Semaglutide vs. Tirzepatide: What the Head-to-Head Trial Showed

For three years, every semaglutide-versus-tirzepatide comparison was arithmetic. Semaglutide produced about 15% weight loss in STEP 1. Tirzepatide produced about 20.9% in SURMOUNT-1. Different trials, different participants, different years — so subtracting one from the other was a guess wearing the clothes of a finding.

That changed in 2025. SURMOUNT-5 put both drugs in the same trial, in the same population, at the same time, and measured them against each other directly. This article covers what it found — and the two situations where the drug that lost is still the better answer.

The one trial that compared them directly

SURMOUNT-5 was an open-label, active-controlled phase 3b trial in 751 adults with obesity or overweight plus at least one weight-related complication, none of whom had type 2 diabetes. Participants were randomised to the maximum tolerated dose of tirzepatide (10 or 15 mg) or the maximum tolerated dose of semaglutide (1.7 or 2.4 mg), injected once weekly for 72 weeks.

The gap in the primary endpoint was about 6.5 percentage points in tirzepatide’s favour, and it held after adjustment for baseline weight, prediabetes status, BMI group and sex. Tirzepatide also won the waist-circumference comparison and, somewhat against expectation, had the lower rate of dropping out over stomach side effects.

Where the headline overstates the result

The press coverage settled on a single number — tirzepatide produced roughly 47% more weight loss — which is a relative difference between two averages, not something any individual should expect. Four things about the trial deserve more weight than they usually get:

• It was open-label. Everyone knew which drug they were taking. In a weight-loss trial where diet and activity are part of the outcome, knowing you were assigned the drug widely reported as stronger is not a neutral fact.
• Neither arm was forced to the top dose. Both groups ran at the *maximum tolerated* dose, so the semaglutide arm includes people who stayed at 1.7 mg rather than the 2.4 mg on the label, and the tirzepatide arm includes people who stayed at 10 mg rather than 15 mg. That mirrors real prescribing, but it is not a maximum-dose-to-maximum-dose comparison.
• The participant mix was unusual. Men were 35% of the trial, higher than most obesity studies, and weight loss ran about 6 percentage points lower in men than women in both arms. Overall results came in below the earlier single-drug trials partly for that reason.
• It is one trial. 751 people, 72 weeks, one design. It is far better evidence than cross-trial subtraction, and it is still one trial.

Why the two drugs behave differently

Semaglutide mimics one hormone, GLP-1. Tirzepatide mimics two, GIP and GLP-1. The prevailing explanation for the extra weight loss is that adding GIP activity does something GLP-1 alone does not — though the mechanism is still argued about, and nobody has shown that the dual action is what produced the SURMOUNT-5 gap.

Does semaglutide beat tirzepatide at anything?

Yes — and this is the part most comparisons skip. On cardiovascular risk reduction, semaglutide has evidence tirzepatide does not.

In the SELECT trial, semaglutide 2.4 mg reduced major adverse cardiovascular events in people with overweight or obesity and established cardiovascular disease but no diabetes. That result earned the branded product a cardiovascular risk-reduction indication — a claim on the label, not an inference. The detail is covered at length in what SELECT proved.

Tirzepatide has nothing equivalent yet. SURPASS-CVOT showed it was non-inferior to dulaglutide — another GLP-1, not placebo — for cardiac events in people with type 2 diabetes. SURMOUNT-MMO, the outcomes trial in obesity without diabetes, is still running. So if the goal is documented cardiovascular benefit rather than maximum weight loss, the lower number on the scale is attached to the stronger label.

One of them comes as a pill

SURMOUNT-5 compared two injections, and the landscape has moved since. An oral semaglutide tablet reached pharmacies in January 2026 — the routine and the indications differ from the injection in ways worth understanding before switching, which is the subject of the pill versus injection comparison.

There is no oral tirzepatide. The oral drug people often confuse with it is orforglipron, a different molecule entirely — a small-molecule GLP-1 with no food or water timing rules, approved in April 2026, and less potent than either injection. If a daily tablet matters more than peak effect, that currently points toward semaglutide or orforglipron, not tirzepatide.

What they cost

Price often decides this before the evidence does. Branded self-pay routes and compounded routes differ by a wide margin, and the numbers move; the current figures are tracked in the GLP-1 Cost Index and explained in how much GLP-1s actually cost.

One structural point: tirzepatide generally costs the same or slightly more than semaglutide through telehealth, so for most people this is not a case of paying more for the stronger drug. At LunaRx, for instance, semaglutide is $139/month and tirzepatide $149/month, both flat.

So which one should you ask about?

The honest framework, given one head-to-head trial:

• Maximum average weight loss — tirzepatide, by about 6.5 percentage points over 72 weeks.
• Documented cardiovascular risk reduction — semaglutide, which has the indication.
• A pill rather than a weekly injection — semaglutide (or orforglipron, a different drug).
• Tolerability — close. GI side effects dominate both; SURMOUNT-5 found slightly fewer people quit tirzepatide over them.
• Coming off at some point — no head-to-head data. What is known about stopping either drug is in GLP-1s after two years.

What the trial cannot tell you is which one works for *you*. Response varies enormously, side-effect tolerance is individual, and switching after a poor response is a recognised path. The choice is a clinician’s to make with your history in front of them.