Retatrutide (GLP-3) Side Effects & Safety

Retatrutide (LY3437943), also known informally as "GLP-3," is an investigational triple-agonist weight loss drug in Phase 3 clinical trials. Like all drugs in the GLP-1 class, it causes side effects — most commonly gastrointestinal symptoms such as nausea, diarrhea, and vomiting. These are generally mild to moderate and tend to occur during dose escalation.

The most comprehensive safety data available comes from the TRIUMPH-4 Phase 3 trial, which followed 445 participants over 68 weeks. This page summarizes what we know from that trial, what remains unknown, and how retatrutide's safety profile compares to existing GLP-1 drugs like semaglutide (Ozempic/Wegovy) and tirzepatide (Mounjaro/Zepbound).

Retatrutide has not been approved by the FDA. All safety data on this page comes from clinical trials with limited sample sizes. The full safety profile will only become clear with larger trials, regulatory review, and post-market surveillance.

Side Effect Rates from TRIUMPH-4

The TRIUMPH-4 trial (announced December 2025) tested retatrutide at 9mg and 12mg doses against placebo over 68 weeks in 445 adults with obesity and knee osteoarthritis. The most commonly reported adverse events were:

Discontinuation rates due to adverse events: 12.2% at 9mg, 18.2% at 12mg, and 4.0% for placebo. Eli Lilly noted that some discontinuations were attributed to "perceived excessive weight loss" rather than intolerable side effects.

There was no significant increase in serious adverse events between the retatrutide groups and the placebo group.

Source: Eli Lilly TRIUMPH-4 press release, December 2025

Gastrointestinal Side Effects

The most common side effects of retatrutide — nausea, diarrhea, constipation, and vomiting — are gastrointestinal (GI) in nature. This is consistent with the entire GLP-1 drug class. GLP-1 receptor activation slows gastric emptying (the rate at which food leaves the stomach), which is central to the drug's appetite-reducing mechanism but also the primary driver of GI symptoms.

Nausea

Nausea was the most frequently reported side effect in TRIUMPH-4, affecting 43.2% of participants at the 12mg dose compared to 10.7% on placebo. Nausea is typically most pronounced during the dose-escalation phase and tends to diminish as the body adjusts to the drug.

Diarrhea

Diarrhea affected approximately one-third of participants at both the 9mg (34.7%) and 12mg (33.1%) doses. This is consistent with rates seen in trials of other GLP-1 drugs and is generally classified as mild to moderate.

Constipation

Constipation occurred in 21.8% (9mg) to 25.0% (12mg) of participants, compared to 8.7% on placebo. Slowed gastric emptying can reduce bowel motility, which contributes to constipation. It may seem contradictory that retatrutide causes both diarrhea and constipation, but different individuals respond differently, and the same person may experience both at different points during treatment.

Vomiting

Vomiting was reported in approximately 20% of participants at both active doses and 0% on placebo. Like nausea, vomiting is more common during dose escalation and tends to improve over time.

Decreased Appetite

Decreased appetite — reported in 18-19% of active-dose participants — is technically a side effect, but also one of the drug's primary mechanisms of action. The distinction matters: in clinical trial reporting, any physiological change that participants report is listed as an adverse event, even when it is the intended therapeutic effect.

Dysesthesia: A New Safety Signal

One finding in TRIUMPH-4 that drew particular attention was dysesthesia — a condition in which normal sensations feel unusual or painful. This could include tingling, burning, numbness, or a heightened sensitivity to touch.

What the data shows

• Dysesthesia was reported in 20.9% of participants at 12mg, 8.8% at 9mg, and just 0.7% on placebo
• The effect is clearly dose-dependent — more than doubling from the 9mg to 12mg dose
• It was described as mild in most cases and rarely led to discontinuation
• It was not reported in retatrutide's Phase 2 trials, making it a new finding that emerged only in the larger, longer Phase 3 study

Why it matters

Dysesthesia can be a symptom of nerve-related conditions such as diabetic neuropathy or multiple sclerosis. In the context of retatrutide, the mechanism behind it is not yet understood. It is unclear whether retatrutide directly affects peripheral nerves, whether the symptom is related to rapid weight loss, or whether some other mechanism is involved.

BMO Capital Markets flagged dysesthesia as a signal worth monitoring in future TRIUMPH program readouts. Whether this finding becomes a significant safety concern or remains a mild, manageable effect will depend on data from the remaining Phase 3 trials and any long-term follow-up studies.

