Retatrutide risks and side effects: what the clinical trials actually show

Retatrutide is an investigational once-weekly injectable that activates the GIP, GLP-1 and glucagon receptors. In the published phase 2 obesity trial, the most common side effects were gastrointestinal (nausea, diarrhea, vomiting, constipation), they were dose-related and mostly mild to moderate, and heart rate rose in a dose-dependent way, peaking at 24 weeks. As of September 2026 retatrutide is not approved by the FDA or the EMA; Eli Lilly has said it plans to file for U.S. approval in the first quarter of 2027.

Method in one line: every figure on this site comes from a peer-reviewed publication, a ClinicalTrials.gov registration, an FDA prescribing document or a dated company announcement, each cited with a clickable DOI or URL on the same page.

Articles

• Retatrutide side effects: every adverse event the trials have reported so far

  Retatrutide side effects by dose from the NEJM and Lancet phase 2 trials and the TRIUMPH phase 3 announcements: GI events, heart rate, dysesthesia, dropouts.

• Retatrutide dosage: the doses the trials actually used, and what is not known

  The exact retatrutide doses and escalation steps used in phase 2 and phase 3 trials, what each produced, what is unknown, why forum charts are not protocols.

• Reta peptide: what retatrutide is, how it works and what the evidence says

  'Reta' is retatrutide (LY3437943), Lilly's investigational GIP, GLP-1 and glucagon triple agonist: what it is, how it works, trial results, unapproved status.

• Retatrutide on Reddit: what the forums report, checked against the trials

  What Reddit reports about retatrutide (weight loss, nausea, heart rate, fatigue, sourcing), checked against the published phase 2 and phase 3 trial data.

• GLP-3 peptide: there is no such hormone, and what people mean when they say it

  'GLP-3' is not a hormone. The term means retatrutide, a GIP, GLP-1 and glucagon triple agonist. What GLP-3 peptide means and the side effects trials report.

• Where to buy retatrutide: what is legal, what the grey market is, and how to read a COA

  Retatrutide is investigational and not sold as a medicine anywhere. What research use only means, how the grey market works, what a COA must show, scam signs.

• Retatrutide results: the weight loss numbers from every trial, by dose and by week

  Every published retatrutide result: mean weight loss by dose at 24 to 104 weeks, responder rates, HbA1c and liver fat, from the papers and TRIUMPH releases.

• Retatrutide FDA approval: where the process stands and what is known about the timeline

  Retatrutide is not FDA approved as of September 2026. TRIUMPH phase 3 status, registry completion dates, Lilly's plan to file in Q1 2027, and the rumors.

• Retatrutide vs tirzepatide and semaglutide: the trial numbers side by side

  Retatrutide versus tirzepatide (Mounjaro, Zepbound) and semaglutide (Wegovy): weight loss, dropout and side effects from the pivotal trials, side by side.

• Retatrutide reconstitution: the laboratory technique, storage rules and the errors that degrade a peptide

  How lyophilized peptides such as retatrutide are reconstituted in a laboratory, how concentration is calculated, how solutions are stored, what degrades them.

• Retatrutide heart rate and arrhythmia: what the trials have measured and what they have not

  What retatrutide trials report on heart rate: a dose-dependent rise peaking at 24 weeks in phase 2, the tirzepatide comparison, arrhythmia data, outcomes trial.

• Retatrutide and alcohol, pregnancy and drug interactions: what the record shows and what it leaves open

  No retatrutide trial covers alcohol, pregnancy or interactions. What the tirzepatide FDA label says on pregnancy, contraception, insulin and oral medications.

Who is behind this

An independent editorial project that reads the primary literature on retatrutide and writes it up in plain language, with a preference for stating what is not known. Read the about page and the editorial policy.