One head-to-head trial, one investigational drug, and why the biggest number is not the answer
Tirzepatide beat semaglutide in the only trial that compared them directly, by 6.5 percentage points of body weight over 72 weeks, and discontinued less often for gastrointestinal side effects. Retatrutide has produced the largest weight-loss figures of the three, but never against a comparator drug — and it is not approved anywhere. On evidence quality the order is semaglutide and tirzepatide first, retatrutide last; on observed weight loss the order reverses. Those are different questions.
| Attribute | Semaglutide | Tirzepatide | Retatrutide |
|---|---|---|---|
| Receptor targets | GLP-1 only | GIP + GLP-1 (dual) | GLP-1 + GIP + glucagon (triple) |
| Best evidence for weight loss [1,3,2,5] | -13.7% at 72 weeks in the head-to-head trial; -14.9% at 68 weeks vs placebo in STEP 1 | -20.2% at 72 weeks in the head-to-head trial | -24.2% at 48 weeks (12 mg, phase 2); -28.3% at 80 weeks reported in TRIUMPH-1 topline |
| Compared directly against the others? [1] | Yes - vs tirzepatide (SURMOUNT-5) | Yes - vs semaglutide (SURMOUNT-5) | No - never trialled against either |
| Approval status [5] | FDA-approved (Ozempic, Wegovy, Rybelsus) | FDA-approved (Mounjaro, Zepbound) | Investigational - US submission planned for Q1 2027 |
| Dosing frequency | Once weekly injection (daily oral tablet also available) | Once weekly injection | Once weekly injection (trial protocol) |
| Brand names | Ozempic, Wegovy, Rybelsus | Mounjaro, Zepbound | LY3437943 |
| Route | Subcutaneous (weekly injection); Oral tablet available (Rybelsus) | Subcutaneous | Subcutaneous (clinical trial formulation only) |
| Frequency | Once weekly (SC); Once daily (oral) | Once weekly | Once weekly |
| Typical dose range | SC: 0.25–2.4 mg/week titrated over 16 weeks; Oral: 3–14 mg/day | 2.5–15 mg/week, titrated every 4 weeks | Phase 2 tested 1, 4, 8, 12 mg weekly SC; optimal dose being determined in Phase 3 |
| Oral bioavailability | Low (~0.4–1% for oral Rybelsus formulation); requires fasting and SNAC absorption enhancer | Not available orally; injectable only (subcutaneous) | — |
| US regulatory status | FDA-approved (Ozempic, Wegovy, Rybelsus). Additional approvals: MASH with fibrosis (Aug 2025), oral 25 mg for weight management (late 2025). 135+ warning letters issued to GLP-1 compounders;… | FDA-approved: Mounjaro (T2D, adults and children 10+) and Zepbound (obesity/overweight with comorbidities, severe OSA including new sleep apnea approval). Compounding fully prohibited for 503B… | Not FDA-approved. Phase 3 TRIUMPH program ongoing (Eli Lilly). TRIUMPH-4 reported 28.7% body weight loss at 12 mg over 68 weeks. Seven Phase 3 readouts expected in 2026. Regulatory submission… |
Semaglutide mimics the GLP-1 hormone by binding to GLP-1 receptors on pancreatic beta cells (glucose-dependent), brain (hypothalamus appetite centers), stomach, and intestines. It stimulates insulin secretion, suppresses glucagon release, slows gastric emptying promoting prolonged fullness, and reduces food intake. A fatty acid chain (C18 diacid) modification enables albumin binding and resistance to DPP-4 degradation, extending its half-life to approximately 160-168 hours (~1 week), allowing once-weekly subcutaneous dosing. Injectable bioavailability is approximately 89%. Oral formulation (Rybelsus) requires administration on empty stomach 30 minutes before food and has lower bioavailability. Developed by Novo Nordisk, first FDA-approved December 5, 2017.
