Retatrutide Side Effects and Safety Profile: What the Trials Have Reported

Important: investigational medicine notice. Retatrutide is an investigational medication. It has not received marketing authorisation from the Medicines and Healthcare products Regulatory Agency (MHRA) or any other regulator, and it cannot be legally prescribed, dispensed or sold in the UK outside of an authorised clinical trial. This article is for information only. It is not medical advice and is not a recommendation to seek, request or use retatrutide. There is no UK Summary of Product Characteristics for retatrutide; the safety information here is drawn entirely from published trials and press releases. If you are considering weight-management medication, speak to your GP or a regulated UK prescriber about MHRA-approved options.

Any time a new medicine produces large headline efficacy numbers, it is worth looking carefully at the safety profile that goes with them. This article walks through what the published trials and reported topline results have said about the side effects and safety of retatrutide — what has been reported, what is typical for the medicine class, and what is being watched in the ongoing Phase 3 programme.

This is a clinical article. The summary is not "retatrutide is safe" or "retatrutide is risky" — neither of those binary statements is meaningful for a medicine still in development. What the evidence supports is a more measured picture: the side-effect profile is broadly typical of the GLP-1 class, gastrointestinal effects dominate and are dose-related, a heart-rate signal is being watched, and the full long-term safety picture across the larger Phase 3 population is still emerging.

A note on what we have to work with

For approved medicines, the most authoritative single source of safety information in the UK is the Summary of Product Characteristics (SmPC) — the official document that describes a medicine's licensed indications, contraindications, warnings and reported adverse events. The SmPC is reviewed by the MHRA before a medicine is authorised and is updated post-marketing as new safety information emerges.

There is no UK SmPC for retatrutide. There is no FDA prescribing information either. The medicine is investigational and has no regulatory label.

What we have instead is:

The published Phase 2 trial in the New England Journal of Medicine (Jastreboff et al., 2023).
The Phase 2a substudy in metabolic dysfunction-associated steatotic liver disease published in Nature Medicine (Sanyal et al., 2024).
Eli Lilly's topline investor releases for the Phase 3 trials that have reported — TRIUMPH-4 (December 2025) and TRANSCEND-T2D-1 (March 2026).

These are credible sources. They are not as detailed or comprehensive as a finalised SmPC. The reporting of adverse events in topline investor releases is, by design, summary-level.

The most common side effects

Across the published Phase 2 and the reported Phase 3 topline results, the most common adverse events with retatrutide are gastrointestinal:

Nausea
Diarrhoea
Vomiting
Constipation

The Phase 2 NEJM paper described these as dose-related (more common at higher doses), mostly mild to moderate in severity, and predominantly occurring during the dose-escalation phase. Starting at a lower dose (2 mg rather than 4 mg) and titrating upwards more gradually partially mitigated these effects.

This profile is broadly consistent with what is reported for currently approved GLP-1 medicines like semaglutide (Wegovy) and tirzepatide (Mounjaro). These effects are a feature of how the medicines work — slowing gastric emptying and acting on appetite-related pathways — rather than a separate toxicity issue.

For most patients on currently approved medicines, these effects ease over time, particularly after the dose-escalation phase is complete and the body adjusts. The same pattern was reported in the Phase 2 trial of retatrutide.

Heart rate

A finding worth specific mention from the Phase 2 trial: retatrutide produced a dose-dependent increase in heart rate that peaked at 24 weeks and declined thereafter. The NEJM paper noted that both glucagon and GLP-1 can have chronotropic (heart-rate) and inotropic (heart-contraction) effects, so this is mechanistically plausible — but it is a finding being examined more closely in the larger Phase 3 trials.

Whether the heart-rate signal represents a transient pharmacological effect that resolves with continued treatment, or a more sustained pattern relevant to long-term safety, will be clarified by the Phase 3 data. It is the most distinctive safety-related observation from the published retatrutide trial data and is appropriate to mention explicitly.

Other observations from the trial data

The Phase 2 NEJM trial reported, beyond the gastrointestinal effects and heart-rate observation:

Cardiometabolic improvements — reductions in waist circumference, blood pressure (systolic and diastolic), HbA1c, fasting glucose, fasting insulin, triglycerides and cholesterol (with the exception of HDL cholesterol).
Reversion to normoglycaemia in 72% of participants who had prediabetes at baseline.

These are favourable findings rather than safety concerns. They are included here for completeness because the full clinical picture of any medicine includes both its adverse events and its beneficial effects on cardiometabolic markers.

The TRIUMPH-4 Phase 3 topline release of 11 December 2025 stated that the safety profile of retatrutide was "consistent with the types of adverse events seen in clinical trials for other incretin-based therapies", and reported clinically meaningful improvements in cardiometabolic risk factors including non-HDL cholesterol, triglycerides and systolic blood pressure.

