Recent advances in neuropsychopharmacology have renewed interest in psychedelics and entactogens as potential treatments for brain disorders, owing to their rapid effects on neural plasticity, cognition, and affect. Classical serotonergic psychedelics primarily act as 5-HT2A receptor agonists with additional activity at 5-HT1A and other serotonin receptors, whereas entactogens such as MDMA enhance monoamine release through transporter interactions. These actions converge on glutamatergic circuits, intracellular signaling cascades and transcriptional programs that can remodel synaptic connectivity. At the cellular level, psychedelics and entactogens acutely induce immediate early genes, including c-Fos, EGR1 and NPAS4, which regulate downstream targets such as BDNF and Arc to promote neuritogenesis, dendritic spinogenesis and synaptogenesis in cortical and hippocampal networks. Complementary epigenetic changes in DNA methylation and histone acetylation may help sustain these transcriptional effects beyond the period of drug exposure. In parallel, these compounds modulate neuroimmune pathways via 5-HT2A and sigma-1-receptors, altering cytokine profiles and microglial activation in a context-dependent manner rather than exerting a uniform antiinflammatory class effect. Preclinical studies and early clinical trials suggest that psychedelics and entactogens can produce rapid and sometimes durable improvements in mood and anxiety symptoms and are being investigated for conditions such as major depressive disorder, post-traumatic stress disorder, obsessive-compulsive disorder, substance use disorder and selected neurodegenerative syndromes. However, most indications remain supported only by small or early-phase studies and evidence for disease modification, particularly in Alzheimer's disease-related dementia, is currently limited to mechanistic hypotheses and preclinical models. This review synthesizes current knowledge on the receptor-level, molecular, genetic and immunological mechanisms of psychedelics and entactogens, linking these processes to neuroplasticity and brain network reorganization. It also discusses emerging therapeutic evidence, safety considerations and evolving regulatory frameworks, highlighting critical knowledge gaps and priorities for future research.
