Retatrutide vs Tirzepatide vs Semaglutide: What the Trial Data Can and Cannot Tell You

All three trials enrolled adults with obesity or overweight and no diabetes. Figures are mean percentage weight change at the top dose.

On the broader treatment-regimen analysis, the gap between retatrutide and tirzepatide is about 4 points, not 6. The estimand matters as much as the drug. The clinical trial results guide in this series explains the two methods.

• Duration. The trials ran 68, 72 and 80 weeks. Retatrutide’s Phase 2 weight curve had not plateaued at 48 weeks, so longer follow-up flatters it.
• Population. TRIUMPH-1 enrolled people with a mean BMI of 40.0 (112.7 kg). STEP-1 averaged a BMI of 37.9 and SURMOUNT-1 38.0. Heavier participants tend to lose more in absolute terms.
• Escalation schedule. Retatrutide stepped up from 2 mg every 4 weeks to 12 mg. Semaglutide reached 2.4 mg by week 16, and tirzepatide reached 15 mg after a 20-week escalation.
• Era and conduct. The trials were run years apart, with different background care and different handling of participants who left treatment.

These differences do not erase retatrutide’s lead. They mean the size of the lead is uncertain until someone runs a direct comparison.

The Phase 2 diabetes trial in The Lancet used dulaglutide 1.5 mg as an active comparator, a GLP-1 drug known more for glucose control than weight loss:

• HbA1c at 24 weeks, 12 mg: down 2.02% versus 1.41% with dulaglutide.
• Weight at 36 weeks, 12 mg: down 16.94% versus 2.02% with dulaglutide.

TRANSCEND-T2D-2 (Lilly topline, 28 October 2026) is the first Phase 3 trial to compare retatrutide with another incretin drug. It enrolled 1,298 adults with type 2 diabetes inadequately controlled on metformin, with or without an SGLT2 inhibitor, and compared retatrutide 9 mg and 12 mg with semaglutide 2 mg (Ozempic) over 80 weeks. From Lilly’s release and coverage:

• HbA1c fell 2.2 points on both retatrutide doses versus 1.8 on semaglutide (baseline 8.3%).
• Weight fell 19.5% (9 mg) and 20.9% (12 mg) versus 9.4% on semaglutide.
• The primary aim was non-inferiority on HbA1c. Superiority was reported for the 12 mg dose under statistical control, so the identical 2.2-point figures do not mean both doses were tested as superior.
• Coverage describes the trial as open-label, so participants and investigators knew which drug was given.

Detailed results are still to be presented and published.

Retatrutide’s tolerability is the main trade-off. In TRIUMPH-1, 11.3% of participants on 12 mg stopped because of adverse events, versus 4.9% on placebo, and across the four TRIUMPH trials the 12 mg figure ranged from 7.7% to 18.2%. The comparator trials reported 7.0% for semaglutide 2.4 mg versus 3.1% on placebo in STEP-1, and 4.3% to 7.1% across tirzepatide doses (6.2% at 15 mg) versus 2.6% on placebo in SURMOUNT-1. Retatrutide’s rate at the top dose is higher than either.

Gastrointestinal events are the main cause, and Lilly describes the profile as consistent with other incretin therapies. Dysaesthesia, an abnormal skin sensation reported by 12.5% on 12 mg in TRIUMPH-1, is not typical of the older drugs. See the side effects guide for the full numbers.

In a summary of Lilly’s third-quarter 2025 call, the company’s chief scientific officer said retatrutide would likely suit people with very high BMI or obesity-related complications that need a high degree of weight loss. BioPharma Dive reports that analysts see the side-effect profile limiting use to people with the most weight to lose.

The three compounds form a ladder of receptor coverage: GLP-1 receptor only, GIP plus GLP-1 receptors, and all three. In cell-based work that makes them a natural set for separating which receptor drives a given readout.

Nordapep supplies retatrutide for laboratory research use only. It is not a medicine and is not for human or veterinary use. Nothing here is a recommendation to use any medicine.