Peptide Science

Retatrutide Side Effects: What the Trials Reported

By UK Peptide Lab•26 July 2026•10 min read

Key Takeaways

The adverse events reported in the phase 2 trials were overwhelmingly gastrointestinal: nausea, diarrhoea, vomiting and constipation.
They were dose-dependent and concentrated during dose escalation rather than spread evenly across the trial.
A dose-dependent increase in heart rate was reported, peaking around week 24 and declining afterwards.
Most events were graded mild to moderate, but a minority of participants discontinued because of them, concentrated in the higher-dose arms.
The largest safety question is not what the trials found, it is what they could not: there is no long-term safety data, no completed phase 3, and no post-marketing surveillance because there is no marketed product.
Every figure here comes from supervised clinical trials with medical oversight and defined escalation. None of it transfers to unsupervised use.

Where this information comes from

Two published phase 2 trials account for nearly everything known about retatrutide's adverse-event profile. The first is the obesity trial reported by Jastreboff and colleagues in the New England Journal of Medicine in 2023, which randomised 338 adults across placebo and several retatrutide dose arms over 48 weeks. The second is the type 2 diabetes trial reported by Rosenstock and colleagues in the Lancet the same year. Earlier phase 1b work by Urva and colleagues established the pharmacokinetics. That is the entire evidence base. Phase 3, the TRIUMPH programme, is ongoing and has not reported. Anyone presenting a fuller safety picture than those trials support is filling gaps with something other than data.

The gastrointestinal profile dominates everything else

The consistent finding across both trials is that retatrutide's adverse events are overwhelmingly gastrointestinal. Nausea, diarrhoea, vomiting and constipation were the most frequently reported events, and the great majority were graded mild to moderate. This is not surprising mechanistically. GLP-1 receptor agonism slows gastric emptying and acts on brainstem regions involved in nausea, and the entire incretin class shares this profile. What retatrutide adds is a third receptor, and it does not appear to add a novel adverse-event category so much as the same one at a magnitude that tracks its overall potency. Two features of the pattern matter more than the raw frequency. First, the events were dose-dependent, appearing more often in higher-dose arms. Second, they concentrated during escalation rather than distributing evenly across the trial, which is why the trials built stepwise increases into their design at all.

Escalation speed changed the outcome

The obesity trial did something useful for interpretation: it included arms that reached the same target dose by different routes, one escalating faster and one slower. The arms that arrived at a given dose more gradually reported fewer gastrointestinal adverse events than the arms that got there quickly, despite ending at the same place. That is a meaningful finding, because it separates the dose itself from the rate of change as a driver of tolerability. It is also the clearest illustration of why trial conditions are not transferable. The escalation schedules were designed, supervised and adjusted by investigators, with participants monitored throughout and able to have their dose held or reduced. That structure is doing part of the work the tolerability numbers reflect.

Heart rate

The obesity trial reported a dose-dependent increase in heart rate. It peaked at around 24 weeks and then declined over the remainder of the 48-week period. Increases in heart rate are documented across the GLP-1 receptor agonist class, so this is not a retatrutide-specific finding. There is a mechanistic reason to pay attention to it here in particular, though: glucagon receptor agonism raises energy expenditure, and the cardiovascular consequences of sustained glucagon receptor activation in humans are less well characterised than those of GLP-1 agonism. That is a gap in knowledge, not a demonstrated problem, and it should be described as one.

Discontinuations

Adverse-event tables can mislead, because they count every recorded event regardless of consequence. A trial in which everybody reports transient mild nausea in week two looks worse on an event count than one in which a handful of people become unwell enough to stop. The more informative figure is discontinuation due to adverse events. In the phase 2 obesity trial a minority of participants stopped for this reason, and those discontinuations were concentrated in the higher-dose arms, matching the dose-dependence seen in the event profile itself. Read against the weight-change outcomes reported in the same trial, that is the actual trade-off the data describes: the arms that produced the largest changes were also the arms people left most often.

What has not been established

This is the part most pages on this subject omit, and it is the more important half. There is no completed long-term safety data. The longest published exposure is the 48-week phase 2 period. Effects emerging over multiple years are simply unknown, and for a mechanism this novel that is a substantial gap rather than a formality. There is no phase 3 result. TRIUMPH is running and has not reported. Phase 2 trials are not powered to detect uncommon adverse events; a problem occurring in one participant per thousand would very likely be invisible in a 338-person study. There is no post-marketing surveillance, because there is no marketed product anywhere in the world. For licensed medicines, a large share of what is eventually known about rare harms comes from this stage. For retatrutide that entire layer of evidence does not exist. And there is no data at all on unsupervised use, on material of uncertain provenance, or in the populations excluded from the trials.

