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Research answer
What are the side effects of retatrutide?
In TRIUMPH-1, the retatrutide phase 3 trial reported by Eli Lilly (checked 23 September 2026), nausea affected 28.6% to 42.4% of participants across the dose arms against 14.8% on placebo, and diarrhoea, constipation and vomiting were also more common. Stopping because of adverse events ranged from 4.1% to 11.3% against 4.9%.
Primara Labs sells some of the compounds covered here. Every figure on this page links to its source and the date it was checked.
Updated 23 September 2026
01
TRIUMPH-1
In TRIUMPH-1, discontinuation because of adverse events was 4.1% on 4 mg, 6.9% on 9 mg and 11.3% on 12 mg, against 4.9% on placebo. The most frequently reported adverse events were nausea in 28.6% on 4 mg, 38.4% on 9 mg and 42.4% on 12 mg against 14.8% on placebo, diarrhoea in 25.2% on 4 mg, 34.1% on 9 mg and 32.0% on 12 mg against 13.5% on placebo, constipation in 23.8% on 4 mg, 25.9% on 9 mg and 26.1% on 12 mg against 10.9% on placebo and vomiting in 10.6% on 4 mg, 22.8% on 9 mg and 25.3% on 12 mg against 4.8% on placebo.
02
TRIUMPH-4 and phase 2
In TRIUMPH-4, discontinuation because of adverse events was 12.2% on 9 mg and 18.2% on 12 mg, against 4.0% on placebo. The most frequently reported adverse events were nausea in 38.1% on 9 mg and 43.2% on 12 mg against 10.7% on placebo, diarrhoea in 34.7% on 9 mg and 33.1% on 12 mg against 13.4% on placebo, constipation in 21.8% on 9 mg and 25.0% on 12 mg against 8.7% on placebo and vomiting in 20.4% on 9 mg and 20.9% on 12 mg against 0.0% on placebo.
Retatrutide phase 2 (Jastreboff): The abstract reports gastrointestinal events as the most common, dose-related and mostly mild to moderate, without rates; rates by arm are in the full text, not read here.
03
The outcomes these rates sit beside
In TRIUMPH-1, a phase 3 trial in adults with obesity, or overweight with at least one weight-related condition, without diabetes (n=2,339), mean body weight fell 28.3% on the 12 mg arm over 80 weeks, against 2.2% lost on placebo (efficacy estimand).
In TRIUMPH-4, a phase 3 trial in adults with obesity or overweight and knee osteoarthritis, without diabetes lasting 68 weeks, WOMAC pain fell 4.5 and 4.4 points from a baseline of 6.0 on 9 and 12 mg, against 2.4 on placebo.
Adverse event rates are reported per trial arm, and the arms are fixed trial doses reached by a scheduled escalation inside the study; the rates describe those trials and nothing outside them.
Sources
Where every figure comes from
- Eli Lilly topline release, 21 May 2026 · checked 23 September 2026
- Eli Lilly topline release, 11 December 2025 · checked 23 September 2026
- Jastreboff AM et al. N Engl J Med 2023;389:514-526 · checked 23 September 2026
Last reviewed 23 September 2026