Tirzepatide (MW ~4813 g/mol, C225H348N48O68) simultaneously activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) receptors. This dual agonism enhances glucose-dependent insulin secretion, suppresses glucagon, slows gastric emptying, increases satiety signaling, and improves insulin sensitivity. The added GIP activity appears to synergistically amplify metabolic effects beyond GLP-1 alone, contributing to superior weight reduction. A C20 fatty acid via hydrophilic linker promotes albumin binding, resulting in a half-life of approximately 5 days, ~80% subcutaneous bioavailability, and 99% albumin binding. Tirzepatide also significantly increases serum adiponectin, an adipokine linked to lipid and glucose metabolism regulation, suggesting potential cardioprotective implications.
Retatrutide simultaneously activates three receptors: GLP-1 (reduces appetite, slows gastric emptying, improves insulin secretion), GIP (enhances insulin sensitivity, glucose control), and glucagon (increases energy expenditure, fat oxidation, thermogenesis). This triple synergy combines reduced caloric intake (GLP-1) plus glucose stability (GIP) plus increased energy expenditure (glucagon), distinguishing it from dual agonists like tirzepatide by adding a glucagon-driven thermogenic component.
Design: Open-label randomised phase 3b trial, 32 sites in the US and Puerto Rico Population: 751 adults with obesity (BMI >=30, or >=27 with a weight-related complication) and without type 2 diabetes Duration: 72 weeks Arms: Tirzepatide 10 or 15 mg weekly vs semaglutide 1.7 or 2.4 mg weekly, both subcutaneous, each titrated to maximum tolerated dose
Mean body-weight change -20.2% with tirzepatide vs -13.7% with semaglutide (p<0.001). Waist circumference -18.4 cm vs -13.0 cm (p<0.001). Gastrointestinal adverse events leading to discontinuation: 2.7% with tirzepatide vs 5.6% with semaglutide.
SURMOUNT-5 is the single randomised trial that put any two of these drugs head-to-head. Over 72 weeks, 751 adults with obesity and without type 2 diabetes were randomised to tirzepatide (10 or 15 mg weekly) or semaglutide (1.7 or 2.4 mg weekly), each titrated to the maximum dose they tolerated. Mean body weight fell 20.2% with tirzepatide and 13.7% with semaglutide, a difference of 6.5 percentage points that was statistically significant (p<0.001). Waist circumference fell 18.4 cm versus 13.0 cm.
The tolerability result cuts against the usual assumption that the stronger drug is the harder one to stay on: gastrointestinal side effects severe enough to stop treatment occurred in 2.7% of the tirzepatide group and 5.6% of the semaglutide group. Both arms were titrated slowly, and most adverse events were mild to moderate and clustered during dose escalation.
Retatrutide has produced the largest weight-loss figures of the three, and it has never been tested against either of them.
Its phase 2 trial randomised 338 adults across six dose arms and placebo for 48 weeks. Mean weight change was -8.7% at 1 mg, -17.1% at 4 mg, -22.8% at 8 mg and -24.2% at 12 mg, against -2.1% for placebo. At the 12 mg dose, every participant lost at least 5% of body weight, 93% lost at least 10% and 83% lost at least 15%. The phase 3 TRIUMPH-1 topline subsequently reported a mean loss of 28.3% at 12 mg over 80 weeks, with 45.3% of participants losing at least 30% of body weight, and TRIUMPH-2 and TRIUMPH-3 met their primary endpoints in populations with type 2 diabetes and established cardiovascular disease.
What none of that establishes is superiority over tirzepatide. A drug that loses 24-28% of body weight in its own placebo-controlled trials and a drug that loses 20.2% against an active comparator are not measured on the same ruler.
Cross-trial comparison is the most common mistake made with this particular trio, and the differences are large enough to change the ranking.
The trial lengths are not the same: 48 weeks for retatrutide's phase 2, 72 weeks for SURMOUNT-5, 80 weeks for TRIUMPH-1. These drugs are still separating from baseline at the point earlier trials stop, so a longer trial flatters its drug. The populations differ too - SURMOUNT-5 excluded people with type 2 diabetes, and diabetes status materially changes how much weight these agents shift. Placebo response, titration schedule, and whether the analysis follows everyone randomised or only those still on drug all move the headline figure by percentage points.