What we don't know about dysesthesia

• Whether symptoms resolve after stopping retatrutide
• Whether specific populations (people with diabetes, neuropathy, or other conditions) are at higher risk
• The underlying mechanism by which retatrutide might cause abnormal nerve sensations
• Whether this effect is unique to retatrutide or could emerge with other triple agonists

Dose-Dependent Side Effects and Titration

Side effects in TRIUMPH-4 were generally dose-dependent — more common and more frequent at 12mg than at 9mg. This pattern is consistent across the GLP-1 drug class: higher doses produce greater efficacy but also more side effects.

The titration schedule

To manage this, retatrutide uses a gradual titration schedule:

1. Start at 2mg once weekly
2. Increase the dose every 4 weeks in stepwise increments
3. Reach the target dose (9mg or 12mg) over several months

This slow escalation gives the body time to adapt to each dose level. Most GI side effects — nausea, vomiting, diarrhea — are concentrated during the early weeks of each dose increase and tend to subside as the body adjusts.

Why titration matters

Without titration, starting directly at a high dose would cause severe GI symptoms in most patients. The titration strategy is the same one used for semaglutide (Ozempic/Wegovy) and tirzepatide (Mounjaro/Zepbound), and it is a major reason why GI side effects in clinical trials, while common, are generally classified as mild to moderate and tolerable for the majority of participants.

How Side Effects Compare to Ozempic and Mounjaro

Retatrutide's side effect profile is broadly similar to other GLP-1 drugs. Here is how nausea rates compare across the landmark trials for each drug:

These numbers suggest that adding a third receptor (glucagon) does not dramatically increase GI side effects compared to existing drugs. Retatrutide's nausea rate at 12mg is comparable to semaglutide at 2.4mg and higher than tirzepatide at 15mg.

However, cross-trial comparisons should be interpreted with caution. These trials had different patient populations, study designs, and duration. Only head-to-head trials can definitively compare safety profiles, and none have been published comparing retatrutide directly to semaglutide or tirzepatide.

The new signal to watch is dysesthesia, which has not been reported as a significant finding in semaglutide or tirzepatide trials.

For a detailed comparison of retatrutide versus existing drugs, see Retatrutide vs Mounjaro vs Ozempic.

Cardiovascular Effects

TRIUMPH-4 reported several positive cardiovascular outcomes in the retatrutide groups:

• Systolic blood pressure reduced by 14.0 mmHg at the 12mg dose
• Improvements in non-HDL cholesterol, triglycerides, and hsCRP (a marker of systemic inflammation)

These cardiometabolic benefits are consistent with effects seen across the GLP-1 drug class and are likely driven by a combination of weight loss and direct metabolic effects of the drug. Semaglutide has already demonstrated cardiovascular risk reduction in the SELECT trial, and whether retatrutide offers similar or greater cardiovascular protection is a question that dedicated cardiovascular outcome trials would need to answer.

Source: Eli Lilly TRIUMPH-4 press release, December 2025

Class-Level Safety Concerns

Retatrutide belongs to the GLP-1 receptor agonist class, and several safety concerns apply broadly to drugs in this category. These have not been specifically flagged as problems in retatrutide trials, but they warrant awareness.

Thyroid cancer risk

All GLP-1 receptor agonists carry a boxed warning about the risk of medullary thyroid carcinoma (MTC). This warning is based on findings in rodent studies where GLP-1 drugs caused thyroid C-cell tumors. Whether this translates to humans remains an open question — recent epidemiological evidence suggests that GLP-1 drugs are not associated with increased thyroid cancer risk in humans, but longer-term data and more studies are needed. People with a personal or family history of MTC or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2) should not use GLP-1 drugs.

Gallbladder events

GLP-1 drugs have been associated with an increased incidence of gallbladder-related events, including gallstones and cholecystitis (inflammation of the gallbladder). Rapid weight loss itself is a known risk factor for gallstones. This has not been specifically flagged in retatrutide trial data, but it remains a theoretical risk given the class association and the significant amount of weight loss retatrutide produces.

Pancreatitis

Pancreatitis (inflammation of the pancreas) has been identified as a potential risk with GLP-1 drugs generally. While the absolute risk appears to be low, patients are typically monitored for symptoms such as severe abdominal pain. The retatrutide Phase 2 trial did not report pancreatitis as a significant finding, but the sample sizes in trials to date have been relatively small.

