Weight & Metabolic
How Much Weight Do People Actually Lose Outside of Clinical Trials?
The short answer
In the pivotal STEP 1 trial, semaglutide produced 14.9% average weight loss over 68 weeks (n=1,961); tirzepatide's SURMOUNT-1 trial produced up to 20.9% at the highest dose over 72 weeks (n=2,539). Real-world cohorts consistently land lower. A 2026 review comparing 127 trials against 26 real-world studies found real-world semaglutide users with diabetes lost 4.7-10.5% of body weight over 6-12 months — well below trial figures — and traced most of the gap to two things: only 13% of real-world semaglutide users ever reached the 2.4 mg trial dose, and 20-50% of people discontinue treatment within a year. Among people who stayed on treatment and reached target doses, real-world results converged much closer to trial results.
Quick answers
- What is it?
- This compares how much weight people lose on semaglutide or tirzepatide in real-world clinical practice versus how much they lost in the controlled pivotal trials.
- Why are people interested in it?
- People starting these drugs want a realistic expectation, not just the headline trial numbers that made the drugs famous.
- What has actually been studied?
- The pivotal STEP 1 and SURMOUNT-1 trials, a 2026 review comparing 127 trials against 26 real-world studies, several individual real-world cohorts from Turkey, Greece, and US electronic health records, and studies tracking dose attainment, treatment persistence, and discontinuation.
- How strong is the evidence?
- Well-documented gap: real-world cohorts consistently show smaller average weight loss than trials, and the review traced most of the gap to two measurable factors — only 13% of real-world semaglutide users reached the trial dose, and 20-50% discontinue within a year.
- What don't we know yet?
- What happens beyond 2-3 years in real-world use, why specifically people stop taking these drugs, how much real-world data undercounts people entirely, and whether the trial-to-real-world gap will keep narrowing as access improves.
- Where can I read the original research?
- Sources at the bottom of this page: Wilding JPH, et al., New England Journal of Medicine, 2021 (STEP 1); and Jastreboff AM, et al., New England Journal of Medicine, 2022 (SURMOUNT-1).
The short answer: Clinical trials of semaglutide and tirzepatide report large, consistent weight loss because trial participants receive close monitoring, structured dose escalation, and strong incentives to stay enrolled. Real-world patients face insurance approvals, cost, side effects, and life — and the data show it. Real-world cohorts generally report smaller average weight loss than trials, largely explained by two measurable factors: fewer people reach the top studied dose, and more people stop treatment altogether.
What the research shows
The trial numbers, for reference
In the pivotal STEP 1 trial, 1,961 adults with overweight or obesity (without diabetes) were randomized to once-weekly semaglutide 2.4 mg or placebo for 68 weeks, alongside lifestyle counseling. Average weight change was -14.9% with semaglutide versus -2.4% with placebo — a result that counted everyone as randomized, regardless of whether they stayed on treatment the whole time.
In SURMOUNT-1, 2,539 adults with obesity were randomized to tirzepatide (5 mg, 10 mg, or 15 mg) or placebo for 72 weeks. Average weight change was -15.0% at 5 mg, -19.5% at 10 mg, and -20.9% at the highest 15 mg dose, versus -3.1% with placebo.
What real-world cohorts actually find
A 2026 narrative review set out to systematically compare this trial evidence against real-world practice, contrasting seven 2026-published meta-analyses of 127 randomized trials (58,976 participants total) against 26 real-world studies identified through a structured PubMed search. In people with type 2 diabetes, real-world semaglutide use over 6-12 months produced 4.7-10.5 kg of weight loss and 1.0-1.3 percentage points of HbA1c reduction — both lower than the trial meta-analytic estimates. In people without diabetes, the picture depended heavily on how the data were analyzed: "persister" analyses that only counted people who stayed on treatment (from the SHAPE real-world study) found semaglutide users lost 14.1% and tirzepatide users lost 16.5% — numbers that closely approximated per-protocol trial results (semaglutide -14.9%, tirzepatide 15 mg -20.9%). But intention-to-treat, all-comer real-world estimates were substantially lower than that.
Several individual real-world cohorts illustrate the range. A nationwide Turkish study of 2,549 patients with obesity found median weight loss at 6 months of 12.6% with semaglutide and 14.4% with tirzepatide — figures reasonably close to trial results, though measured over a shorter window. A Greek multicenter study of 260 patients on low-to-moderate dose tirzepatide, with 80% completing 6 months of therapy, found 94.2% achieved at least 5% weight loss, 77.9% achieved at least 10%, 47.1% achieved at least 15%, and about 20% achieved 20% or more. A US electronic-health-record study that propensity-matched 20,678 tirzepatide and semaglutide patients found mean maximum weight loss over two years of 14.7% with tirzepatide versus 10.8% with semaglutide, with tirzepatide patients nearly twice as likely to be "high responders" (≥15% weight loss in year one: 42.6% vs. 21.6%).
Why the gap: dose, and dropout
The 2026 review identified two factors that explained much of the difference between trial and real-world results. First, dose attainment: just 13% of real-world semaglutide users reached the 2.4 mg dose used in the STEP trials, and only 25.9% of real-world tirzepatide users reached the 15 mg dose used in SURMOUNT-1. Since these drugs show a clear dose-response relationship for weight loss, a population that never reaches the studied dose will not match the studied result.
