PMID-40353578 — SURMOUNT-5: Tirzepatide vs Semaglutide Head-to-Head for Obesity

[DRAFT — authored 2026-04-18. Requires Medical Director review.]

Citation

Aronne LJ, Horn DB, le Roux CW, Ho W, Falsey R, Stefanski A, Wharton S, Forrester T, Diniz Behn C, Bunck MC, Kushner R. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity. N Engl J Med. 2025;393(1):26-36. doi: 10.1056/NEJMoa2416394. PMID: 40353578.

Study Design

Phase: 3b (head-to-head active-comparator trial)
Design: Multicenter, randomized, open-label (double-blind at investigator-reader level for body composition), parallel-group, active-comparator
Randomization: 1:1 to tirzepatide (titrated to MTD up to 15 mg weekly SC) vs. semaglutide (titrated to MTD up to 2.4 mg weekly SC)
Blinding: Open-label for participants and site clinicians; blinded adjudication of some endpoints
Duration: 72 weeks
N: 751 adults with obesity without T2DM

Population

Inclusion criteria:
- BMI ≥30 kg/m²
- Without type 2 diabetes
- No prior tirzepatide or semaglutide exposure
- Adults ≥18 years
Exclusion criteria:
- T2DM
- Prior GLP-1 exposure in preceding 6 months
- MTC/MEN 2 history
- Bariatric surgery history
Demographics:
- Mean age: ~45 years
- Mean baseline weight: ~113 kg
- Mean BMI: ~39 kg/m²
- Female: ~65%

Intervention

Tirzepatide arm: 2.5 mg → 15 mg MTD over 20+ weeks (standard Zepbound titration)
Semaglutide arm: 0.25 mg → 2.4 mg MTD over 16+ weeks (standard Wegovy titration)
Lifestyle counseling: Standardized across both arms

Outcomes

Primary

Percent change in body weight from baseline to week 72

Results (efficacy estimand, least-squares mean):

Tirzepatide: −20.2% (95% CI, −21.4 to −19.1)
Semaglutide: −13.7% (95% CI, −14.9 to −12.6)
Treatment difference: −6.4 percentage points favoring tirzepatide (P < 0.001)

Secondary

≥15% weight reduction:
- Tirzepatide: ~65%
- Semaglutide: ~40%
≥20% weight reduction:
- Tirzepatide: ~32%
- Semaglutide: ~16%
≥25% weight reduction:
- Tirzepatide: ~15%
- Semaglutide: ~5%
Waist circumference change: −18.4 cm (tirzepatide) vs −13.0 cm (semaglutide) (P < 0.001)
Cardiometabolic parameters: favored tirzepatide for waist circumference and lipid profile; similar blood pressure improvements
Patient-reported physical function (IWQOL-Lite-CT): numerically favored tirzepatide

Key Findings

SURMOUNT-5 is the first and to date most definitive head-to-head comparison of tirzepatide and semaglutide — the two most-prescribed obesity pharmacotherapies in 2026. The results confirm that dual incretin agonism (tirzepatide) produces greater weight loss than GLP-1 monotherapy (semaglutide) at maximum tolerated doses in a population with obesity without T2DM. The ~6.4 percentage-point difference translates to clinically meaningful additional weight loss — approximately 7–8 kg more weight loss on average in this cohort.

Clinical implications:

For patients whose primary goal is maximum weight reduction and who have not responded adequately to semaglutide, switching to tirzepatide is the evidence-based next step before declaring pharmacotherapy failure.
For prescribers choosing initial therapy when both are accessible, tirzepatide offers a statistical advantage but requires considering individual patient factors (tolerability profile, cost, insurance coverage, supply stability).
Semaglutide remains the class leader for cardiovascular outcomes (SELECT trial, PMID: 37952131) and kidney outcomes in T2DM (FLOW, PMID: 38785209); tirzepatide's CV outcomes data (SURPASS-CVOT) remain awaited as of 2026.

Post-hoc 10-year CVD risk modeling (not hard outcomes): A separate post-hoc analysis of SURMOUNT-5 (PMID: 40980721, European Heart Journal Open 2025) applied a 10-year ASCVD risk-calculation model to the trial's weight-loss and cardiometabolic results to estimate the corresponding relative difference in projected 10-year ASCVD risk between arms. This is modeling-based estimation, not hard cardiovascular outcome data. Hard CV outcomes for tirzepatide await SURPASS-CVOT readout.

Limitations (Author-acknowledged)

Open-label design — participants and site clinicians knew their assignment. Adjudicated secondary endpoints (some body composition, adverse events) were blinded, but subjective outcomes are susceptible to bias.
72-week duration — long-term maintenance and cardiovascular/kidney outcome differences not captured.
Non-T2DM cohort only — head-to-head in T2DM is SURPASS-2 (PMID: 34170647), which used different primary endpoints.
No blinded active-arm dosing; different devices/schedules preclude a fully blinded design.
Funded by Eli Lilly (manufacturer of tirzepatide).
Cost-effectiveness not addressed.

Evidence Level

Level Ib (Oxford CEBM) — large, adequately-powered Phase 3b active-comparator RCT with low-to-moderate risk of bias (open-label design is the primary bias source).

Linked Peptides

Orchestrator Notes

Funded by Eli Lilly; direct manufacturer sponsorship of a head-to-head trial against a competitor product is noted and should be weighed in interpretation.
As of 2026, SURMOUNT-5 is the single most influential trial in obesity pharmacotherapy selection for patients without T2DM where both agents are accessible.
Post-hoc analyses continue to emerge; see PMID: 40980721 for the 10-year CVD risk reduction modeling.
Practical interpretation: if tirzepatide is accessible, the efficacy argument favors it; if semaglutide is accessible and the patient has established cardiovascular disease, semaglutide's SELECT trial CV benefit may reasonably inform the choice despite slightly lower weight-loss potency.

PMID-40353578 – SURMOUNT-5 Tirzepatide vs Semaglutide