Muscle and Bone Considerations

A persistent concern with all weight loss interventions — drugs, surgery, or caloric restriction — is the loss of lean body mass (muscle) alongside fat. When the body loses a substantial amount of weight, some of that loss inevitably comes from muscle tissue, not just fat.

What we know

• All GLP-1 drugs cause some degree of lean mass loss alongside fat loss. In semaglutide trials, approximately 40% of weight lost was lean mass in some analyses.
• Retatrutide's glucagon receptor activation is hypothesized to have muscle-sparing properties. Glucagon promotes lipolysis (fat breakdown) and thermogenesis (increased energy expenditure), which could theoretically shift the body's energy sourcing toward fat rather than muscle. However, this hypothesis has not been confirmed with body composition data from Phase 3 trials.
• No published retatrutide trial has reported detailed body composition analysis (e.g., DEXA scans) distinguishing fat loss from lean mass loss.

What we don't know

• Whether retatrutide's triple-agonist mechanism results in a better fat-to-lean-mass loss ratio than semaglutide or tirzepatide
• Long-term effects on bone mineral density, which can decline with significant weight loss
• Whether resistance training during retatrutide treatment meaningfully preserves lean mass (this is the standard clinical recommendation for all weight loss interventions, but has not been specifically studied in retatrutide trials)

What We Don't Know Yet

Retatrutide is still in clinical development. While TRIUMPH-4 provides the best safety data available, there are significant gaps in our knowledge:

• Long-term safety beyond 68 weeks: TRIUMPH-4 is the longest published trial. We do not know what happens with years of continuous use, which is the likely treatment duration for a chronic obesity medication.
• Safety in larger populations: TRIUMPH-4 enrolled only 445 participants. Rare side effects — those occurring in fewer than 1 in 100 or 1 in 1,000 people — may not appear until the drug is used by thousands or millions of patients.
• Dysesthesia resolution: It is unknown whether the abnormal nerve sensations seen in TRIUMPH-4 resolve after stopping the drug, persist, or progress.
• Drug interactions: Retatrutide's interactions with other medications have not been extensively characterized. Since it slows gastric emptying, it could theoretically affect the absorption of oral medications.
• Specific populations: Safety data in pregnant or breastfeeding women, elderly patients (over 75), adolescents, and people with severe kidney or liver disease is limited or nonexistent.
• Weight regain after stopping: Like other GLP-1 drugs, weight regain after discontinuation is expected, but the rate and extent are not yet characterized for retatrutide.

These gaps will begin to close as additional TRIUMPH program trials report results throughout 2026 and as the FDA conducts its own review of the complete safety database. For the latest trial results and timeline updates, see Clinical Trials & Results and FDA Approval Timeline.

Frequently Asked Questions

Sources

• Eli Lilly and Company. (2025). Lilly's retatrutide achieved significant weight loss and pain relief in adults with obesity and knee osteoarthritis. Press release.
• Jastreboff, A.M., et al. (2023). Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. New England Journal of Medicine. DOI: 10.1056/NEJMoa2301972
• Wilding, J.P.H., et al. (2021). Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). New England Journal of Medicine. DOI: 10.1056/NEJMoa2032183
• Jastreboff, A.M., et al. (2022). Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). New England Journal of Medicine. DOI: 10.1056/NEJMoa2206038
• Lincoff, A.M., et al. (2023). Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes (SELECT). New England Journal of Medicine. DOI: 10.1056/NEJMoa2307563
• ClinicalTrials.gov: NCT04881760 (Phase 2, Obesity), NCT04867785 (Phase 2, T2D)

Medical Disclaimer

The content on glp3.wiki is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. Retatrutide is an investigational drug that has not been approved by the U.S. Food and Drug Administration (FDA) or any other regulatory agency.

Do not use this information to make decisions about your health without consulting a qualified healthcare provider. The clinical trial data cited on this page reflects results from controlled research settings with limited sample sizes and may not reflect real-world outcomes.

Side effects, risks, and safety data described here are based on published clinical trial results and press releases available as of February 2026. As new data emerges from ongoing Phase 3 trials and regulatory review, the understanding of retatrutide's safety profile may change. This page will be updated as new information becomes available.

If you are experiencing side effects from any medication, contact your healthcare provider. If you are considering weight loss medication, talk to your doctor about currently approved options. For information about enrolling in retatrutide clinical trials, visit ClinicalTrials.gov.

This site is not affiliated with Eli Lilly and Company or any pharmaceutical manufacturer.