiotics, microbiota transplantation, or glycine/sarcosine supplementation, hold promise as novel therapeutic avenues for cocaine addiction, likely acting through multi-target regulatory mechanisms.

Nicotine (Tobacco), predominantly consumed via combustible tobacco products and electronic cigarettes (e-cigs), is the most widely used addictive substance globally. It exerts potent neuropharmacological effects on the CNS and exhibits a high potential for dependence. Exposure to nicotine significantly influences the microbiota of the oral, GI, and respiratory systems.93−96 E-cigs, often perceived as a safer alternative to traditional cigarettes, pose non-negligible risks to microbiota.

Both e-cigs and conventional cigarettes alter the oral microbiome.97 Tobacco smoke can alter the pH and oxygen tension in the oral cavity, inducing a localized anaerobic environment that promotes pathogenic microbial colonization and drives the establishment of a pro-inflammatory microenvironment.98^,99 Under these conditions, smokers exhibit significant enrichment of pathogenic genera such as Veillonella, Leptotrichia, and Prevotella, while protective taxa, including Haemophilus and Neisseria are markedly reduced.100 A study investigating the oral microbiota of tobacco smokers in the Middle East further validated the aforementioned microbial alterations and revealed associations between nicotine dependence levels and specific microbial abundances.101 The study found that the relative abundances of Streptobacillus hongkongensis, Fusobacterium massiliense, and Prevotella bivia were significantly elevated in the oral microbiota of individuals with high nicotine dependence. Compared to never-smokers, the oral microbiota of smokers exhibited significant enrichment in the tricarboxylic acid (TCA) metabolic pathway. Notably, TCA such as citric acid in the TCA cycle are potent chelators of magnesium ions, and their excessive accumulation may exacerbate host magnesium deficiency. Magnesium plays a key role in nicotine addiction by inhibiting dopamine secretion, glutamate-mediated N-methyl-D-aspartate (NMDA) receptor activation, and the synthesis of substance P and nitric oxide.102^,103 The high nicotine dependence group also showed significant enrichment in xanthosine utilization; xanthosine, a catabolic product of purine nucleotides, is involved in metabolic pathways related to caffeine biosynthesis. This mechanism may explain the increased demand for caffeine intake among smokers, reflecting a nicotine-caffeine synergistic addiction.104

Similarly, e-cigs use disrupts the structure of the oral microbiome. Studies have shown that e-cigs increase the abundance of opportunistic pathogens such as Candida albicans and Porphyromonas gingivalis, and promote the proliferation of pathogenic bacteria, including Streptococcus mutans, P. gingivalis, and Fusobacterium.105 E-cigs aerosols contain high levels of reactive aldehydes and carbonyl compounds, which can induce oxidative stress, damage host cells, and disturb microbial homeostasis.105 Moreover, e-cigs use reduces levels of antimicrobial proteins in saliva—such as lysozyme and lactoferrin—thereby weakening the natural immune defense of the oral cavity.106 Notably, P. gingivalis and Fusobacterium nucleatum play critical roles in the development and progression of periodontal disease. These pathogens can upregulate multiple pro-inflammatory cytokines, including TNF-α, interferon-γ (IFN-γ), IL-1β, IL−6, and IL−8, thereby activating host inflammatory responses.107^,108 This chronic inflammatory state is not confined to the oral cavity100; it may also affect the CNS via systemic circulation, triggering neuroinflammation or directly modulating the brain's reward pathways, thereby increasing sensitivity to addictive substances and promoting the development and progression of addictive behaviors.109 Compared to never-smokers, traditional smokers exhibit broad reductions in microbial diversity across oral and pulmonary sites (152 differentially abundant taxa), whereas e-cigs users show alterations primarily confined to the oral cavity (17 differential taxa compared to smokers), with no significant shifts in the lung microbiome and only 21 overlapping taxa affected in both oral and pulmonary compartments.110 The use of e-cigs is significantly associated with oral microbial dysbiosis and increased biofilm accumulation. Nevertheless, although its harm is lower than that of conventional tobacco, it still poses a substantial threat to health.110

