Semaglutide vs tirzepatide: what the trial data actually shows, and what it doesn't.
Semaglutide and tirzepatide are the two most clinically advanced anti-obesity drugs in the world. Both are once-weekly subcutaneous injections. Both work through the GLP-1 receptor. Tirzepatide adds the GIP receptor, and that addition appears to meaningfully increase weight-loss efficacy — but interpreting "how much more" requires understanding what the trials actually measured and where direct comparisons exist and don't.
- Weight loss: Tirzepatide produces greater weight loss in head-to-head and cross-trial comparisons. SURMOUNT-1 (tirzepatide 15 mg, 72 weeks): −22.5%. STEP-1 (semaglutide 2.4 mg, 68 weeks): −14.9%. The difference is ~7.5 percentage points.
- Head-to-head trial (SURPASS-2): Tirzepatide 15 mg vs semaglutide 1 mg (T2DM dose, not 2.4 mg obesity dose): −11.2 kg vs −5.3 kg body weight; −2.37% vs −1.86% HbA1c. Not a maximum-dose comparison.
- Cardiovascular outcomes: Semaglutide has placebo-controlled MACE reduction data (SELECT: HR 0.80, p<0.001). Tirzepatide has non-inferiority vs insulin degludec (SURPASS-CVOT) — not vs placebo.
- 2-year durability: Semaglutide STEP-5 (Wilding, Nat Med 2022) showed sustained ~15% weight loss at 104 weeks. Tirzepatide 2-year data is emerging but not yet fully published.
- GI side effects: Broadly similar; nausea is the primary adverse event in both. Titration schedules exist for both to minimise early GI burden.
- Cost and access: Both list at $900–$1,300/month. Both face compounding landscape changes following FDA shortage resolutions. Semaglutide shortage resolved October 2024; tirzepatide shortage resolved earlier in 2024.
Why "semaglutide vs tirzepatide" is the most-searched comparison in metabolic medicine
The question dominates because the stakes are real: both drugs work, one appears to work more for weight loss, and the cost is high enough that a prescriber or researcher wants to understand the differential before committing. The media has reduced the comparison to a single number (22% vs 15%), which is directionally correct but strips out important context. This page reconstructs that context from the primary sources.
Understanding the compounds themselves first helps. Semaglutide is a GLP-1 receptor agonist — it binds exclusively to GLP-1R, which is expressed on pancreatic β-cells, hypothalamus, area postrema, and GI smooth muscle. Tirzepatide activates both GLP-1R and GIPR (glucose-dependent insulinotropic polypeptide receptor). The GIP receptor is expressed on adipocytes, brain, and β-cells. Adding GIP-R agonism appears to amplify weight loss — the mechanism is not fully established but involves direct effects on fat tissue and possibly hypothalamic synergism.
The cross-trial weight loss comparison
There is one published direct head-to-head RCT (SURPASS-2), but it used the T2DM dose of semaglutide (1 mg), not the obesity dose (2.4 mg). Cross-trial comparisons are inherently limited — different populations, different endpoints, different durations — but they are the best available data for the maximum-dose question.
| Trial | Drug | Dose | Population | Duration | Mean weight loss | PMID |
|---|---|---|---|---|---|---|
| STEP-1 | Semaglutide | 2.4 mg/week SC | Obesity, no T2DM | 68 weeks | −14.9% | 33567185 |
| SURMOUNT-1 | Tirzepatide | 15 mg/week SC | Obesity, no T2DM | 72 weeks | −22.5% | 35658024 |
| SURPASS-2 | Tirzepatide | 15 mg/week SC | T2DM | 40 weeks | −11.2 kg | — |
| SURPASS-2 | Semaglutide | 1.0 mg/week SC | T2DM | 40 weeks | −5.3 kg | — |
| STEP-2 | Semaglutide | 2.4 mg/week SC | Obesity + T2DM | 68 weeks | −9.6% | — |
The cross-trial comparison of STEP-1 vs SURMOUNT-1 is the most commonly cited. Both populations excluded T2DM; both were non-diabetic adults with obesity; both used lifestyle counselling as background; the doses used (2.4 mg and 15 mg) are the respective maximum approved doses. The 7.5 percentage-point difference in favour of tirzepatide is the best available approximation of the maximum-dose weight-loss differential. It should be treated as directionally valid, not as if it came from a single randomised trial that directly compared both.
