Does Retatrutide Cause Diarrhoea? Trial Data

Retatrutide, an investigational triple receptor agonist developed by Eli Lilly, has no UK marketing authorisation and is available only within authorised clinical trials. A common question arising from early trial data is whether retatrutide causes diarrhoea. Phase 2 findings published in the New England Journal of Medicine in 2023 identified gastrointestinal adverse events, including diarrhoea, nausea and vomiting, among the most frequently reported effects across active dose groups. This article summarises what trial data show about these effects, who may be more susceptible, how serious they appear to be, and the risks of seeking retatrutide outside a formal clinical trial.

What is retatrutide and what is its regulatory status in the UK?

Retatrutide has no UK marketing authorisation and can only be supplied within an authorised clinical trial. It is an investigational GLP-1, GIP and glucagon triple receptor agonist developed by Eli Lilly, currently being studied in the Phase 3 TRIUMPH programme.

Retatrutide holds no marketing authorisation in the UK or anywhere else, and it can be supplied only within an authorised clinical trial.^[1][2] It is an investigational medicine developed by Eli Lilly that acts simultaneously on three hormone receptors: the glucagon-like peptide-1 (GLP-1) receptor, the glucose-dependent insulinotropic polypeptide (GIP) receptor, and the glucagon receptor.^[2][3] This triple agonist mechanism is designed to influence appetite regulation, energy expenditure and glucose metabolism, distinguishing it pharmacologically from currently authorised dual agonists such as tirzepatide (Mounjaro) and single agonists such as semaglutide (Wegovy, Ozempic).

Because retatrutide is not authorised, it cannot be marketed, advertised or sold to the public. The MHRA has not issued a marketing authorisation, and NICE has therefore not evaluated it for NHS use. No UK launch date can be given, and no timeline for authorisation is available. Clinical research is ongoing through the TRIUMPH programme of Phase 3 trials, which are investigating retatrutide in adults with obesity and related conditions, and through the TRANSCEND-T2D-1 study in type 2 diabetes.^[4][11] Findings from these trials will form part of any future regulatory submission.

Anyone who encounters retatrutide offered for sale online, outside a formal clinical trial, should be aware that such a product is not a licensed medicine and may be falsified or counterfeit. If you are interested in licensed weight-management treatments, a GP can discuss the options currently available on the NHS or through regulated private services.

Who is most likely to experience digestive side effects in trials?

Participants receiving higher doses of retatrutide and those in the dose-escalation phase are most likely to experience gastrointestinal side effects, including diarrhoea. These effects are consistent with GLP-1 receptor agonist class pharmacology and were more frequent early in treatment.

Trial data suggest that gastrointestinal side effects, including diarrhoea, nausea and vomiting, are more common in participants receiving higher doses of retatrutide and during the dose-escalation phase of treatment. This pattern is consistent with the pharmacology of GLP-1 receptor agonism, which slows gastric emptying and alters gut motility. Because retatrutide also activates the glucagon receptor, its overall effect on the gastrointestinal tract may differ in character or intensity from that of single or dual agonists, though direct head-to-head comparisons between trials are not valid given differences in study design, populations and duration.

In the Phase 2 trial published in the New England Journal of Medicine in 2023, gastrointestinal adverse events were among the most frequently reported findings across all active dose groups.^[2][3] Participants who escalated to higher doses experienced these events more often than those on lower doses, and events were generally more frequent early in treatment.^[2][3] Phase 3 readouts from the TRIUMPH programme and the TRANSCEND-T2D-1 study are now available and are expected to provide a fuller picture of the gastrointestinal safety profile across larger and more diverse populations; the Phase 2 findings should therefore be understood as an early signal rather than a definitive characterisation.

For GLP-1 receptor agonist medicines as a class, gastrointestinal symptoms are a recognised feature, and trial protocols for retatrutide have used structured titration schedules to manage tolerability. Whether individual factors such as a prior history of gastrointestinal conditions influence the risk of pronounced symptoms with retatrutide specifically has not been established in published retatrutide data; this remains a general class consideration pending further evidence.