Compelling evidence indicates that specific components of tobacco smoke induce gut microbiota dysbiosis. Carbon monoxide (CO) alters the gut microbiome by promoting the proliferation of bacterial species that express iron-uptake-related molecules.111 Furthermore, heavy metals (e.g., cadmium, arsenic, chromium, iron, mercury, nickel) present in cigarette smoke may, upon ingestion, cause gut microbial dysbiosis and impair intestinal epithelial cell transport function, oxidative stress status, and inflammatory responses.112^,113 Cigarette smoke also contains highly toxic volatile organic compounds (VOCs), such as benzene, which experimental studies demonstrate can shift the overall structural architecture of the gut microbiota.114 A large population-based cohort study was conducted, encompassing 288 never-smokers, 267 former smokers, and 203 current smokers,115 and found that the three groups exhibit no significant differences in alpha diversity, whereas beta diversity reveals significant intergroup variation, indicating that smoking status reshapes the overall microbial community structure. Compared to never-smokers, current smokers exhibit elevated relative abundance of Actinobacteria and reduced levels of Firmicutes and Proteobacteria. Former smokers show higher relative abundance of Bacteroidetes and Tenericutes compared to current smokers, alongside decreased Verrucomicrobia. Notably, no significant differences are observed between never-smokers and former smokers in these phylum-level compositional changes, suggesting that long-term smoking cessation (≥6 y) can restore the microbiota to a non-smoker state. Fan et al. conducted a two-sample Mendelian randomization analysis and found that smoking reduces the abundance of Peptococcus, Actinobacteria, and Bifidobacterium. Notably, reduced Actinobacteria abundance correlates with increased daily cigarette consumption, while decreased Bifidobacterium abundance is associated with earlier smoking initiation age.116Peptococcus participates in Trp and tyrosine metabolism, pathways closely related to neurotransmitter precursor synthesis, thereby contributing to nicotine addiction.117 Actinobacteria modulate the 5-HT pathway and kynurenine pathway through Trp metabolism, playing crucial regulatory roles in nicotine addiction.116Bifidobacterium influences smoking behavior through multiple mechanisms. First, it transmits signals to the brain via the VN to regulate dopamine levels, counteracting smoking-induced euphoria or withdrawal distress.59 Second, it activates the Chromogranin A/Adrenergic Receptor α2A (CGA/ADRα2A) cascade and modulates the Trp Hydroxylase-Opioid Receptor (TRP/TPH-OR) pathway to promote 5-HT biosynthesis in colonic enterochromaffin cells.118^,119 Third, it affects neurotransmitters, including GABA and norepinephrine.120^,121 Furthermore, Bifidobacterium metabolites and components exhibit close associations with the CNS: SCFAs influence microglial morphology and function,122 acetate demonstrates therapeutic potential for preventing cognitive impairment,123 and PGN can penetrate the blood-brain barrier to transmit information.124 Overall, alterations in metabolites resulting from gut microbiota dysbiosis affect the central nervous system, potentially eliciting fear or anxiety-like emotions and inducing depression, thereby increasing the risk of smoking initiation or cessation failure.

Nicotine exposure (conventional cigarettes and e-cigs) induces microbiota dysbiosis, exhibiting sex-specific differences. Male nasal microbiota exhibits significantly higher α-diversity than females, with enrichment of Abiotrophia and Finegoldia genera, while females are characterized by Yaniella enrichment. Smokers and e-cigs users show elevated IL−8 expression in nasal lavage fluid, accompanied by relatively reduced levels of lactoferrin and IgA, suggesting that microbiota dysbiosis may drive inflammatory responses.125 Mouse experiments have demonstrated that sex modulates nicotine's effects on gut microbiota. Nicotine reduces α-diversity only in female mice, with no changes in males. It alters community structure in both sexes and drives convergence between them. Odoribacter significantly decreases in females, while the Firmicutes/Bacteroidetes ratio significantly decreases in males but shows no change in females. Chi et al. directly demonstrated the sex-specific effects of nicotine exposure on gut microbiota and gut-brain axis metabolic signaling in male and female mice.126 Nicotine-exposed male mice exhibit increased abundance of Turicibacteraceae, while female mice show significant reductions in Christensenellaceae and Anaeroplasmataceae. Female mice demonstrate comprehensive elevations in neurotransmitters, including glutamate, GABA, glycine, and serine, potentially enhancing nicotine's rewarding effects. Male mice show significant decreases in glycine, serine, and aspartate, potentially attenuating NMDA receptor activation and reducing nicotine dependence.127−129 Nicotine-induced metabolite changes may contribute to sex differences in nicotine addiction through alterations in the balance of neurotransmitter pools, consistent with clinical observations of higher smoking cessation difficulty in women and greater nicotine intake in men.126^,130