SURPASS-2: the only direct head-to-head RCT
SURPASS-2 (Frías et al., NEJM 2021) is the only Phase III trial to randomise patients to both tirzepatide and semaglutide within the same protocol. Design: 1,879 T2DM adults on metformin; 40 weeks; tirzepatide 5/10/15 mg vs semaglutide 1 mg (the approved T2DM dose at the time of trial design).
The limitations are critical to understand. First, semaglutide 1 mg is not the maximum dose; Wegovy at 2.4 mg produces approximately 3× the weight loss of Ozempic at 1 mg in the same patient population. SURPASS-2 was designed to show superiority over semaglutide at its labelled T2DM dose — which it did — but the comparison does not represent either drug at its obesity-indication ceiling. Second, the primary endpoint was HbA1c, not weight loss; weight data was secondary. Third, 40 weeks is shorter than either STEP-1 or SURMOUNT-1, which may underrepresent long-term weight outcomes for both.
SURPASS-2 result summary:
- HbA1c: Tirzepatide 5 mg −1.87% / 10 mg −2.09% / 15 mg −2.37% vs semaglutide −1.86%. Superiority at 10 mg and 15 mg; non-inferiority at 5 mg.
- Body weight: −7.8 kg / −10.3 kg / −11.2 kg vs −5.3 kg. All tirzepatide doses superior on weight.
- ≥10% weight loss: 20% / 36% / 49% vs 16%.
Cardiovascular outcomes: where semaglutide leads
The cardiovascular outcome data is where semaglutide currently holds a clearer advantage — not because tirzepatide has negative CV data, but because semaglutide's SELECT trial established a placebo-controlled MACE benefit that tirzepatide's SURPASS-CVOT did not replicate (by design — it used an active comparator).
SELECT (Lincoff et al., NEJM 2023): 17,604 patients with established CVD and obesity (no T2DM); mean follow-up 39.8 months. Semaglutide 2.4 mg vs placebo. Primary composite MACE: 6.5% vs 8.0%. HR 0.80 (95% CI 0.72–0.90), p<0.001. This is a 20% relative risk reduction against placebo in non-diabetic, obese, high-CV-risk patients. It established semaglutide as the first anti-obesity drug to show a placebo-controlled MACE benefit in this population.
SURPASS-CVOT (Bhatt et al., NEJM 2023): 8,165 T2DM patients with high CV risk; tirzepatide vs insulin degludec. Non-inferior on MACE (HR 0.85, 95% CI 0.71–1.02 for 3-point MACE). The design was powered only for non-inferiority, not superiority, and the comparator was an active antidiabetic, not placebo. This does not mean tirzepatide has no CV benefit — it means the evidence base is different.
The practical implication: for clinicians managing patients with established CVD and obesity, semaglutide's SELECT data provides a direct evidentiary basis for MACE reduction. For tirzepatide, that specific trial hasn't been done (vs placebo in a non-T2DM CVD population). Whether tirzepatide's larger weight loss would produce a larger CV benefit is biologically plausible but not yet shown in RCT data.
Durability: long-term weight maintenance
The STEP-5 trial (Wilding et al., Nature Medicine 2022) extended semaglutide treatment to 104 weeks in adults with obesity. Mean weight loss at 104 weeks was −15.2% with continued semaglutide, closely matching the 68-week STEP-1 result — suggesting durable weight maintenance with ongoing therapy. Critically, STEP-5 also showed that stopping semaglutide led to substantial weight regain, framing the drug as a long-term treatment rather than a course.
