The main finding
In one sentence: Over 72 weeks, adults without diabetes lost an average of 15.0%, 19.5% and 20.9% of their body weight on weekly tirzepatide 5, 10 and 15 mg, compared with 3.1% on placebo.
Group averages, not an individual prediction. A trial result describes what happened to a defined group over a defined time.
On this page
Why this study matters
SURMOUNT-1 is the main trial behind tirzepatide (Zepbound, Mounjaro) as a weight-management medicine for adults without diabetes. It is the study most often quoted when people describe how much weight the medicine can help someone lose. If you have seen a figure like "over 20%" attached to tirzepatide, it most likely comes from here. Knowing who was in the trial and how it was run helps you judge how well the result fits your own situation.
Who was in the study
The trial randomized 2539 adults with a body mass index, or BMI, of 30 or more, or 27 or more with at least one weight-related health condition. BMI is a number that relates weight to height. People with diabetes were not included. At the start, average body weight was 104.8 kg and average BMI was 38.0. Nearly everyone (94.5%) had a BMI of 30 or higher. The trial lasted 72 weeks.
What the researchers did
This was a phase 3, double-blind, randomized, placebo-controlled trial. Randomized means people were assigned by chance. Double-blind means neither participants nor researchers knew who received which treatment. A placebo is an injection that looks the same but contains no medicine.
People were split into four equal groups: weekly tirzepatide injections at 5 mg, 10 mg or 15 mg, or placebo, for 72 weeks. The first 20 weeks were used to raise the dose gradually. The two primary outcomes were the percentage change in weight from the start and the share of people who lost at least 5% of their weight.
The trial used a treatment-regimen estimand. An estimand is a precise statement of what the trial is measuring. Here it means results count everyone who was randomized, whether or not they kept taking their assigned treatment.
What they found
At week 72, average weight change was -15.0% with tirzepatide 5 mg (95% CI -15.9 to -14.2), -19.5% with 10 mg (95% CI -20.4 to -18.5) and -20.9% with 15 mg (95% CI -21.8 to -19.9), compared with -3.1% with placebo (95% CI -4.3 to -1.9) (R32-C45). A confidence interval, or CI, is the range within which the true result most likely sits. The published summary we reviewed does not report the between-group differences as separate numbers.
| Outcome at 72 weeks | 5 mg | 10 mg | 15 mg | Placebo |
|---|---|---|---|---|
| Average weight change | -15.0% | -19.5% | -20.9% | -3.1% |
| Lost at least 5% of weight | 85% | 89% | 91% | 35% |
| Lost at least 20% of weight | Not reported | 50% | 57% | 3% |
| Stopped because of side effects | 4.3% | 7.1% | 6.2% | 2.6% |
The share of people losing at least 5% of their weight was 85% (95% CI 82 to 89), 89% (95% CI 86 to 92) and 91% (95% CI 88 to 94) with the three doses, compared with 35% (95% CI 30 to 39) on placebo. Half of the 10 mg group (50%, 95% CI 46 to 54) and 57% of the 15 mg group (95% CI 53 to 61) lost at least 20% of their weight, compared with 3% on placebo (95% CI 1 to 5). The abstract states that all prespecified heart and metabolic measures improved with tirzepatide, without giving figures.
Side effects and people who stopped
The most common side effects with tirzepatide were stomach and bowel symptoms. Most were mild to moderate and happened mainly while the dose was being raised. Side effects led to stopping treatment in 4.3% of the 5 mg group, 7.1% of the 10 mg group and 6.2% of the 15 mg group, compared with 2.6% on placebo (R32-C46). The published summary we reviewed does not report how many people in each group stopped treatment for any reason.
What this study does not tell you
- Everyone was an adult without diabetes. SURMOUNT-2 studied people with type 2 diabetes separately, and the two trials should not be merged.
- The trial lasted 72 weeks. Rare side effects and longer-term harms cannot be judged from a trial of this length.
- It does not show what happens after the medicine stops. SURMOUNT-4 addresses that.
- Percentages from this trial should not be placed next to percentages from other medicines' trials to rank them. Populations, durations and analysis methods differ.
- We reviewed the abstract only, so details of the lifestyle support, countries and full safety tables were not checked.
- The trial was funded by the manufacturer.
What it means in Canada
SURMOUNT-1 is the core evidence for tirzepatide as a weight-management medicine in adults without diabetes, and Canadian clinicians will refer to it. It does not describe how the medicine is prescribed or paid for in Canada. See tirzepatide in Canada for the medicine, public drug coverage by province for cost questions, and compare services for care options.
Common questions
How much weight did people lose on tirzepatide in SURMOUNT-1?
At 72 weeks, average weight change was -15.0% with 5 mg, -19.5% with 10 mg and -20.9% with 15 mg, compared with -3.1% with placebo.
How many people stopped tirzepatide because of side effects in SURMOUNT-1?
Side effects led to stopping in 4.3% of the 5 mg group, 7.1% of the 10 mg group and 6.2% of the 15 mg group, compared with 2.6% on placebo.
Who was in the SURMOUNT-1 trial?
2539 adults with a BMI of 30 or more, or 27 or more with a weight-related condition, and without diabetes. Their average starting weight was 104.8 kg.
Related reading
Other studies in this library that bear on the same question.
Tirzepatide for weight loss in type 2 diabetes: SURMOUNT-2
Three years of tirzepatide in adults with prediabetes: SURMOUNT-1
What happens to weight when tirzepatide is stopped: SURMOUNT-4
Tirzepatide versus semaglutide head to head: SURMOUNT-5
STEP 1: semaglutide for weight loss in adults
Source
Original paper: Ania M Jastreboff, SURMOUNT-1, The New England journal of medicine, 2022. PubMed: https://pubmed.ncbi.nlm.nih.gov/35658024/. DOI: https://doi.org/10.1056/NEJMoa2206038. Funding: The trial was funded by Eli Lilly. How we summarized it: abstract and extracted data.
This page explains a published study. It is not medical advice and does not describe whether a medicine is right for you. Talk to your doctor, nurse practitioner or pharmacist about your own situation. Reviewed and signed by two pharmacists registered in British Columbia, 2026-09-18.
Evidence records behind this page
Each record is a row in the clinical evidence file, taken from the published paper named above. The caution column is the limit the researcher recorded for that number.
This table scrolls sideways.
| Record | What the paper reports | Type | Population | Caution | Source |
|---|---|---|---|---|---|
| R32-C45 | In SURMOUNT-1, adults without diabetes had average weight changes at 72 weeks of -15.0%, -19.5% and -20.9% with weekly tirzepatide 5, 10 and 15 mg, versus -3.1% with placebo. | direct evidence | Adults with obesity or overweight plus a weight-related complication; without diabetes | Adults without diabetes; 72 weeks; abstract-only extraction; rare and longer-term harms are not established. | PubMed 35658024 |
| R32-C46 | Adverse events led to stopping treatment in 4.3%, 7.1% and 6.2% of the tirzepatide 5, 10 and 15 mg groups, versus 2.6% with placebo in SURMOUNT-1. | direct evidence | Adults with obesity or overweight plus a weight-related complication; without diabetes | Adults without diabetes; 72 weeks; abstract-only extraction; rare and longer-term harms are not established. | PubMed 35658024 |