Currently, therapeutic strategies targeting gut microbiota for nicotine addiction remain underdeveloped. Probiotics, such as the aforementioned Bifidobacterium, can improve nicotine dependence by regulating SCFAs and neurotransmitters. Additionally, Bacteroides xylanisolvens can degrade nicotine in the gut, reducing nicotine accumulation in the intestine and delaying the progression of non-alcoholic fatty liver disease (NAFLD) in smokers and animal models.131 These findings suggest that gut microbiota can serve as an auxiliary therapeutic strategy for nicotine addiction, potentially alleviating nicotine dependence and associated metabolic and inflammatory disorders.

Opioids which include compounds that interact with mu, delta, and kappa opioid receptors on nerve cells, have extensive effects throughout the body.132 Their initial interactions often occur in the GI tract.133 Opioid use, including agents like tramadol, methadone, and morphine, markedly disrupts gut microbiota composition, leading to reduced bacterial diversity and shifts in microbial abundance. The analgesic tramadol demonstrates dose- and time-dependent bactericidal activity against E.coli, Staphylococcus epidermidis, Staphylococcus aureus, and Pseudomonas aeruginosa in vitro,134 while subcutaneous injections of tramadol in mice inhibited S. aureus growth but not P. aeruginosa.135 Similarly, methadone shows antimicrobial effects against S. aureus, P. aeruginosa, and Streptococcus marcescens in vitro.136 In contrast, morphine significantly alters gut microbiota composition after short-term exposure.137 In murine studies, chronic morphine exposure was associated with the selective depletion of beneficial genera such as Bifidobacteria and Lactobacillus, contributing to microbial dysbiosis.138−140 In addition, a decreased abundance of beneficial gut bacteria Bacteroidaceae and Ruminococcaceae was observed in tramadol-, methadone-, and morphine-exposed patients,141 while heroin or prescription opioid users exhibited reduced microbiota diversity, with lower levels of Roseburia and Bilophila.142 These shifts in gut microbiota composition due to opioids correlated with alterations in intestinal motility, permeability, and inflammation.143 These changes may further interact with the brain's reward circuits, influencing opioid-related behaviors and withdrawal symptoms.144 Germ-free mice did not exhibit typical withdrawal symptoms, while mice with TLR2 gene knockout demonstrated a reduced withdrawal response, implicating that targeting the TLR2-mediated gut microbiota could provide a promising approach to alleviate withdrawal symptoms and improve treatment outcomes for morphine use disorder.145 Furthermore, morphine withdrawal induces sex-specific alterations in gut microbiota composition and metabolic profiles. Following morphine withdrawal, female mice exhibit a significant increase in the abundance of Proteobacteria, accompanied by disrupted Trp metabolism. This dysregulation manifests as activation of the kynurenine pathway and accumulation of neurotoxic metabolites, thereby exacerbating anxiety-like behaviors. In contrast, male mice demonstrate relative enrichment of Bacteroidota alongside reduced abundance of Actinobacteria and Firmicutes. These microbial shifts primarily correlate with diminished locomotor activity, while anxiety-like behaviors remain unaltered. This behavioral phenotype is closely associated with inflammatory response activation.146 These findings underscore the complex relationship between opioids and gut microbiota, highlighting the gut microbiota as a potential target for mitigating opioid-related GI and neurological effects.