Reading the comparisons honestly

Comparisons between retatrutide, semaglutide and tirzepatide circulate constantly, including for tolerability. They should be treated with care. No head-to-head trial exists. Every comparison you will encounter places numbers from separate trials side by side, and those trials differed in population, duration, escalation schedule, background care and adverse-event reporting conventions. Cross-trial comparison produces figures that carry an air of precision they have not earned. What can be said at the level of category is that the three share an adverse-event profile dominated by gastrointestinal effects, dose-dependent in each case. Ranking them against each other on tolerability requires a study nobody has run.

What this page is, and is not

This is a summary of adverse events reported in published clinical trials of retatrutide, written because the question is asked constantly and answered badly nearly everywhere, usually by pages more interested in the efficacy figures. It is not medical advice, it is not a safety assessment, and it is not a protocol. It contains no instruction about quantities, schedules or administration, and deliberately so. Every figure described here was produced under medical supervision, with defined escalation, monitoring and the ability to hold or reduce exposure. None of that structure exists outside a trial, and the numbers do not transfer without it. Retatrutide supplied by UK Peptide Lab is a lyophilised research chemical for in-vitro laboratory use only. It is not a licensed medicine in the UK or anywhere else, and is not supplied for human consumption, veterinary use, or any therapeutic purpose. For the legal position, see our guide to whether peptides are legal in the UK.

Related Products

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Disclaimer: This article is for research and educational purposes only. All information provided is not intended as medical advice. UK Peptide Lab products are not for human consumption and are sold strictly for laboratory research use only.

Frequently Asked Questions

What are the side effects of retatrutide?

In the published phase 2 trials the reported adverse events were predominantly gastrointestinal: nausea, diarrhoea, vomiting and constipation. They were dose-dependent, most were graded mild to moderate, and they clustered during dose escalation rather than being spread evenly across the trial. A dose-dependent increase in heart rate was also reported, peaking around week 24 and declining afterwards. This is trial data from a supervised setting, not a description of what happens outside one.

Does retatrutide cause nausea?

Nausea was among the most frequently reported adverse events in both phase 2 trials, and its frequency tracked with dose. The trials also found that escalation speed mattered: arms that reached a given dose through a slower stepwise increase reported fewer gastrointestinal events than arms that arrived at the same dose faster. That pattern is consistent across the incretin drug class rather than unique to retatrutide.

Is retatrutide safe?

That question cannot be answered yet, and any source telling you otherwise is overstating what exists. Retatrutide has completed phase 2 and is in phase 3. There is no completed long-term safety dataset, no regulatory approval in any country, and no post-marketing surveillance, because there is no marketed product to survey. What can be said is narrower: across roughly a year of supervised trial exposure the adverse events reported were mostly gastrointestinal and mostly mild to moderate. Absence of a signal in a phase 2 trial is not evidence of long-term safety.

Does retatrutide affect heart rate?

The phase 2 obesity trial reported a dose-dependent increase in heart rate that peaked at around 24 weeks and then declined over the remainder of the 48-week period. Heart rate increases have been observed across the GLP-1 receptor agonist class, so this is not specific to retatrutide, though retatrutide's glucagon receptor activity is a mechanistically plausible additional contributor given its effect on energy expenditure.

How do retatrutide side effects compare to semaglutide or tirzepatide?

The categories overlap heavily: gastrointestinal events dominate for all three, are dose-dependent for all three, and concentrate during escalation for all three. Direct comparison beyond that is not supportable, because there has never been a head-to-head trial. Comparing adverse-event rates across separate trials with different populations, durations, escalation schedules and reporting conventions produces numbers that look precise and are not.

Why did people drop out of the retatrutide trials?

A minority of participants discontinued because of adverse events, and those discontinuations were concentrated in the higher-dose arms, consistent with the dose-dependence of the gastrointestinal profile. Discontinuation rates are the most useful single number in an adverse-event table, because they measure events severe enough that someone stopped, rather than events merely recorded.

References

[1] Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial (Jastreboff et al.). New England Journal of Medicine (2023). View →
[2] Retatrutide (LY3437943), a GIP, GLP-1 and glucagon receptor agonist, in people with type 2 diabetes: a phase 2 trial (Rosenstock et al.). The Lancet (2023). View →
[3] LY3437943, a novel triple GIP, GLP-1 and glucagon receptor agonist: phase 1b multiple-ascending-dose trial (Urva et al.). The Lancet (2022). View →