What are the side effects of retatrutide?
Retatrutide's Phase 2 AE profile is dominated by GI events — nausea, diarrhea, vomiting — similar to other GLP-1 drugs but more pronounced at high doses.
Updated May 25, 2026 · 4 min read
Retatrutide's side effect profile, based on its Phase 2 trial published in the New England Journal of Medicine in 2023, is dominated by gastrointestinal events — exactly what you'd expect from a drug that activates GLP-1 receptors. The main question is whether the third agonist target (glucagon) adds meaningfully to the side effect burden compared to GLP-1/GIP dual agonists like tirzepatide. The short answer: it may, particularly at higher doses.
The Phase 2 GI Profile
In the Phase 2 trial, the most commonly reported adverse events were:
- Nausea — the most frequent, reported by a substantial proportion of participants at higher doses
- Diarrhea — second most common
- Vomiting — less common than nausea but still notable at higher doses
- Constipation — reported at rates similar to other GLP-1 drugs
- Decreased appetite — technically an adverse event in the trial data, though for most patients it's the desired effect
These events were dose-dependent — higher retatrutide doses produced more GI adverse events — and frontloaded — most occurred in the first several weeks of treatment and particularly in the week after a dose escalation, then subsided. This pattern is consistent with how semaglutide and tirzepatide behave.
The trial used a titration protocol that started at 1 mg and ramped up every 4 weeks, specifically designed to limit GI adverse events by giving the gut time to adapt.
How It Compares to Tirzepatide
Side-by-side comparison is imperfect because the trials weren't designed head-to-head, the populations differed, and the dose ranges don't map cleanly. With that caveat:
| Side effect | Retatrutide (Phase 2) | Tirzepatide (SURMOUNT-1) |
|---|---|---|
| Nausea | More pronounced at max dose | Moderate, dose-dependent |
| Vomiting | Higher rate at max dose | Lower than semaglutide |
| Diarrhea | Common | Common |
| Constipation | Present | Present |
| Discontinuation due to GI AEs | Higher at highest doses | Lower |
The glucagon agonism in retatrutide is the likely reason for the somewhat higher GI burden at peak doses. Glucagon affects gut motility and can amplify the gastric emptying effects already produced by GLP-1 signaling.
The Glucagon Component: Extra Considerations
Retatrutide's third mechanism — glucagon receptor agonism — adds some considerations not present in semaglutide or tirzepatide:
Heart rate. GLP-1 drugs as a class tend to modestly increase resting heart rate (typically by 2–4 bpm in clinical trials). Glucagon agonism can have similar effects. The Phase 2 trial monitored cardiovascular parameters; no significant cardiac safety signals emerged, but Phase 3 will provide the statistical power to detect subtler effects.
Blood glucose variability. Glucagon raises blood sugar (it's the counter-regulatory hormone to insulin). In people without diabetes, this effect is largely buffered by intact insulin secretion — retatrutide's GLP-1 component also enhances insulin release, partially counterbalancing the glucagon effect. In people with T2D, the interaction is more complex; Phase 3 trials include a T2D population specifically to clarify this.
Hypoglycemia. Despite the blood sugar concerns above, hypoglycemia was uncommon in Phase 2, as expected in people without diabetes. The insulin-stimulating GLP-1 effects are glucose-dependent — meaning they don't push insulin release when blood sugar is already normal or low.
What Phase 3 Is Measuring
The TRIUMPH Phase 3 program, underway as of early 2026, will provide a substantially larger safety dataset. The specific areas being monitored:
- Cardiovascular outcomes (MACE, heart rate, blood pressure)
- Longer-term GI tolerability at maintenance doses
- Pancreatic and hepatic enzymes
- Kidney function
- Thyroid C-cell effects (required for all GLP-1 class drugs based on rodent study data)
Phase 3 will almost certainly confirm the GI dominance of the AE profile. The more important question is whether any of the glucagon-related theoretical risks materialize at scale.
Practical Expectations
If retatrutide is eventually approved and you start on it:
- Expect nausea and possible GI disruption in the first few weeks of each dose increase
- The titration schedule will be slow — this is likely to be even more gradual than tirzepatide's given the higher GI burden at max doses
- People who've tolerated tirzepatide well tend to tolerate retatrutide, based on the overlapping mechanism; people who struggled on tirzepatide may find it harder
- The side effects, while more pronounced at maximum dose, are manageable at moderate doses for most people — and the weight loss at moderate doses still substantially exceeds what semaglutide produces