SURMOUNT-5: Tirzepatide vs. Semaglutide Head-to-Head — The Complete Data

The first direct comparison trial between the two dominant GLP-1 drugs is now published. Tirzepatide produced 20.2% average weight loss versus 13.7% for semaglutide at 72 weeks — a 47% greater relative reduction. Here's every endpoint, the safety data, and the caveats you should know.

Until SURMOUNT-5, the comparison between tirzepatide (Zepbound/Mounjaro) and semaglutide (Wegovy/Ozempic) was always indirect — different trials, different patient populations, different timepoints. In May 2025, the New England Journal of Medicine published the first head-to-head trial, and the result was not ambiguous. Tirzepatide produced significantly greater weight loss, waist circumference reduction, and cardiovascular risk improvement at every measured timepoint.

That said, this is a comparison between two genuinely effective drugs. Semaglutide's 13.7% mean weight loss far exceeds anything achievable with prior non-surgical interventions. The question SURMOUNT-5 answers is not whether semaglutide works — it does — but whether tirzepatide's dual GIP/GLP-1 mechanism produces meaningfully better outcomes.

Trial Design

SURMOUNT-5 (NCT05822830) was a Phase 3b, open-label, randomized trial conducted across 32 sites in the United States and Puerto Rico. It enrolled 751 adults with obesity (BMI ≥30) or overweight (BMI ≥27) with at least one weight-related complication, excluding patients with diabetes. Participants were randomized 1:1 to receive either tirzepatide or semaglutide, titrated to maximum tolerated doses, for 72 weeks.

The open-label design is worth noting. Participants and investigators knew which drug was being administered. This was a deliberate choice — the trial was designed as a Phase 3b comparator study, not a blinded registration trial. It was funded by Eli Lilly, which manufactures tirzepatide. NEJM peer review is substantive, and the data was collected under standard protocols, but the funding source and open-label design are relevant context for interpreting the results.

Primary and Key Secondary Endpoints

Tirzepatide was superior to semaglutide on the primary endpoint and all five key secondary endpoints. The 6.5-percentage-point gap in weight loss is both statistically significant and clinically meaningful — translating to an additional 17 pounds of weight loss on average.

Cardiovascular Risk Implications

A post-hoc analysis published in the European Heart Journal Open examined predicted 10-year cardiovascular disease risk using SURMOUNT-5 patient-level data. The average baseline 10-year CVD risk score was 9.3%. Tirzepatide was associated with a significantly greater reduction in predicted 10-year CVD risk compared to semaglutide: an absolute reduction of 2.4% versus 1.4% (P < 0.001). However, this is a modeled prediction based on risk factor changes, not an observed difference in actual cardiovascular events. Neither drug has been tested against the other in a cardiovascular outcomes trial.

Safety Comparison

Most adverse events in both groups were gastrointestinal and occurred during dose escalation. The trial was not designed to detect differences in adverse event rates between the two drugs. Both treatments had similar overall safety profiles consistent with their respective prior trial programs.

One observation from clinical practice: semaglutide tends to produce more nausea and constipation, while tirzepatide more commonly causes diarrhea with relatively less nausea. SURMOUNT-5 did not formally compare these profiles, and individual responses vary.

What This Means for Patients

SURMOUNT-5 provides the first controlled evidence that tirzepatide's dual GIP/GLP-1 mechanism produces meaningfully greater weight loss than GLP-1 agonism alone. For patients choosing between the two drugs, the efficacy difference is real — but it exists alongside differences in cost, insurance coverage, side effect profiles, and individual response. Neither drug is universally superior for every patient.

The trial does not resolve the broader question of whether greater weight loss translates to proportionally greater improvements in hard clinical outcomes (cardiovascular events, mortality, cancer risk). That answer will require different trials with different endpoints.