Growing public interest in incretin-based therapies for weight loss has been accompanied by increasing availability of research peptides purchasable online. Products may be used without medical assessment or counselling regarding adverse effects, sick-day management or appropriate follow-up. In addition, composition, purity and dosing accuracy may be uncertain. Retatrutide is a novel triple agonist of the glucose-dependent insulinotropic polypeptide, glucagon-like peptide-1 and glucagon receptors currently being investigated for obesity and type 2 diabetes mellitus, but has no established role in type 1 diabetes mellitus. We report a man in his mid-30s with longstanding type 1 diabetes mellitus who presented with severe vomiting, diarrhoea, hyperglycaemia, ketonaemia and acute kidney injury shortly after self-administering an online-purchased product marketed as retatrutide for weight loss. His partner, who had taken the same preparation, also developed gastrointestinal symptoms. Both had also consumed a potential foodborne source of infection. In type 1 diabetes mellitus, where patients remain dependent on exogenous insulin to suppress ketogenesis, gastrointestinal illness may rapidly precipitate ketosis when insulin is omitted and oral intake is reduced. Although diabetic ketoacidosis did not develop, peak ketonaemia reached 4.3 mmol/L in the setting of dehydration and insulin omission, requiring intravenous insulin and fluid replacement. During recovery, recurrent hypoglycaemia required intravenous dextrose and insulin dose adjustment, highlighting challenges of glycaemic management in acutely unwell patients exposed to agents with incretin and glucagon receptor activity. Stool culture subsequently grew Shigella flexneri, introducing diagnostic uncertainty regarding the relative contributions of gastrointestinal infection and retatrutide exposure, and complicating counselling regarding future retatrutide use. While causation cannot be established, the temporal relationship, similar symptoms in another exposed individual, and recognised gastrointestinal adverse-effect profile of retatrutide suggest that exposure may have contributed to symptom severity or amplified the metabolic consequences of infection. This case highlights diagnostic and management challenges created by unregulated research peptides obtained outside healthcare pathways, particularly in patients with type 1 diabetes mellitus where evidence to guide management is limited and clinicians in non-specialist settings may be unfamiliar with these agents. As access expands through online vendors and social media, clinicians should routinely enquire about prescribed and non-prescribed agents when assessing unexplained illness or metabolic decompensation.