Opioids induce gut microbiota dysbiosis and significantly disrupt the metabolic homeostasis of SCFAs. Chronic morphine administration in mice markedly reduces fecal levels of butyrate and propionate,147 while individuals undergoing methadone maintenance treatment exhibit a synchronous decline in fecal acetate, propionate, and butyrate.148 These alterations are closely associated with structural disruption of the microbiota, characterized by a significant reduction in the abundance of SCFA-producing core taxa, including Firmicutes and Akkermansia muciniphila.149 Critically, antibiotic-mediated depletion of the gut microbiota exacerbates the reduction in SCFAs following morphine exposure, demonstrating that opioids directly suppress SCFA biosynthesis by perturbing microbial ecological stability.150 SCFAs play a pivotal role in mediating opioid-induced tolerance, addiction, and withdrawal. Opioids compromise intestinal epithelial barrier integrity by disrupting tight junctions, leading to increased paracellular permeability, as evidenced by elevated FITC-dextran flux. This barrier dysfunction permits bacterial translocation and subsequent systemic inflammation.147 Conversely, both butyrate and other metabolites derived from A. muciniphila, distinct from SCFAs, have been shown to restore epithelial barrier function, thereby preventing microbial translocation and dampening systemic inflammatory cascades. Consistent with this, methadone maintenance treatment is associated with elevated plasma levels of pro-inflammatory cytokines IL−6 and TNF-α.148 SCFAs counteract this inflammation primarily through their activity as HDAC inhibitors, which downregulate inflammatory gene expression and modulate transcriptional programs within reward-related brain regions such as the NAc, including key immediate-early genes like Egr2 and Egr4.150 Furthermore, SCFAs can cross the blood-brain barrier to directly influence dopaminergic signaling or indirectly modulate central reward circuitry via vagal afferent pathways.151 Interventions targeting the microbiome or supplementing SCFAs effectively reverse opioid-induced behavioral deficits. Oral administration of butyrate prevents the development of morphine-induced antinociceptive tolerance147 Supplementation with a mixture of SCFAs completely restores morphine-induced CPP deficits caused by antibiotic-mediated microbiota depletion.150 In a fentanyl self-administration model, SCFA repletion normalizes reward pathway function and reverses the enhanced motivation and drug-seeking behavior caused by microbiota disruption.152 Moreover, the gut microbiota, acting via SCFAs, regulates withdrawal severity: antibiotic-induced microbiota depletion attenuates naloxone-precipitated morphine withdrawal symptoms, whereas exogenous SCFA supplementation reduces fentanyl-seeking behavior, indicating that SCFA levels are a critical determinant of withdrawal intensity and relapse vulnerability.152^,153

Opioids exacerbate irinotecan (CPT−11)-induced GI toxicity by increasing the abundance of β-glucuronidase-producing bacteria and resulting in SN−38 accumulation in the small intestine.154 Opioid use disrupts gut microbiota homeostasis, compromises intestinal immunity and increases susceptibility to infections.155−157 Studies have demonstrated a marked increase in chemokine expression in the intestinal tissue of morphine-treated mice, accompanied by elevated neutrophil infiltration that contributes to gut microbiota dysbiosis.158 Opioids can increase IL−2, IL−4, IL−6, IL−10, TNF-α and IFN-γ. subsequently, the intestinal barrier was disrupted by activating the TLR2/4 pathway, leading to persistent bacterial transmission159 and leakage of microbial components such as LPS, lipoteichoic acid (LTA), and endotoxins into the systemic circulation. The latter binds to receptors on the circulating immune cells to initiate cytokine production.160 The cytokines not only disrupt the tight junction of gut epithelia but also into the blood circulation and deposited into the CNS138^,140 thereby regulating the functional connectivity of brain regions related to behavioral changes such as intoxication and withdrawal.161 Alterations in the thalamus have been identified in rats exposed to morphine. Changes in gut microbiota diversity and bacterial abundances precede these anatomical features, and emerging neuroinflammation is a crucial link mediating communication between the gut and the thalamus.162 In microbiota-depleted rodents, increased recruitment of Fos⁺ neuronal ensembles was identified in the basolateral amygdala (BLA) and central nucleus of the amygdala (CeA) in an oxycodone intoxication and withdrawal model. The BLA connects with the CeA, which plays an integral role in withdrawal. Increased Fos⁺ neurons in the BLA send projections to the NAc, a central brain area underlying reward.163 Microglia in the NAc exhibit higher expression of TLR2 after chronic morphine treatment. This activation of microglia was markedly inhibited, accompanied by attenuated behavioral signs of morphine withdrawal in TLR2 knockout mice.164