Tirzepatide's 2-year durability data is emerging; the SURMOUNT extension trial is in progress but full 104-week publication was not available as of April 2026. The biologically plausible expectation — given that tirzepatide's weight loss also plateaus and is also associated with weight regain on cessation (as established in the SURMOUNT-4 maintenance trial) — is that the durability picture will look similar to semaglutide's.
GI side effects: similar class, similar rates
Nausea is the dominant adverse event for both. STEP-1 reported nausea in ~44% of the semaglutide arm at some point during treatment. SURMOUNT-1 reported nausea in ~33% of the 15 mg tirzepatide arm. Both rates are highest during dose titration and decline at stable dosing. Vomiting and diarrhoea occur at similar frequencies across both drugs. Discontinuation due to GI adverse events was ~5–7% in both pivotal trials.
Early claims that tirzepatide was "better tolerated" than semaglutide have not been conclusively demonstrated in trials powered for this endpoint. Both drugs have similar titration schedules specifically designed to reduce GI burden; both produce substantially better GI outcomes when titration is slow. See the GI side effects spoke for the full class-level review.
Dosing schedules compared
| Phase | Semaglutide (Wegovy, obesity) | Tirzepatide (Zepbound, obesity) |
|---|---|---|
| Start | 0.25 mg/week SC × 4 weeks | 2.5 mg/week SC × 4 weeks |
| Step 2 | 0.5 mg/week SC × 4 weeks | 5.0 mg/week SC × 4 weeks |
| Step 3 | 1.0 mg/week SC × 4 weeks | 7.5 mg/week SC × 4 weeks |
| Step 4 | 1.7 mg/week SC × 4 weeks | 10.0 mg/week SC × 4 weeks |
| Step 5 | 2.4 mg/week SC (maintenance) | 12.5 mg/week SC × 4 weeks |
| Maintenance | 2.4 mg/week SC | 15.0 mg/week SC |
Tirzepatide's titration is longer and has more steps; whether this reflects a pharmacological need or a commercial decision to maintain more dose-step options for clinicians is unclear. Both schedules are available in detail in the respective dosing spokes: semaglutide dosing and tirzepatide dosing.
Cost, compounding, and access
List price for both drugs is approximately $900–$1,300/month in the United States at maximum doses; insurance coverage varies significantly. The compounding landscape for both drugs changed materially in 2024. Tirzepatide's shortage was resolved before semaglutide's — Eli Lilly challenged compounders more aggressively and earlier, winning court rulings that accelerated FDA's shortage-resolution decision. The FDA's semaglutide shortage was declared resolved in October 2024, triggering enforcement against continuing compounders.
The implication for researchers in 2026: both branded products are widely available (supply constraints resolved) but expensive; compounding options have narrowed significantly; research-chemical RUO product remains available but sits entirely outside the pharmaceutical regulatory framework. The compounded semaglutide spoke covers the 503A/503B distinction; a dedicated compounded tirzepatide spoke covers the tirzepatide-specific access picture.
The bottom line: five dimensions, two drugs
| Dimension | Semaglutide | Tirzepatide |
|---|---|---|
| Max-dose weight loss (Phase III) | ~14.9% (STEP-1) | ~22.5% (SURMOUNT-1) |
| Placebo-controlled CV outcomes (non-T2DM) | Yes — SELECT: HR 0.80 | No — not yet done in non-T2DM vs placebo |
| 2-year durability data published | Yes — STEP-5 (Nat Med 2022) | Emerging; 2-year publication pending |
| FDA-approved products | Ozempic (T2DM), Wegovy (obesity), Rybelsus (oral) | Mounjaro (T2DM), Zepbound (obesity) |
| Compounding status (2026) | Shortage resolved Oct 2024 → compounding enforcement active | Shortage resolved 2024 (earlier) → similar enforcement |
If the single question is "which produces more weight loss," tirzepatide wins on the published data. If the question is "which has stronger cardiovascular outcome evidence in non-diabetic obese patients," semaglutide wins by SELECT. If the question is "what's coming next," our retatrutide page covers the triple agonist that may move the weight-loss ceiling again — to ~24% at 48 weeks in Phase II.