SURMOUNT-5: Tirzepatide vs Semaglutide Head-to-Head (2026)

For years the question “is Zepbound or Wegovy better for weight loss?” could only be answered indirectly, by lining up separate trials run in different patients. SURMOUNT-5 settled it directly. It was the first large, randomized, head-to-head trial to give one group tirzepatide (Zepbound) and another group semaglutide (Wegovy), each pushed to its maximum tolerated dose, and follow them for 72 weeks. The result was clear: tirzepatide produced a mean weight loss of 20.2% versus 13.7% for semaglutide — a 6.5-percentage-point advantage — and beat semaglutide at every responder threshold, including more than double the rate of people losing at least 25% of their body weight (Aronne 2025 ^[1]). This article is a detailed walk through SURMOUNT-5's exact numbers: the primary endpoint, the responder ladder, the side-effect head-to-head, and what the trial does and doesn't tell you about choosing between the two drugs. For the broader picture across the full SURMOUNT and STEP trial families, see tirzepatide vs semaglutide: the head-to-head look at SURMOUNT and STEP.

The headline result in one line

At 72 weeks, adults with obesity (but not diabetes) lost a mean of 20.2% of their body weight on tirzepatide and 13.7% on semaglutide — about 22.8 kg (50 lb) versus 15.0 kg (33 lb) in absolute terms (Aronne 2025 ^[1]). Tirzepatide was statistically superior on the primary endpoint, and the gap widened the further out on the “big loss” scale you looked. This was a true head-to-head: 751 participants randomized 1:1, each drug titrated to its highest tolerated maintenance dose, both paired with the same lifestyle-support program, open-label design, 72 weeks (Aronne 2025 ^[1]).

How SURMOUNT-5 was built

• Design: Phase 3b, multicenter, open-label, randomized controlled trial across 32 sites in the United States and Puerto Rico (Aronne 2025 ^[1]).
• Who: 751 adults with obesity (BMI ≥30) or overweight (BMI ≥27) plus at least one weight-related complication, without type 2 diabetes. Mean age about 45; this was a weight-management population, not a diabetes population (Aronne 2025 ^[1]).
• Arms: randomized 1:1 to once-weekly subcutaneous tirzepatide (n≈375) or once-weekly subcutaneous semaglutide (n≈376) (Aronne 2025 ^[1]).
• Doses: each drug was titrated to its maximum tolerated maintenance dose — tirzepatide 10 mg or 15 mg weekly; semaglutide 1.7 mg or 2.4 mg weekly. That “max tolerated” design matters: it pits each drug at its strongest approved obesity dose, not a fixed low dose (Aronne 2025 ^[1]).
• Duration: 72 weeks of treatment, the same window used in the pivotal SURMOUNT-1 and STEP trials, so the magnitudes are comparable to those studies (Aronne 2025 ^[1]).
• Funding: sponsored by Eli Lilly, the maker of tirzepatide — a real consideration when reading any head-to-head, though the trial was published in The New England Journal of Medicine with its standard peer review (Aronne 2025 ^[1]).

The primary endpoint: mean weight loss

The primary endpoint was the percent change in body weight from baseline to week 72. Tirzepatide delivered a mean reduction of 20.2%; semaglutide delivered 13.7%. The treatment difference of roughly 6.5 percentage points favored tirzepatide and was highly statistically significant (Aronne 2025 ^[1]). In absolute weight, that was about 22.8 kg lost on tirzepatide versus 15.0 kg on semaglutide (Aronne 2025 ^[1]).

For context, those numbers track closely with each drug's own pivotal monotherapy trial: tirzepatide's SURMOUNT-1 reported up to ~20.9% at the 15 mg dose, and semaglutide's STEP-1 reported ~14.9% at 2.4 mg. SURMOUNT-5 is valuable precisely because it reproduced that gap within the same trial, same patients, same protocol — removing the “different populations” objection that dogs cross-trial comparisons (Aronne 2025 ^[1]).

The responder ladder: where tirzepatide pulls away

Mean weight loss is one number; what patients often care about is “what are my chances of a big result?” SURMOUNT-5 reported the share of people hitting each weight-loss threshold, and the tirzepatide advantage grew at every higher rung (all comparisons P<0.001) (Aronne 2025 ^[1]):

Read the bottom row: on tirzepatide, roughly 1 in 3 participants lost at least a quarter of their body weight, versus about 1 in 6 on semaglutide — nearly double the rate (Aronne 2025 ^[1]). The pattern is consistent all the way up: the harder the target, the wider the gap. That is the practical heart of SURMOUNT-5 — not just “more weight on average,” but a meaningfully better chance of a transformative result.