The gut microbiota influences DNA methylation patterns.165 Choline-utilizing gut bacteria decrease DNA methylation in various brain regions by competing for choline, leading to increased anxiety in mice.166 Morphine alters gut microbiota related to DNA methylation.140^,166 Morphine use has been related to a decrease in Lactobacillus levels in the gut,167 which has been shown to attenuate abdominal pain and alleviate depression and anxiety by sustaining levels of IFN-γ168 and mediating the expression of the opioid receptor gene (OPRM) in intestinal epithelial cells via NF-κB-dependent mechanism. DNA methylation of the OPRM promoter correlates with opioid use,169 and is influenced by a single nucleotide polymorphism (SNP) at position + 188 of the OPRM gene promoter.170 This SNP is significantly associated with the onset and treatment of opioid addiction.171 These results imply that opioid addiction leads to complex changes in DNA methylation levels, partly mediated by the microbiota. Nevertheless, the linkage between the microbiota and DNA methylation-mediated opioid addiction and withdrawal symptoms remains insufficiently understood.

Interventions such as probiotics, prebiotics, SCFA supplements, and FMT can improve opioid-induced microbial dysbiosis, reduce withdrawal symptoms, delay the development of tolerance, and alleviate comorbid psychiatric and psychological symptoms. Probiotics enriched with Bifidobacteria and Lactobacillaceae significantly modulate gut inflammation in morphine-treated mice. This is a modulation that prolongs the analgesic effects of morphine and attenuates the development of antinociceptive tolerance.140 Oral administration of Lactobacillus acidophilus NCFM induces the expression of μ-opioid and cannabinoid receptors in intestinal epithelial cells, thereby exerting gut analgesic effects comparable to those of morphine.172 Crawford et al. found that the ketogenic diet alleviate abnormal pain behaviors induced by chronic morphine treatment in male mice by restoring SCFA-producing bacteria.173 This dietary approach may serve as an adjunctive strategy for analgesic management in patients receiving long-term opioid therapy. Additionally, studies indicate that supplementing with a diet rich in n-3 polyunsaturated fatty acids (n-3 PUFAs) during opioid exposure and withdrawal significantly increases the abundance of Bifidobacterium and Allobaculum. Among these, Bifidobacterium enhances opioid bioavailability by degrading glucuronides in the intestinal lumen. This action thereby improves withdrawal outcomes. In addition, n-3 PUFA-induced enrichment of Allobaculum is closely associated with reduced anxiety-like behaviors and may exert neuroprotective effects by suppressing anxiety-promoting taxa such as Akkermansia.174 Collectively, n-3 PUFAs may alleviate opioid-induced behavioral and neurophysiological abnormalities through modulation of gut microbiota composition. Furthermore, Xie et al. reported that molecular hydrogen, particularly coral calcium hydride 200 (CCH 200), can alleviate depressive and anxiety-like symptoms in individuals with opioid addiction and modulate gut microbiota composition. It facilitates the extinction of morphine-associated behaviors and reduces the risk of relapse. The underlying mechanism may involve decreasing the Firmicutes/Bacteroidetes ratio and increasing the relative abundance of beneficial bacteria such as Lactobacillus and Alloprevotella, thereby significantly ameliorating morphine-induced gut microbiota dysbiosis.175 These findings suggest that the gut microbiota may serve as a key potential mechanism through which molecular hydrogen exerts its therapeutic effects in opioid addiction.

