Retatrutide vs Tirzepatide vs Semaglutide: The 2026 Data, Side by Side
⚖ Key Takeaway
With TRIUMPH-1 data now published, we can compare all three GLP-1-class heavyweights using Phase 3 numbers. Retatrutide leads on weight loss. Tirzepatide leads on breadth of approval. Semaglutide leads on real-world data and cardiovascular outcomes. Here’s the full side-by-side.
For the first time, we have Phase 3 registrational data on all three major incretin-class obesity drugs in matched populations (adults with obesity/overweight without type 2 diabetes). The comparison is no longer theoretical.
Head-to-Head Efficacy Comparison
| Retatrutide | Tirzepatide | Semaglutide 2.4 mg | |
|---|---|---|---|
| Trial | TRIUMPH-1 | SURMOUNT-1 | STEP-1 |
| Mechanism | Triple: GIP + GLP-1 + Glucagon | Dual: GIP + GLP-1 | Single: GLP-1 |
| N | 2,339 | 2,539 | 1,961 |
| Duration | 80 wk (104 ext) | 72 wk | 68 wk |
| Max Dose Weight Loss | 28.3% (30.3% ext) | 22.5% | 14.9% |
| ≥20% Achievers | >60% (est.) | 57% | 32% |
| Route | SC injection, weekly | SC injection, weekly | SC injection, weekly |
| FDA Status | Investigational | Approved (Zepbound) | Approved (Wegovy) |
| Availability | Research only | Prescribed | Prescribed |
What the Numbers Don’t Tell You
The Glucagon Difference
Retatrutide’s third receptor — glucagon — is what separates it mechanistically. Glucagon increases energy expenditure through hepatic lipid oxidation and thermogenesis. This may explain two TRIUMPH-1 observations: the larger absolute weight loss and the absence of a weight-loss plateau at 104 weeks. Semaglutide and tirzepatide both plateau between 60–72 weeks, suggesting metabolic adaptation catches up with their mechanisms. Glucagon agonism may override that adaptation.
The Safety Tradeoff
More receptors means more side effects. Retatrutide’s GI adverse event rates (nausea 42.4%, diarrhea 32.0% at 12 mg) exceed tirzepatide’s (nausea ~25%, diarrhea ~17% at 15 mg in SURMOUNT-1) and semaglutide’s (nausea ~44% but at a much slower titration). The dysesthesia signal (~10% at high doses) is unique to retatrutide and not seen with the other two.
The Muscle-Loss Question
All three drugs carry the lean-mass-loss concern. Roughly 20–40% of weight lost on GLP-1-class agents can be lean mass rather than fat. With retatrutide driving the deepest weight loss, the absolute amount of muscle potentially lost is also the largest. TRIUMPH-1’s detailed body-composition data hasn’t been published yet — this will be critical when it appears.
The Access Reality (June 2026)
| Retatrutide | Tirzepatide | Semaglutide | |
|---|---|---|---|
| Brand Name | None yet | Zepbound / Mounjaro | Wegovy / Ozempic |
| Prescription? | No — investigational | Yes | Yes |
| Cost (brand) | N/A | ~$1,060/mo | ~$1,350/mo |
| Compounded? | No | Proposed 503B exclusion | Proposed 503B exclusion |
| Research? | Yes | No | No |
The irony of the current regulatory environment: the FDA’s crackdown on compounded semaglutide and tirzepatide (through the proposed 503B exclusion closing June 29) has increased interest in retatrutide — a compound that can’t be prescribed at all. The enforcement wave is pushing curious patients toward research-grade compounds of a drug that hasn’t even completed Phase 3 yet.
Which One Wins?
It depends on the criteria:
- Maximum weight loss: Retatrutide (but it’s investigational)
- Breadth of evidence: Semaglutide (14 STEP trials + SELECT cardiovascular outcome trial)
- Available today: Tirzepatide and semaglutide (both FDA-approved)
- Cost-accessible: TrumpRx executive order brought brand GLP-1 pricing down to ~$199/mo for some patients
- Cardiovascular outcomes: Semaglutide (SELECT showed 20% MACE reduction)
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Retatrutide produced 28.3% weight loss at 80 weeks in TRIUMPH-1, beating tirzepatide (22.5% in SURMOUNT-1) and semaglutide (14.9% in STEP-1). However, retatrutide is investigational and not yet available by prescription.
No. Retatrutide is investigational. Eli Lilly is expected to file a New Drug Application in late 2026 to early 2027, with potential approval in 2027-2028. Only semaglutide (Wegovy/Ozempic) and tirzepatide (Zepbound/Mounjaro) are currently FDA-approved.
Retatrutide activates three receptors (GIP, GLP-1, and glucagon) compared to tirzepatide’s two (GIP, GLP-1) and semaglutide’s one (GLP-1). The glucagon component increases energy expenditure through thermogenesis and may explain the larger weight loss and lack of plateau.
The FDA proposed permanently excluding semaglutide, tirzepatide, and liraglutide from the 503B Bulks List (public comment closes June 29, 2026). If finalized, large-scale compounding of these drugs would end permanently, even in future shortages.
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