How much weight will you lose on a GLP-1? What the trials — and real life — show

“How much weight will I lose?” is the first question almost everyone asks — and the honest answer is a range, not a number. The trials behind these prescription medicines produced some of the largest average weight losses recorded for a weight-management drug. But an average is a fact about a group, not a promise to you, and the gap between the trial headline and a real person’s year can be wide. Here’s the whole picture, including the parts the ads leave out.

What the big trials actually found

Two landmark trials set the expectations, both in adults with obesity or overweight and without diabetes, both against placebo plus lifestyle support.

• Semaglutide 2.4 mg (Wegovy) — STEP 1, 1,961 adults over 68 weeks: mean weight change −14.9%, versus −2.4% on placebo. About 15% of body weight, on average.
• Tirzepatide (Zepbound) — SURMOUNT-1, 2,539 adults over 72 weeks: mean change of about −15.0% at 5 mg, −19.5% at 10 mg, and −20.9% at 15 mg, versus −3.1% on placebo. (You’ll often see “up to 22.5%” quoted — that’s a different, more optimistic statistical estimand that assumes full adherence. The numbers here are the more conservative, more honest ones.)

Those are real, and they’re why these drugs changed the field. But read them as what the average participant achieved, not as your target.

The averages hide enormous variation

The single most misleading thing about a mean is that almost no one is the mean. In STEP 1, the spread was huge:

Weight lost	Share of people on semaglutide
≥ 5%	~86%
≥ 10%	~69%
≥ 15%	~51%
≥ 20%	~32%

So about half reached 15% or more, and roughly a third reached 20% — genuinely striking. But look at the other end: around one in seven didn’t even reach 5%. These are the non-responders, and they were in the trial too. Real results run from more than 25% loss to almost nothing, and there is currently no reliable way to predict in advance which end you’ll land on.

Why real life comes in lower than the trials

Trial numbers describe near-ideal conditions: free medication, structured support, and people who completed a full dose escalation. Outside a trial, the average is lower. In a large clinical cohort of roughly 7,900 adults, mean weight loss at one year was about 9% — meaningfully below the ~15% trial figure. The reason isn’t that the drug stopped working; it’s mostly dose and duration:

• People who stayed on the medicine lost about 12%;
• those who discontinued later lost about 7%;
• those who stopped early lost about 4%.

Most were also on lower maintenance doses than the trial’s top dose. The lesson isn’t “the trials lied” — it’s that the headline number belongs to people who reached the full dose and kept going, and that reaching it is where much of real life diverges.

It largely comes back if you stop

Weight loss on these drugs is a treatment effect, not a cure. In the STEP 1 extension, participants regained about two-thirds of the weight they had lost within a year of stopping. In STEP 4, people who switched to placebo at week 20 began regaining, while those who continued kept losing. This is one of the most important and least-advertised facts about the whole class, and it’s why the decision to start is really a decision about a longer-term plan, made with a clinician — not a short course.

Which one takes off more weight?

Head-to-head, tirzepatide edged out semaglutide: in SURMOUNT-5, a 72-week trial comparing them directly, tirzepatide averaged −20.2% versus semaglutide’s −13.7%. So on the current evidence tirzepatide (Mounjaro/Zepbound) produces somewhat more weight loss on average than semaglutide (Ozempic/Wegovy) — though both are highly effective, tolerability and access differ, and “more on average” still isn’t a promise for any one person. We compare them in more depth on the semaglutide vs tirzepatide page.

A note on brands: the biggest weight-loss numbers come from the weight-management doses — Wegovy (semaglutide 2.4 mg) and Zepbound (tirzepatide). The diabetes versions at their usual doses — Ozempic, Mounjaro — tend to produce somewhat less, because the dose and the studied population differ.

What this means if it’s you

The useful version of “how much will I lose” isn’t a percentage — it’s a set of levers and a range. On today’s evidence, GLP-1 and GIP/GLP-1 medicines produce clinically significant weight loss for most people who take them at an effective dose (that part is well established). But your specific result isn’t predictable, real-world averages run below the trial headlines, and the weight tends to return if you stop.

It also helps to remember what the number is for. Clinically, weight loss is a means to health — better blood sugar, blood pressure, and cardiometabolic risk, and relief for things like sleep apnea and joints — not a score in itself. Even a loss in the 5–10% range delivers meaningful metabolic benefit, so landing below the trial average is often still a real success, not a failure. Two things shape whether the loss you get is good weight loss: staying at an effective dose, and protecting muscle with enough protein and some resistance training, since a share of any weight lost is lean mass.

And these are prescription medicines with real indications: the trials studied adults with obesity or overweight who met specific criteria, and they are not intended for cosmetic weight loss in people without a medical indication. Some people shouldn’t take them at all or need extra care — in pregnancy, with a personal or family history of medullary thyroid cancer or MEN 2, or with a history of pancreatitis (see the side-effects rundown). Whether to start, which medicine, and what a realistic goal looks like is a conversation with a clinician who can weigh your history — not a figure to lift from a study. You can use our progress tool to understand the normal range of results, real-world included, rather than to grade yourself against a trial curve.

Frequently asked questions

How much weight will I lose on Ozempic or Wegovy? In the STEP 1 trial, adults on the weight-management dose of semaglutide (Wegovy 2.4 mg) lost about 15% of body weight on average over 68 weeks — but that’s a group mean. About half lost 15% or more, while roughly one in seven lost less than 5%. Real-world averages are lower (around 9% at a year), largely because many people stop early or don’t reach the full dose. Ozempic (the diabetes brand of semaglutide) is usually dosed lower and tends to produce somewhat less. None of this predicts your individual result.

How much weight will I lose on Mounjaro or Zepbound? In SURMOUNT-1, tirzepatide (Zepbound) averaged roughly 15% at 5 mg up to about 21% at 15 mg over 72 weeks — versus about 3% on placebo. As with semaglutide, individual results vary widely and real-world loss tends to run lower than the trial. The diabetes brand (Mounjaro) at typical doses generally produces somewhat less than the weight-management dosing.

Why am I losing less weight than the trials say? Very common, and usually not a sign anything is wrong. Trial averages reflect people who reached the full maintenance dose and stayed on it with structured support; real-world loss averages lower (~9% at a year). Dose, how long you’ve been on it, and your own biology all matter. If your loss has stalled, our page on the weight-loss plateau explains why — and it’s worth raising with your prescriber rather than self-adjusting.

Will I keep the weight off if I stop? Mostly not, on current evidence: about two-thirds of lost weight was regained within a year of stopping semaglutide in the STEP 1 extension. These medicines work while you take them. Whether, when, and how to stop is a clinical decision — see what happens when you stop.

Which causes more weight loss, Ozempic or Mounjaro? In a direct head-to-head trial (SURMOUNT-5), tirzepatide averaged more weight loss than semaglutide (−20.2% vs −13.7%). So tirzepatide (Mounjaro/Zepbound) tends to produce more on average — but both are highly effective, and the right choice depends on tolerability, cost, access and your history, not the average alone.