Trial participants who experience gastrointestinal symptoms should follow the guidance provided by their trial team. Staying well hydrated is important if diarrhoea or vomiting occurs. It is important to note that trial participants are a selected population: individuals who are pregnant, breastfeeding or planning pregnancy are excluded, as are those with certain comorbidities, and contraception requirements apply within trial protocols. The side-effect profile observed in a trial population may therefore not fully reflect what would be seen across a broader clinical population if the medicine were eventually authorised.

How serious are the gastrointestinal effects seen in clinical trials?

Gastrointestinal effects in the Phase 2 trial were generally mild to moderate, with most participants continuing treatment, though discontinuation rates were higher in the highest-dose groups. Severe or persistent abdominal pain should be reported to the trial team immediately.

In clinical trial reports, diarrhoea and other gastrointestinal effects associated with retatrutide have generally been described as mild to moderate in severity, with most participants continuing treatment. However, a proportion of participants in the Phase 2 trial did discontinue due to gastrointestinal adverse events, and this rate was higher in the groups receiving the highest doses.^[2][3] These findings are reported as outcomes of a specific trial population and cannot be taken as a prediction of what any individual would experience.

The UK SmPC for authorised GLP-1 receptor agonists notes that acute pancreatitis has been reported, including post-marketing reports of necrotising pancreatitis and reports with a fatal outcome, though no risk magnitude is established.^[14][15] While retatrutide is not yet authorised and has no UK SmPC, its mechanism of action includes GLP-1 receptor agonism, and the class-level signal for pancreatitis is relevant context for researchers and clinicians involved in its trials. Participants in retatrutide trials who develop persistent or severe abdominal pain should report this to their trial team immediately, following the study protocol.

Gastrointestinal symptoms that are severe, persistent or accompanied by signs of significant dehydration warrant prompt clinical assessment. Within a trial setting, participants should follow the escalation guidance provided by the trial team. NHS 111 is available for urgent medical advice outside trial hours. Trial participants should also inform any emergency or urgent-care clinician that they are enrolled in a clinical trial, and should carry their trial card or study contact details so that the treating clinician can liaise with the trial team if needed.

Suspected side effects arising during participation in an authorised clinical trial should be reported to the trial team under the study protocol, as the trial has its own pharmacovigilance and safety-reporting procedures.

What are the risks of obtaining retatrutide outside a clinical trial?

Any retatrutide obtained outside an authorised clinical trial is unlicensed and may be falsified, contaminated or mislabelled, as the MHRA has not assessed it for quality, safety or efficacy. No pharmacy or clinic can legally supply retatrutide outside a clinical trial setting in the UK.

Obtaining retatrutide outside an authorised clinical trial carries serious risks, because any product sold or supplied through unofficial channels is not a licensed medicine and may be falsified, contaminated or mislabelled. Retatrutide has no UK marketing authorisation, which means it has not been assessed by the MHRA for quality, safety or efficacy in a regulatory context. A product sold online under this name provides no guarantee of what it actually contains, and using an unverified injectable substance carries risks of infection, incorrect dosing and unpredictable adverse effects.

The absence of a licensed product also means there is no approved patient information leaflet and no regulated dosing schedule. There is no product-specific pharmacovigilance system covering retatrutide in the general population, because the medicine exists only within clinical trials. However, if you experience a suspected adverse reaction after using any product obtained outside regulated channels, including an unlicensed or suspected counterfeit product, this can be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk. You should also seek prompt medical assessment and disclose the exact product and source to the clinician treating you.

The MHRA has highlighted the risks associated with unlicensed weight-management products circulating online, and the same concerns apply to any investigational medicine offered outside its trial framework.^[18]

If you are concerned about your weight or metabolic health, a GP is the appropriate first point of contact. Licensed treatments for weight management and type 2 diabetes are available in the UK, and a GP or specialist can advise on eligibility and the options that are currently authorised and accessible. Decisions about NHS commissioning of specific treatments are made by Integrated Care Boards (ICBs) based on NICE guidance and local priorities.^[19] No pharmacy or clinic can legally supply retatrutide outside a clinical trial setting.

Frequently Asked Questions