Even pooling only the direct comparisons leaves real uncertainty. A 2025 meta-analysis of seven studies and 28,980 participants found a standardised mean difference of 0.75 (95% CI 0.52 to 0.92) favouring tirzepatide over semaglutide - but reported heterogeneity above 90%, meaning the underlying studies disagreed with each other substantially. The direction is consistent; the exact size is not settled.
The mechanistic story is genuinely different across the three, and it explains both the efficacy trend and the side-effect trend.
Semaglutide is a GLP-1 receptor agonist: it slows gastric emptying, increases satiety and improves glucose-dependent insulin secretion. Tirzepatide adds GIP receptor agonism, which appears to improve both weight and glycaemic outcomes beyond GLP-1 alone and may reduce the nausea burden at equivalent efficacy. Retatrutide adds a third arm, glucagon receptor agonism, which raises energy expenditure rather than only suppressing intake - a different lever, and the reason its weight-loss curve is steeper.
More receptor targets also means more physiology to perturb. Retatrutide's phase 2 trial recorded dose-dependent increases in heart rate that peaked at 24 weeks before declining, alongside the dose-related gastrointestinal effects common to the whole class. Whether that matters clinically over years, in people with cardiovascular disease, is exactly what the phase 3 programme exists to answer, and why regulatory review is not a formality.
Two of these drugs can be prescribed today and one cannot. Semaglutide is FDA-approved as Ozempic, Wegovy and Rybelsus; tirzepatide as Mounjaro and Zepbound. Retatrutide is investigational, with a US regulatory submission planned for the first quarter of 2027 - meaning any retatrutide sold outside a clinical trial today is, by definition, not a pharmaceutical product supplied through an approved channel.
Semaglutide is the only one of the three with an oral formulation. It also has the longest real-world safety record and the most counterfeit exposure - the FDA has issued warnings about falsified semaglutide, and there is no approved generic. For research-grade material of any of the three, third-party purity testing on the specific lot is the only meaningful quality signal, because none of these are simple molecules to synthesise correctly.
By reported weight loss, retatrutide is highest (-24.2% at 48 weeks in phase 2, -28.3% at 80 weeks in TRIUMPH-1 topline), then tirzepatide (-20.2% at 72 weeks), then semaglutide (-13.7% over the same 72 weeks). But only tirzepatide and semaglutide were compared in the same trial. Retatrutide's figures come from trials against placebo, over different durations and in different populations, so they cannot be ranked against the other two with confidence.
Yes. SURMOUNT-5 randomised 751 adults with obesity and without type 2 diabetes to tirzepatide or semaglutide for 72 weeks. Tirzepatide produced 20.2% mean weight loss versus 13.7% for semaglutide (p<0.001), and lower gastrointestinal discontinuation (2.7% vs 5.6%).
No. Retatrutide is investigational. Its phase 3 TRIUMPH programme is reporting results, and Eli Lilly has said it plans a US regulatory submission in the first quarter of 2027. It cannot be prescribed as an approved obesity medicine today.
That is not what the head-to-head trial found. In SURMOUNT-5, gastrointestinal adverse events leading to discontinuation were less common with tirzepatide (2.7%) than semaglutide (5.6%), despite greater weight loss. Both drugs produce mostly mild-to-moderate gastrointestinal effects concentrated during dose escalation.
Because the trials differ in length (48, 72 and 80 weeks), in whether participants had type 2 diabetes, in titration schedule, in placebo response and in how dropouts are analysed. Each of those shifts the headline percentage by whole points. Even a meta-analysis restricted to direct tirzepatide-versus-semaglutide comparisons reported heterogeneity above 90%, meaning the studies disagreed substantially among themselves.
Glucagon receptor agonism increases energy expenditure, whereas GLP-1 and GIP agonism mainly reduce intake and improve glucose handling. That extra lever is the mechanistic reason retatrutide's weight-loss curve is steeper. It also adds physiological effects to monitor: the phase 2 trial recorded dose-dependent heart-rate increases that peaked at 24 weeks and then declined.
Full profiles: Semaglutide, Tirzepatide, Retatrutide.