What is not yet known

A complete safety profile of any medicine emerges over time, not at the moment of regulatory submission. Several questions about retatrutide's safety remain open:

Long-term safety across years. Phase 2 ran for 48 weeks. Phase 3 TRIUMPH trials are running for around 68 weeks. Real-world safety over 2, 5 or 10 years on the medicine will only become clear after sustained post-marketing use, assuming the medicine is approved.

Safety in specific populations. The published Phase 2 trial excluded participants with type 2 diabetes. Phase 3 trials include participants with type 2 diabetes (TRIUMPH-2, TRANSCEND-T2D-1), with knee osteoarthritis (TRIUMPH-4), and with other conditions — but the smaller subgroups within these trials may not be sufficient to characterise safety in narrower populations (older adults, ethnic minorities, those with kidney disease, those on specific concomitant medications).

Drug interactions. A full understanding of how retatrutide interacts with other medicines comes from dedicated pharmacokinetic studies and from post-marketing observation. This is part of what the MHRA reviews at the marketing authorisation stage.

Rare adverse events. Adverse events that occur at very low frequency — say 1 in 5,000 or 1 in 10,000 patients — typically only become visible through post-marketing surveillance. The TRIUMPH programme has enrolled around 5,800 participants, which is large but not sufficient to fully characterise very rare events.

Effects in pregnancy and breastfeeding. As with most medicines, the trial populations exclude pregnant and breastfeeding individuals. Information here typically comes from post-marketing data and dedicated registries.

Class-related safety considerations to keep in mind

Because retatrutide acts on the same hormone receptors as currently approved GLP-1 medicines, some class-related considerations from those medicines may also be relevant. These have not all been specifically established for retatrutide and would be assessed in the MHRA's review at the time of any submission.

The general class considerations for GLP-1 medicines, as set out in the SmPCs for Wegovy and Mounjaro and the GOV.UK GLP-1 information, include:

Risk of pancreatitis — uncommon but reported, with clinicians typically advised to monitor for and discontinue at signs of pancreatitis.
Gallbladder-related events — cholelithiasis (gallstones) and cholecystitis (gallbladder inflammation) reported at higher frequency than placebo in trials of GLP-1 medicines.
Diabetic retinopathy progression in some populations with type 2 diabetes, when blood-sugar control improves rapidly.
Hypoglycaemia when used together with insulin or insulin secretagogues in adults with type 2 diabetes.
Specific contraindications — for example, semaglutide and tirzepatide are not licensed in adults with a personal or family history of medullary thyroid carcinoma or with multiple endocrine neoplasia syndrome type 2, based on findings in rodent studies. Whether equivalent contraindications would apply to retatrutide is a matter for the regulatory submission.

These are points a regulated UK prescriber would consider when discussing any medicine in this class with a patient.

Safety of unlicensed retatrutide is a separate concern

Everything above relates to retatrutide as administered in authorised clinical trials, under medical supervision, with clinical monitoring. The safety profile of unlicensed products sold online as "retatrutide" is a different matter entirely — and is the subject of MHRA enforcement action. Counterfeit or unlicensed pens may contain incorrect doses, contaminants, or no active ingredient at all. The risks are not those of retatrutide as a medicine; they are the risks of an unregulated product of unknown quality.

This is covered in detail in a separate article in this series, Unlicensed Retatrutide in the UK.

The bottom line

The retatrutide safety profile reported in published Phase 2 and Phase 3 topline results is broadly consistent with the GLP-1 medicine class: gastrointestinal effects dominate, dose-related and mostly mild to moderate, predominantly during dose-escalation. A dose-dependent increase in heart rate that peaked at 24 weeks and declined thereafter is a distinctive finding being examined further. There is no UK Summary of Product Characteristics for retatrutide; the medicine is not approved.

If you are considering weight-management medication, the place to begin is a conversation with your GP or a regulated UK prescriber, who will assess your individual circumstances and discuss the benefits and risks of the MHRA-approved options available to you.

Sources and further reading

Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. New England Journal of Medicine. 2023;389(6):514–526.
Sanyal AJ, Kaplan LM, Frias JP, et al. Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial. Nature Medicine. 2024.
Eli Lilly investor release, 11 December 2025: Lilly's triple agonist, retatrutide, delivered weight loss of up to an average of 71.2 lbs along with substantial relief from osteoarthritis pain in first successful Phase 3 trial (TRIUMPH-4 topline).
Eli Lilly investor release, 19 March 2026: Lilly's triple agonist, retatrutide, demonstrated significant reductions in A1C and weight in first Phase 3 trial for treatment of type 2 diabetes (TRANSCEND-T2D-1 topline).
UK Government / MHRA: GLP-1 medicines for weight loss and diabetes: what you need to know (GOV.UK publication).
MHRA: Yellow Card scheme for reporting suspected side effects.