FMT has emerged as a promising therapeutic strategy in the treatment of opioid addiction.176 FMT restores the diminished butyrate synthesis capacity in the intestines of mice that have been chronically exposed to morphine. Butyrate enhances intestinal barrier function and suppresses systemic inflammation; it also maintains opioid receptor sensitivity, thereby effectively delaying the development of morphine analgesic tolerance.177 Furthermore, transplantation of gut microbiota from morphine-naïve mice significantly alleviates naloxone-precipitated withdrawal symptoms. The underlying mechanisms include restoration of intestinal barrier integrity, reduction of bacterial translocation, and inhibition of TLR2/4-mediated inflammatory pathway activation.140 Notably, transplantation of microbiota from morphine-withdrawn mice induces anxiety-like behaviors in recipient animals; conversely, FMT ameliorates anxiety phenotypes by reshaping gut microbial balance, downregulating aberrant expression of serotonin receptors (e.g., Htr6) in the amygdala, and reversing Trp metabolic dysregulation.146 However, current explorations of FMT as a treatment for opioid addiction are limited to preclinical studies, and variations in individual microbiota may contribute to differences in opioid tolerance and withdrawal symptoms. The mechanisms underlying the interaction between GBA and opioids are illustrated in Figure 2.

Methamphetamine (METH) is a highly addictive neurotoxic substance with primary effects on the brainstem, where it induces addiction through mechanisms involving oxidative stress, activation of transcription factors, DNA damage, excitotoxicity, BBB impairment, microglial activation, and apoptotic pathways.178 METH abuse has a profound impact on the profiles and functions of gut microbiota. METH exposure alters the α and β diversity of gut microbiota in mice, increasing the relative abundance of pathogenic microbiota and reciprocally decreasing the beneficial bacterial taxa.179 Studies on METH-treated rats have shown a marked decrease in propionate-producing bacteria such as Phascolarctobacterium, along with an increase in the abundance of the Ruminococcaceae family, Proteobacteria, and Fusobacteria.179 METH exposure is associated with a slight increase in overall microbial biodiversity, with a significantly higher abundance of Rumen cocciz compared to control groups, and is linked to METH-induced neurotoxicity and cognitive impairments.180 Changes in microbiota abundance in METH-exposed mice were significantly correlated with depression-like behaviors and neurotoxicity.181 On the one hand, gut microbiota produces metabolites that indirectly regulate the gut-brain axis. Microbial metabolites such as Trp and SCFAs play a role in connecting to the gut-brain axis, thereby regulating neural function.182^,183Anaerostipes both produces SCFAs and facilitates Trp metabolism.184^,185 METH withdrawal resulted in a reduction of Anaerostipes, thereby inhibiting the transmission of Trp microbial metabolic signaling mediated by the Aryl hydrocarbon receptor (AhR) pathway through immune response, and contributing to the development of anxiety and depression-like behaviors.183 A similar study showed that enhanced microbial Trp metabolism by Lactobacillus reuteri reduced anxiety-like behavior by upregulating IL−22 levels in the colon.186

Microbes release SCFAs that regulate G-protein-coupled receptors to mediate neurotransmitter release (i.e., dopamine).187 Microbial dysbiosis contributes to METH-related anxiety and depression by reducing microbiota-derived SCFAs.188 Alpha synuclein can be translocated from the stomach to the brain more easily when there is a shortage of SCFAs, which can result in increased intestinal permeability.189^,190 Supplementing SCFAs can alleviate METH-induced neurotoxicity and behavioral abnormalities through the Wnt/β-Catenin/GSK3β signaling pathway and anti-inflammatory effects, revealing the neuroprotective potential of the gut microbiota.191 Microbiota-derived SCFAs maintain intestinal barrier function by inhibiting macrophage infiltration and M2 macrophage phenotype switching,192 the latter is associated with METH-induced immune impairment.193 Microbiota-derived SCFAs could modulate METH-induced anxiety- and depression-like behaviors via the Sigma−1 receptor (SIGMAR1)/BDNF/tyrosine kinase B (TrkB) pathway.188 SIGMAR1 is expressed throughout the brain, and it shows protective effects against multiple mental disorders, including anxiety and depression.194 SIGMAR1 may serve as one of the key mediators in the bidirectional communication of the gut-brain axis. SIGMAR1 knockout induced dysbiosis of the gut microbiota with lower expression of neurotrophic factors.195 BDNF and its receptor TrkB are crucial anti-anxiety and anti-depression factors.196 Prebiotic and probiotic treatments restored cognitive function by reverting BDNF and α-synuclein levels in the hippocampus.197Lactiplantibacillus and Bifidobacterium downregulated gut dysbiosis by increasing the hippocampal BDNF-positive cell population and BDNF level, leading to the alleviation of depression and cognitive impairment.198 Furthermore, SCFA-producing bacteria (i.e., Bacillus licheniformis) can regulate gut microbiota composition and increase dopamine levels in the brain.199

