Semaglutide vs tirzepatide: what the trials actually show

Semaglutide and tirzepatide are the two molecules behind almost every GLP-1 program you'll see advertised. People ask which one is 'stronger,' and the honest answer is that you have to look at what the trials measured — the dose, the patients, and the length of the study all change the number. Here's what the pivotal evidence actually shows, in plain English.

Semaglutide: the STEP-1 trial

The headline evidence for semaglutide in weight management is STEP-1, published in the New England Journal of Medicine in 2021. In adults with overweight or obesity who did not have diabetes, once-weekly semaglutide 2.4 mg produced a mean weight reduction of about 14.9% over 68 weeks, compared with about 2.4% on placebo, alongside diet and exercise¹. That result is what put semaglutide on the map for weight loss and underpins the FDA-approved brand Wegovy⁴.

Tirzepatide: the SURMOUNT-1 trial

Tirzepatide's counterpart is SURMOUNT-1, published in NEJM in 2022. In adults with obesity (again, without diabetes), once-weekly tirzepatide produced mean weight reductions of roughly 15% to 21% over 72 weeks depending on the dose (5, 10, or 15 mg), versus about 3% on placebo². Those are among the largest reductions reported in this class, and they support the FDA-approved brand Zepbound⁵.

The catch with cross-trial comparisons

It's tempting to line up 14.9% against 21% and declare a winner, but STEP-1 and SURMOUNT-1 were separate studies with different patients, different durations, and different designs. You can't cleanly subtract one number from the other. To actually compare the two molecules, you need a head-to-head trial — and there is one, though it's in a different population.

The head-to-head: SURPASS-2

SURPASS-2, published in NEJM in 2021, compared tirzepatide directly against semaglutide — but in people with type 2 diabetes, and it used semaglutide at 1 mg, the diabetes dose. On that footing, tirzepatide produced greater reductions in both A1c and body weight than semaglutide³. That's a real, direct comparison and it favors tirzepatide. The important caveat: SURPASS-2 tested semaglutide at 1 mg, not the 2.4 mg dose used for weight management, so it isn't a clean comparison of the two drugs at their obesity doses.

So which is 'stronger'?

Taken together, the evidence leans toward tirzepatide delivering larger average weight loss — the SURMOUNT-1 numbers are high, and the one head-to-head trial favored it, even at semaglutide's lower diabetes dose²³. But 'average' is doing a lot of work. Individual response varies widely, tolerability differs from person to person, and the two molecules have different dosing schedules and side-effect experiences. The trials tell you what happened across thousands of patients; they don't tell you what will happen for you.

What this means when you're shopping

Most telehealth desks offer both molecules, often at different prices — tirzepatide usually costs more, and our tirzepatide price ranking tracks who's actually cheapest on that molecule this cycle and where it ships. Which one you end up on should be a clinical decision, not a checkout decision, so use this evidence to ask your prescriber better questions rather than to self-select. When you're comparing providers, our 5-minute checklist and the difference between compounded and brand-name matter more to your experience than a few percentage points on a trial chart. You can see how every desk prices each molecule in our provider briefs. And if you'd rather skip the needle entirely, the emerging oral GLP-1 pills are the next chapter of this story. Weight loss also isn't the only outcome these trials measure anymore — see what GLP-1s do for your heart, kidneys, and sleep apnea for the separate, dedicated trials behind those added indications. And if you're wondering what comes after this pair, retatrutide's phase 2 data is the most-discussed candidate, though it's still years from any approval. None of this is medical advice.