What the Trial Data Says About GLP-1 Weight Loss, and What It Does Not

You've probably seen the numbers: people losing 15%, 20%, sometimes more of their body weight on a GLP-1 medication. Those figures are real, but they come from specific clinical trials with their own rules, and the fine print changes what the headline means.

Where the percentages come from

The most-cited number for semaglutide comes from STEP 1, a 68-week trial of 1,961 adults with obesity or overweight who did not have diabetes. Everyone in the trial, including the placebo group, also got counseling on diet and activity. The group taking semaglutide lost an average of 14.9% of their body weight. The placebo group, on counseling alone, lost 2.4%. About 86% of the semaglutide group reached at least 5% weight loss, compared with 31% on placebo.

The tirzepatide numbers come from a separate trial, SURMOUNT-1, which ran 72 weeks in 2,539 adults, also without diabetes. Across the three doses tested, average weight loss ran from 15.0% on the lowest to 20.9% on the highest, against 3.1% for placebo. Tirzepatide works on two hormone receptors instead of one, so its results describe that drug specifically and shouldn't be read as a GLP-1 number.

Both trials measured a gap between two groups, not a fixed outcome. The placebo participants weren't doing nothing: they were also being coached on eating and exercise, and they still lost some weight. The number usually quoted in ads is the bigger group's average, stripped of that comparison.

There's a second layer of averaging inside these figures. One way of reporting counts everyone who was randomized, including people who stopped the drug early, in the same average as people who stayed on it as planned. Another narrows the analysis to people who stayed on treatment. SURMOUNT-1 reported both, and the figures above are the version that counts everyone as randomized, which is the more conservative of the two. Neither is wrong; they answer slightly different questions about the same 72 weeks. When you see a percentage quoted somewhere else, it is worth knowing which version it came from.

What happens if you stop

A separate trial, STEP 4, is more revealing about what these numbers mean over time. It took 803 people who had already spent 20 weeks reaching the full semaglutide dose, then split them: some kept taking the drug, others were switched to placebo, for another 48 weeks. The group that kept taking semaglutide lost another 7.9% on top of what they'd already lost. The group switched to placebo regained 6.9%. Those are two very different lines on the same chart, starting from the same place.

A 2026 review pooled six randomized trials with a combined 3,236 participants who had stopped a GLP-1 medication, and modeled the pattern of regain. By one year after stopping, about 60% of the weight lost during treatment had come back. Past that point there were no trial data to work from, so the authors extrapolated: their model puts the plateau at roughly 75% of the weight lost on treatment. Their own reading is that part of the benefit persists and much of it does not.

None of these trials tell you what happens if someone stays on the drug for five or ten years, and none of them settle whether the regain pattern would look different under a different stopping plan. What they show is that the plateau isn't the end of the story, which is why the discontinuation studies matter as much as the headline percentages.

Who was actually studied

Trial populations are chosen on purpose, and that matters for reading the results. STEP 1 and SURMOUNT-1 both excluded people with diabetes, so neither trial's numbers describe how these drugs perform in someone managing type 2 diabetes alongside weight loss, a very common real-world case. Participants also had to meet BMI thresholds, agree to a structured trial protocol, and in STEP 4's case, survive a 20-week run-in that filtered out people who didn't tolerate the drug or couldn't stick with dose escalation. A trial population selected this way tends to look more adherent and more tolerant than a general group of patients, which is one reason a trial average may not describe what happens outside a trial.

Intake for a telehealth program works differently from a trial protocol. The Embody GLP-1 program uses a short online intake followed by a licensed provider's individual review, rather than a fixed set of trial inclusion criteria, before deciding whether a prescription is appropriate.

What the data doesn't cover

One more limitation applies across all of this: every trial cited here tested the branded, FDA-approved product, dosed and monitored under a research protocol. None of it was generated on a compounded version of semaglutide or tirzepatide, and compounded versions aren't FDA-approved products. A compounded medication is a question to raise directly with a licensed provider; the trial data above doesn't speak to it.

STEP 1 reported two different kinds of result: the mean percentage change, and the share of participants who reached at least 5% weight loss. Most of the copy built around this drug class quotes the first and leaves the second out, even though the trial published both.

Reading a program's page after research like this puts the numbers in a different light. The collection breaking down the evidence behind GLP-1 claims covers what the studies do and don't say, if you want to go further before deciding anything.

For anyone weighing a prescription telehealth option, the Embody GLP-1 program page lays out how its intake and provider review work, separate from any specific outcome number.

Sources

• Wilding et al., STEP 1, NEJM 2021, figures read from the American College of Cardiology trial summary of the same publication, https://www.acc.org/latest-in-cardiology/clinical-trials/2021/02/18/19/23/step-1. 1,961 adults with obesity/overweight, no diabetes; semaglutide 2.4 mg weekly vs. placebo, both with lifestyle intervention; 68 weeks; mean weight change -14.9% vs. -2.4%, 86.4% vs. 31.5% reached at least 5% weight loss; limitation: excluded diabetes, self-selected volunteers, industry-funded.
• Jastreboff et al., SURMOUNT-1, NEJM 2022, figures read from a peer-reviewed summary of the same trial, https://pmc.ncbi.nlm.nih.gov/articles/PMC9606350. 2,539 adults with obesity/overweight, no diabetes; tirzepatide 5/10/15 mg weekly vs. placebo; 72 weeks; weight loss 15.0%, 19.5% and 20.9% by dose vs. 3.1% placebo, counting all randomized participants; limitation: dual-mechanism drug, not directly comparable to GLP-1-only results.
• Wilding et al., STEP 4, JAMA 2021, https://jamanetwork.com/journals/jama/fullarticle/2777886. 803 adults with obesity/overweight who had reached target dose; continued semaglutide 2.4 mg vs. switch to placebo; 48 additional weeks; weight change -7.9% (continued) vs. +6.9% (switched); limitation: shows discontinuation within the trial window, not indefinite long-term outcomes.
• FDA statement, "FDA clarifies policies for compounders as national GLP-1 supply begins to stabilize", https://www.fda.gov/drugs/drug-alerts-and-statements/fda-clarifies-policies-compounders-national-glp-1-supply-begins-stabilize. 503A pharmacies and 503B outsourcing facilities; states that compounded drugs are not approved by FDA and do not receive FDA premarket review for safety, effectiveness and quality; limitation: a shortage wind-down policy that can change, so the status should be rechecked before republishing.
• Budini et al., "Trajectory of weight regain after cessation of GLP-1 receptor agonists," eClinicalMedicine 2026, full text read at https://pmc.ncbi.nlm.nih.gov/articles/PMC13043475. Six randomized trials pooled, 3,236 participants who stopped a GLP-1 receptor agonist; no active intervention; follow-up varies by pooled study; about 60% of the weight lost was regained by one year, with a modelled plateau at 75.3% (95% CI 68.9 to 81.6); limitation: meta-regression across heterogeneous studies, and the plateau is extrapolated beyond 52 weeks rather than observed.

Educational content only. Not medical advice, diagnosis, or treatment. Talk to a licensed clinician before starting, changing, or stopping any medication, peptide, or supplement.