Waist circumference moved too

Beyond the scale, waist circumference — a marker of visceral fat — fell more on tirzepatide as well: a least-squares mean reduction of about 18.4 cm versus 13.0 cm with semaglutide (P<0.001) (Aronne 2025 ^[1]). A post-hoc analysis of SURMOUNT-5 also estimated a larger projected 10-year cardiovascular-disease risk reduction with tirzepatide, driven by the greater improvements in weight and related risk factors (Mamas 2025 ^[2]) — though that is a modeled projection, not a measured cardiovascular-outcomes result.

The safety and tolerability head-to-head

A bigger effect would mean little if it came with worse side effects — so the tolerability comparison matters. The reassuring finding for tirzepatide is that its larger weight loss did not come at the cost of worse gastrointestinal tolerability. In both arms, gastrointestinal effects were the most common adverse events, were mostly mild-to-moderate, and clustered during the dose-escalation phase (Aronne 2025 ^[1]).

Two things stand out. First, fewer people stopped tirzepatide for side effects — both overall (6.1% vs 8.0%) and specifically for GI events (2.7% vs 5.6%) — and vomiting was less common on tirzepatide (15.0% vs 21.3%) (Aronne 2025 ^[1]). Second, the one place semaglutide looked better was injection-site reactions, which were far more common with tirzepatide (8.6% vs 0.3%) (Aronne 2025 ^[1]). Serious adverse events were uncommon in both arms (4.8% vs 3.5%), and there were no treatment-attributed deaths (Aronne 2025 ^[1]).

What SURMOUNT-5 does NOT tell you

• It was open-label. Both patients and clinicians knew which drug was given. For an objective endpoint like body weight that's a modest concern, but it can influence behavior and side-effect reporting (Aronne 2025 ^[1]).
• It was Lilly-sponsored. The maker of the winning drug funded and designed the trial. The NEJM peer review and the consistency with prior independent trials are reassuring, but the funding is part of the context (Aronne 2025 ^[1]).
• It enrolled people without diabetes. SURMOUNT-5 was a weight-management population; results may differ in people with type 2 diabetes, where the weight-loss magnitudes for both drugs are generally smaller.
• It measured weight, not hard cardiovascular outcomes. Semaglutide has a completed cardiovascular-outcomes trial (SELECT) showing reduced major adverse cardiac events; tirzepatide's comparable outcomes trial (SURMOUNT-MMO) is still ongoing. The CV-risk advantage seen here is a modeled projection, not measured events (Mamas 2025 ^[2]).
• It says nothing about cost, coverage, or supply. “More weight loss” is not the only input to a real-world choice; price, insurance, and which drug you can actually get also matter.

What it means for choosing between them

On the single question SURMOUNT-5 was built to answer — which produces more weight loss at maximum tolerated dose over 72 weeks — the answer is tirzepatide, and not by a little: 20.2% vs 13.7% on average, with about double the chance of losing 25% or more (Aronne 2025 ^[1]). It also had fewer GI-driven dropouts. That makes tirzepatide a reasonable first choice when maximizing weight loss is the priority and the patient can access and afford it.

But “better in the trial” isn't the same as “better for you.” Semaglutide remains highly effective, has the more mature cardiovascular-outcomes evidence base (SELECT), and may be the more available or more affordable option depending on insurance and supply. Injection-site reactions were also more common with tirzepatide. The right drug is the one that fits your goals, your other conditions, your tolerance, and what you can actually obtain — a decision to make with a prescriber, not from a trial table alone. If you're already on one and considering the other, see switching between GLP-1 medications.

Bottom line

SURMOUNT-5 is the head-to-head trial people had been waiting for, and it gave a decisive primary result: at 72 weeks, tirzepatide (Zepbound) produced 20.2% mean weight loss versus 13.7% for semaglutide (Wegovy), with a wider advantage at every higher responder threshold and roughly double the rate of ≥25% weight loss (31.6% vs 16.1%) — all while showing fewer GI-related discontinuations, though more injection-site reactions (Aronne 2025 ^[1]). It does not address cost, coverage, hard cardiovascular outcomes, or people with diabetes, so it informs the choice without making it. Verified primary publication: Aronne et al, New England Journal of Medicine 2025;393(1):26-36, PMID 40353578.

This article is educational and is not medical advice. Every number above is taken from the published SURMOUNT-5 trial report and its post-hoc analysis as indexed in PubMed, verified against the live PubMed database and the published trial coverage before publication. Decisions about starting, stopping, or switching a GLP-1 medication should be made with your prescriber.