Trial review
SURMOUNT-1 Tirzepatide Trial Explained
SURMOUNT-1 randomised 2,539 adults with obesity, or overweight with a complication, and without type 2 diabetes, to tirzepatide 5, 10 or 15 mg weekly or placebo for 72 weeks. Mean weight reduction was 15.0%, 19.5% and 20.9% against 3.1% with placebo on the treatment-regimen estimand. The effect rose with dose.
The two estimands, and why the difference matters
SURMOUNT-1 published its result twice, using two different analytical approaches, and the figures differ by about one and a half percentage points at every dose.
The treatment-regimen estimand includes every randomised participant regardless of whether they kept taking the drug: 15.0% at 5 mg, 19.5% at 10 mg, 20.9% at 15 mg, against 3.1% with placebo. It answers "what happens if this treatment is prescribed."
The efficacy estimand estimates the effect assuming continued treatment: 16.0%, 21.4%, 22.5% against 2.4%. It answers "what happens if this treatment is taken as intended."
Both are legitimate. Neither is more honest than the other. What is not legitimate is quoting the higher figure without saying which analysis produced it, which is standard practice in the marketing built on this trial. Our charts use the treatment-regimen figures because they are the more conservative and the closer to what a reader should expect.
Who was studied, and who was not
Adults with a BMI of 30 or above, or 27 or above with at least one weight-related complication such as hypertension, dyslipidemia, obstructive sleep apnoea or cardiovascular disease. Critically, without type 2 diabetes — weight outcomes for GLP-1 drugs are typically smaller in people with diabetes, so this result does not transfer to that population.
Participants received the drug alongside a reduced-calorie diet and increased physical activity, with the structured support a trial provides. That support is part of what produced the result and is not part of most telehealth programmes.
Adverse events and discontinuation
At the 15 mg dose: nausea in 29%, diarrhoea 23%, constipation 17%, vomiting 13%, dyspepsia 10%. Most were mild to moderate and concentrated during dose escalation. Seven per cent discontinued because of an adverse event.
That last figure is the one worth carrying into a purchasing decision. Roughly one in fourteen people at the top dose stopped because they could not tolerate it — which is a strong argument against committing twelve months of prepayment before your first injection.
What this trial does not establish
It does not predict your outcome. A 20.9% mean contains people who lost far more and people who lost almost nothing.
It does not apply to compounded preparations, microdoses or oral formulations. It studied an FDA-approved subcutaneous injection at 5, 10 and 15 mg. The dose-response relationship it demonstrates is in fact the strongest argument against assuming a roughly 1 mg microdose produces comparable results.
It does not establish what happens on stopping — that is SURMOUNT-4 — or long-term outcomes beyond the trial window, though the three-year prediabetes analysis extends the picture considerably.
Funding
Eli Lilly, which manufactures tirzepatide, funded and analysed the trial. That is normal in drug development and does not make the results false: it is a large, registered, peer-reviewed study published in the New England Journal of Medicine. It belongs in the citation regardless.
| Design | Phase 3, randomised, double-blind, parallel-group, placebo-controlled |
|---|---|
| Population | Adults with obesity (BMI 30 or above), or overweight (BMI 27 or above) with at least one weight-related complication, without type 2 diabetes |
| Sample size | 2,539 randomised |
| Intervention | Tirzepatide once weekly |
| Comparator | Placebo |
| Duration | 72 weeks |
| Primary endpoint | Co-primary: percentage change in body weight from baseline at week 72, and the proportion achieving at least 5% weight reduction |
| Main result | Treatment-regimen estimand: −15.0% (5 mg), −19.5% (10 mg), −20.9% (15 mg) versus −3.1% placebo. Efficacy estimand: −16.0%, −21.4%, −22.5% versus −2.4% |
Show this figure as a table
| Step | Stage | What happens |
|---|---|---|
| 1 | Who was eligible | Adults with obesity (BMI 30 or above), or overweight (BMI 27 or above) with at least one weight-related complication, without type 2 diabetes |
| 2 | What they received | Tirzepatide once weekly |
| 3 | What it was compared against | Placebo |
| 4 | For how long | 72 weeks |
| 5 | What was measured first | Co-primary: percentage change in body weight from baseline at week 72, and the proportion achieving at least 5% weight reduction |
| 6 | What the group average was | Treatment-regimen estimand: −15.0% (5 mg), −19.5% (10 mg), −20.9% (15 mg) versus −3.1% placebo. Efficacy estimand: −16.0%, −21.4%, −22.5% versus −2.4% |
| 7 | What it does not tell you | A group average is not a prediction for any individual reader. |
Show this figure as a table
| Item | Weight change | Evidence |
|---|---|---|
| Tirzepatide 5 mg | 15% | Verified |
| Tirzepatide 10 mg | 20% | Verified |
| Tirzepatide 15 mg | 21% | Verified |
| Placebo | 3% | Verified |
| Question | Position |
|---|---|
| Average effect in the studied population | Reported above Verified |
| Your individual outcome | Not predicted by any trial |
| Effect of a compounded preparation | Not studied — this was an approved product |
| Effect at doses outside the protocol | Not studied |
| Effect after stopping | Only SURMOUNT-4 tested withdrawal |
| Long-term safety beyond the window | Outside the observation period |
Questions readers actually ask
Why do I see different SURMOUNT-1 numbers on different sites?
The trial published two estimands. The treatment-regimen analysis gives 15.0/19.5/20.9%; the efficacy analysis gives 16.0/21.4/22.5%. Marketing usually quotes the higher set without saying which.
Does this apply to compounded tirzepatide?
No. The trial studied an FDA-approved subcutaneous injection at 5, 10 and 15 mg.
How many people stopped because of side effects?
Seven per cent at the 15 mg dose discontinued because of an adverse event.
Related on this site
- Research hubResearch
- SURMOUNT-3 Tirzepatide Trial ExplainedResearch
- SURMOUNT-4: Weight Maintenance and Tirzepatide WithdrawalResearch
- SURMOUNT-5: Tirzepatide vs Semaglutide Head to HeadResearch
- How rankings are computedCore & Trust
- All-in cost toolTools
- Price records, machine-readableData
- Tirzepatide vs Semaglutide: What the Head-to-Head Trial ShowsJournal
- Tirzepatide and Type 2 Diabetes PreventionResearch
- Tirzepatide and PCOS: What Evidence Exists?Research
GLP-1 Tirzepatide Review. “SURMOUNT-1 Tirzepatide Trial Explained.” S.J Partners LLC, 2026-07-24. https://glp1tirzepatidereview.com/research/surmount-1/
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