On the other hand, gut microbiota can increase neuromodulators in the human brain, including dopamine produced by Bacillus.200 These neurotransmitters mediate gut-brain signals and regulate cognition and behavior by communicating with the limbic system.201 Dopamine is best known for its role in the reward system, where it plays a fundamental part in reward learning and prediction.202 Microbiota-derived dopamine in the gut can regulate the production of certain cytokines such as IL−4 and IFN-γ, thereby activating immune responses through natural killer cells.203 Furthermore, gut microbiota are major contributors to dopamine bioavailability in the enteric and central nervous systems.204 Metabolites produced by gut microbiota are associated with dopamine functioning through modulation of dopaminergic synaptic transmission. In rodents, Prevotella exerts protective properties against degeneration of tyrosine hydroxylase-containing dopaminergic neurons by secreting hydrogen sulfide.205 These findings indicate that gut microbes alter the level and function of striatal dopamine by regulating cytokine-mediated inflammation.

Additionally, gut microbiota can influence the cellular metabolic function of microglia. The transformation of microglial M2 to M1 and the subsequent change of proBDNF–p75^NTR/mBDNF–TrkB signaling lead to a reduction of hippocampal neurogenesis and deteriorated spatial learning and memory in METH-exposed mice.206 Gut-derived bacterial LPS enter the circulating blood, activate microglia in the brain and consequently attenuate METH craving after METH withdrawal207; this process is associated with the activation of the TLR4/MyD88/NF-κB signaling pathway.208 LPS administration increases intestinal permeability, allowing endotoxins to travel towards the midbrain. This causes neuroinflammatory effects, which lead to a decrease of dopamine levels.209 LPS administration can influence brain activity by producing reactive oxygen species (ROS) through NADPH oxidase activity and result in microglial activation and dopaminergic neuronal loss.210

Supplementing gut microbial metabolites, such as SCFAs, or the peroxisome proliferator-activated receptor γ (PPARγ) selective agonist pioglitazone, can effectively alleviate METH-induced neurotoxicity and behavioral abnormalities. SCFAs exert their effects by modulating the Wnt/β-catenin/GSK3β signaling pathway, restoring intestinal barrier function, and suppressing neuroinflammatory responses in the CNS. Moreover, both SCFAs and pioglitazone improve METH-induced neuroinflammation, anxiety-, and depression-like behaviors, as well as restore intestinal function, through VN-dependent mechanisms.191 METH withdrawal significantly reduces key gut microbial genera associated with TRP metabolism, including Akkermansia, Bacteroides, Faecalibaculum, Desulfovibrio, and Anaerostipes. This leads to a marked decrease in indole derivatives produced through the TRP metabolic pathway. A high-TRP diet effectively suppresses METH-induced depressive- and anxiety-like behaviors, and supplementation with indole−3-acetic acid (IAA), indole−3-pro-pionic acid (IPA), or indole compounds also ameliorates METH-induced behavioral abnormalities. Notably, these protective effects are significantly attenuated in AhR knockout mice, highlighting the critical regulatory role of the “microbiota-indole-AhR signaling axis” in METH-associated emotional disturbances.183 FMT enhances the expression of 5-HT receptor subtype 4 (5-HT4R) in the NAc and hippocampus of mice. Activation of 5-HT4R not only attenuates intestinal inflammation but also modulates dopamine release, effectively reducing METH-induced CPP behavior.211 Furthermore, the abundance of Rikenellaceae exhibits a positive correlation with anxiety and depressive symptoms in METH users, suggesting that restoring this microbial population through FMT may alleviate withdrawal-related emotional disturbances and improve mental health.212 Transplantation of gut microbiota from METH users into germ-free mice induces anxiety- and depression-like behaviors, whereas metformin treatment significantly ameliorates these behavioral abnormalities. Mechanistic studies indicate that metformin likely modulates gut microbiota composition, thereby affecting neuroinflammatory responses and neurotransmitter imbalances during METH withdrawal and participating in its pathophysiological processes.212 These findings underscore the pivotal role of gut microbiota in emotional disorders caused by METH addiction and provide a theoretical foundation for novel microbiota-targeted therapeutic strategies. The mechanisms underlying the interaction between GBA and METH are summarized in Figure 3. Tables 1 and 2 summarize the microbial effects observed in SUD and interventions targeting the GBA, respectively.