Second, persistence. A claims-based study of 33,607 commercially insured people without diabetes who newly started a weight-loss-indicated GLP-1 drug between 2021 and mid-2024 found that 1-year treatment persistence rose sharply over that window — from 33.2% among people who started in 2021 to 60.9% among those who started in the first half of 2024, likely reflecting the resolution of drug shortages and improved side-effect management. Even at its most recent, best measurement, persistence was 60.9% — meaning roughly 4 in 10 new users had stopped filling prescriptions within a year. Tirzepatide's persistence rate, in the years it had been available (2023-2024), ran higher, around 64-65%.
What happens when people stop
A retrospective study of 7,938 patients who discontinued semaglutide or tirzepatide within 3-12 months of starting found average weight loss to the point of discontinuation was -8.4% when the drug was prescribed for obesity, and -4.4% when prescribed for type 2 diabetes. In the year after stopping, average weight change was a modest +0.5% for the obesity group (essentially flat, though with wide individual variability) and -1.3% for the diabetes group (continued loss, likely reflecting continued lifestyle changes or other medications). Within that first year off treatment, 19.6% of patients restarted the same drug and 35.2% moved to some other obesity treatment — a lifestyle-modification visit, a different medication, or, rarely, bariatric surgery.
What we don't know yet
What happens beyond 2-3 years in real-world use. Most real-world cohorts described here run 6 months to 2 years. Longer-term weight trajectories, on or off these drugs, remain thinly documented outside of trial extension studies.
Why, specifically, people stop taking these drugs. Cost, insurance coverage changes, side effects, plateaued results, and supply shortages are all plausible drivers, but most real-world cohorts don't consistently capture the reason for each discontinuation.
How much real-world data undercounts people entirely. Electronic-health-record and insurance-claims studies can only see prescriptions filled through the tracked system — they miss people who source medication through other channels, and may miscount people who transfer care between health systems.
Whether the trial-to-real-world gap will keep narrowing. As access, affordability, and side-effect management continue to change, it's unclear how much of the current gap between trial and real-world results is structural versus a snapshot of an early, still-maturing period of widespread use.
Who, specifically, reaches trial-level results in real-world care. Aggregate cohort averages are available; individual-level predictors of who ends up in the "reached target dose, stayed on treatment" group versus who doesn't are not well characterized.
Sources
- Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. New England Journal of Medicine, 2021;384(11):989-1002. DOI
- Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine, 2022;387(3):205-216. DOI
- Bloomgarden Z. Real-World vs. Randomized Trial Evidence for Incretin-Based Therapies: A Narrative Review. Journal of Diabetes, 2026;18(7):e70248. DOI
- Venkatakrishnan AJ, Murugadoss K, Soundararajan V. Weight-loss dynamics with tirzepatide versus semaglutide. PNAS Nexus, 2026;5(6):pgag171. DOI
- Gasoyan H, et al. Obesity Treatments and Weight Changes in Clinical Practice After Discontinuation of Semaglutide or Tirzepatide. Diabetes, Obesity & Metabolism, 2026;28(6):4795-4805. DOI
- Marshall LZ, Gleason PP, Friedlander N, Farley J, Urick BY. Trends in 1-year persistence and adherence among initiators of high-potency, weight loss-indicated glucagon-like peptide 1 receptor agonists. Journal of Managed Care & Specialty Pharmacy, 2026;32(3):281-291. DOI
- Hepşen S, et al. Real-World Comparison of Short-Term Adverse Events, Treatment Persistence, and Efficacy of Semaglutide and Tirzepatide: A Nationwide Multicenter Study. Obesity Facts, 2026. DOI
- Angelopoulos N, et al. Six-Month Real-World Effectiveness, Persistence and Safety of Low-to-Moderate Dose Tirzepatide in Adults With Obesity: A Multicentre Observational Study. Diabetes, Obesity & Metabolism, 2026;28(7):6113-6120. DOI
Article records maintained via PubMed. Peptide Proof reports what studies found. It does not provide medical advice, dosing guidance, or treatment recommendations.
Topic: Weight & Metabolism | Evidence Index: semaglutide · tirzepatide
Sources
- Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1)
- New England Journal of Medicine, 2021 · n=1961 · DOI
- Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1)
- New England Journal of Medicine, 2022 · n=2539 · DOI
- Real-World vs. Randomized Trial Evidence for Incretin-Based Therapies: A Narrative Review
- Journal of Diabetes, 2026 · DOI
- Weight-loss dynamics with tirzepatide versus semaglutide
- PNAS Nexus, 2026 · n=20678 · DOI
- Obesity Treatments and Weight Changes in Clinical Practice After Discontinuation of Semaglutide or Tirzepatide
- Diabetes, Obesity & Metabolism, 2026 · n=7938 · DOI
- Trends in 1-year persistence and adherence among initiators of high-potency, weight loss-indicated GLP-1 receptor agonists
- Journal of Managed Care & Specialty Pharmacy, 2026 · n=33607 · DOI
- Real-World Comparison of Short-Term Adverse Events, Treatment Persistence, and Efficacy of Semaglutide and Tirzepatide
- Obesity Facts, 2026 · n=2549 · DOI
- Six-Month Real-World Effectiveness, Persistence and Safety of Low-to-Moderate Dose Tirzepatide in Adults With Obesity
- Diabetes, Obesity & Metabolism, 2026 · n=260 · DOI
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