Retatrutide Side Effects: What Research Reports

TL;DR

- Retatrutide is an investigational triple agonist that activates the GLP-1, GIP, and glucagon receptors; the largest published human data comes from a phase 2 obesity trial (Jastreboff et al., N Engl J Med 2023; doi:10.1056/NEJMoa2301972; PMID 37366315). - In that trial, the most commonly reported adverse events were gastrointestinal, mainly nausea, diarrhea, vomiting, and constipation (Jastreboff et al., 2023). - The adverse events were dose-dependent: they showed up more often as the studied dose rose from 1 mg to 12 mg once weekly (Jastreboff et al., 2023). - The same trial reported weight reduction of up to about 24.2 percent at 48 weeks, which is why the compound draws research interest (Jastreboff et al., 2023). - Retatrutide is not an approved medicine; every figure here describes what a trial reported, not guidance for personal use. A personal dose decision belongs with a qualified clinician.

What side effects did the retatrutide phase 2 trial report?

The phase 2 trial reported that gastrointestinal events were the most common adverse events, and that they became more frequent at higher studied doses (Jastreboff et al., 2023).

The gastrointestinal events reported most often were nausea, diarrhea, vomiting, and constipation (Jastreboff et al., 2023). The trial characterized most of these events as mild to moderate rather than severe (Jastreboff et al., 2023). This pattern, where stomach and gut effects lead the adverse event list, is the through-line of the published safety picture.

Because retatrutide has only reached phase 2 in the sources cited here, the long-term human safety record is still limited. Phase 3 studies are underway, but their results are not part of the data referenced on this page.

Why does retatrutide cause gastrointestinal side effects?

The gut pattern traces to how incretin drugs work: they act on GLP-1 receptors in the gut and the brain, shifting appetite and satiety signaling, which commonly brings on nausea and related effects.

Drugs in this class slow the rate at which the stomach empties, and they also act centrally on brain regions that regulate appetite and nausea. A 2025 review (Jalleh et al., J Clin Endocrinol Metab 2025;110(1):1-15; doi:10.1210/clinem/dgae719) notes that the link between slowed stomach emptying and symptoms like nausea is weak, and that the nausea seen with these drugs appears to be driven mainly by central, brain-level signaling rather than by gastric transit alone. Retatrutide activates the GLP-1 and GIP receptors, the same targets behind other incretin therapies. Semaglutide, a GLP-1 receptor agonist, produced about 15 percent mean weight loss in its pivotal trial and carried a gut-led side effect profile (Wilding et al., N Engl J Med 2021; doi:10.1056/NEJMoa2032183). Tirzepatide, which activates GLP-1 and GIP, produced up to about 22.5 percent weight loss and showed the same gut-predominant pattern (Jastreboff et al., N Engl J Med 2022; doi:10.1056/NEJMoa2206038). Retatrutide's gut effects sit in the same family (Jastreboff et al., N Engl J Med 2023; doi:10.1056/NEJMoa2301972).

What makes retatrutide different from other incretin drugs?

Retatrutide adds glucagon receptor agonism to the GLP-1 and GIP activity found in tirzepatide, which makes it a triple hormone receptor agonist (Jastreboff et al., 2023).

The added glucagon arm is the defining feature. Where semaglutide acts on one receptor and tirzepatide acts on two, retatrutide engages three. The comparison below places the three compounds side by side using their pivotal trial results.

The trial also reported dose-related increases in heart rate that peaked at around 24 weeks and then declined by week 48 (Jastreboff et al., 2023). This heart rate observation is one reason cardiovascular measures are tracked closely in incretin research.

Were the retatrutide side effects dose-dependent?

Yes: the trial reported that adverse events, especially gastrointestinal ones, grew more frequent as the studied dose increased (Jastreboff et al., 2023).

The ranges below reflect what published studies and commonly studied research protocols report. This is educational, not a prescription or a personal recommendation.

In the trial, participants received retatrutide by weekly subcutaneous injection at one of several maintenance doses, or placebo. The dose groups are shown below, framed as a research protocol, not as a schedule for personal use.

These numbers describe what a trial studied. They are not a recommendation, and they do not tell any reader what to take, when, or how. Any decision about a personal dose belongs with a qualified clinician.

How serious were the reported retatrutide side effects?

The trial characterized most reported adverse events as mild to moderate, with gastrointestinal complaints leading the list (Jastreboff et al., 2023).

Retatrutide remains investigational. The phase 2 trial gives the clearest published human picture so far, but it followed 338 participants for 48 weeks, which is short relative to the timelines used to judge long-term safety (Jastreboff et al., 2023). Larger and longer phase 3 trials are in progress, and their safety results are not included in the sources cited here. Until those read out, any statement about rare or long-term effects would go beyond the published data.