Despite the significant therapeutic potential that gut microbiota offers for SUD, several critical limitations remain. Emerging evidence suggests that the gut microbiota may regulate addictive behaviors in a sex-dependent manner. However, research focusing on specific populations such as adolescents and the elderly remains insufficient, limiting the generalizability and potential for personalized applications of current findings.

Most studies rely heavily on animal models, which fail to fully replicate the complexity of human genetic diversity, environmental exposure histories, and individualized drug-use patterns. Therefore, caution must be exercised when extrapolating findings from animal experiments to human populations. Although 16S rRNA and metagenomic sequencing are widely used, the taxonomic identification, abundance quantification, and functional annotation of microbiota in these analyses are significantly influenced by multiple factors. These factors include sample purity, gene copy number variations, sequencing depth, and reference database completeness.

At the clinical level, a cross-sectional study demonstrates limited effectiveness of dynamic changes in gut microbiota in modulating addictive behaviors. Furthermore, studies’ reliance on fecal specimens limits accurate representation of mucosa-adherent microbiota composition and associated microenvironmental alterations along the GI tract. Moreover, the composition of the gut microbiota is influenced by multiple factors, including genetic background, dietary patterns, and geographic environment. Individuals with SUD often exhibit lifestyle disturbances, including altered dietary habits, increased psychological stress, comorbid conditions, and polysubstance abuse, which in turn significantly impact the diversity and functional profile of the gut microbiota. This makes it difficult to isolate the effects of addictive substances on gut microbiota composition in clinical studies.

Although interventions such as probiotics and FMT have shown certain regulatory effects and therapeutic potential in animal models, FMT faces substantial scientific and ethical barriers. Key limitations include that the safety and efficacy of FMT in treating addiction in humans remain unverified. Moreover, non-standardized donor screening protocols, uncontrolled transmission of viral communities, and the potential risk of antibiotic resistance gene transfer pose addressable yet critical issues for clinical FMT. Advancing research for addiction treatment via FMT critically depends on interlinked strategies. These include elucidating the mechanistic functions of specific microbial consortia, developing biobanked donor strain repositories with standardized preparation protocols, and ultimately creating synthetic microbial capsule therapeutics. As research in this field progresses, equal attention must be given to patient-centered care and ethical responsibility. In animal experimentation, the “3Rs” principle (Replacement, Reduction, and Refinement) should be rigorously applied to minimize suffering. In human studies, participants’ rights to informed consent, privacy, and autonomy must be protected, especially when working with vulnerable populations such as those with mental illness or substance dependence. Furthermore, enhanced ethical evaluation and social support systems are crucial.

Based on existing evidence, the gut microbiota plays a predominant role in modulating substance abuse through several key mechanisms, including the regulation of metabolic and signaling processes by microbial metabolites, immune activation mediated by gut microbes, and neuroendocrine modulation via the HPA axis and VN. Preclinical and clinical studies highlight the potential of microbiota-targeted interventions, such as probiotics, prebiotics, FMT, and dietary modifications, in restoring gut homeostasis and enhancing addiction treatment outcomes. Future research should focus on elucidating the cellular and molecular mechanisms by which gut microbiota contribute to addiction and its comorbidities. Identifying key microbial metabolites, neurotransmitter precursors, and immune modulators will be essential in developing targeted therapies. Additionally, precision medicine approaches tailored to individual gut microbiota profiles could optimize treatment efficacy, given that genetics, diet, sex, and lifestyle significantly shape microbial composition.