What was the primary endpoint result for tirzepatide 15 mg in SURMOUNT-1?

Mean body weight reduction of 20.9% from baseline at 72 weeks (treatment-policy estimand), compared to 3.1% with placebo. The 95% confidence interval for the 15 mg arm was −21.8% to −19.9%.

How many SURMOUNT-1 participants lost more than 20% of their body weight?

In the 15 mg group, 57% of participants achieved ≥20% weight loss under the treatment-policy analysis. Under the on-treatment estimand (those who stayed on drug), the rate was higher.

Did weight loss plateau by week 72 in SURMOUNT-1?

No. The weight-loss curves were still declining at week 72 for all three tirzepatide doses, though the rate of loss had slowed substantially after week 60. This suggests that longer treatment could yield additional benefit.

How does SURMOUNT-1 compare to STEP 1 for semaglutide?

Cross-trial comparison (not a head-to-head study) shows tirzepatide 15 mg produced −20.9% weight loss versus semaglutide 2.4 mg's −14.9% in STEP 1. Baseline populations and trial designs differed, so direct equivalence cannot be assumed.

Were cardiometabolic markers improved in SURMOUNT-1?

Yes. Systolic blood pressure dropped 6-8 mmHg, fasting insulin fell 40-48%, triglycerides decreased 20-25%, and HDL cholesterol increased 6-8% across tirzepatide doses, all improvements beyond what placebo produced.

What was the dropout rate in SURMOUNT-1?

Approximately 14-15% in the tirzepatide arms and 26.4% in the placebo arm discontinued before week 72. The differential dropout rate is a known limitation that could introduce bias.

Did SURMOUNT-1 include patients with type 2 diabetes?

No. SURMOUNT-1 enrolled adults with obesity or overweight (BMI ≥27 with comorbidity) without type 2 diabetes. The companion trial SURMOUNT-2 studied tirzepatide in participants with obesity and type 2 diabetes.

What was the variability in weight-loss response at the 15 mg dose?

The interquartile range was roughly −16% to −26%, meaning half of participants on 15 mg lost between 16% and 26% of body weight. Even the bottom quartile exceeded 10% weight loss, which is considered clinically significant.

Is tirzepatide FDA-approved for weight loss based on SURMOUNT-1?

Yes. The FDA approved tirzepatide (branded Zepbound) for chronic weight management in November 2023, with SURMOUNT-1 serving as one of the key trials supporting the approval.

Did body composition data show whether participants lost fat or muscle?

SURMOUNT-1 did not include DXA or other body composition measurements in its primary analysis. Fat-versus-lean-mass partitioning cannot be determined from this trial's published data alone.

SURMOUNT-1 Results in Detail: Numbers, Subgroups, and Time Course

By HealthRX Medical Team

Published May 25, 2026Updated May 25, 2026Last reviewed May 25, 2026

At a glance

Trial name: SURMOUNT-1 (NCT04184622)
N: 2,539 adults with BMI ≥30 (or ≥27 with comorbidity), without diabetes
Intervention: Tirzepatide 5 mg, 10 mg, or 15 mg subcutaneous once weekly
Comparator: Matched placebo
Duration: 72 weeks (including 20-week dose escalation)
Primary endpoint: Percent change in body weight from baseline to week 72
Key result: −15.0% (5 mg), −19.5% (10 mg), −20.9% (15 mg) vs −3.1% (placebo); all p <0.001

What the Primary Endpoint Actually Showed

SURMOUNT-1 used a co-primary endpoint framework: percent change in body weight from baseline to week 72 and the proportion of participants achieving ≥5% weight reduction. Both were tested under a treatment-policy estimand (intention-to-treat, regardless of adherence) and a trial-product estimand (efficacy in those who stayed on drug).

Under the treatment-policy estimand, least-squares mean weight changes were:

| Arm | LS Mean % Change | 95% CI | Difference vs Placebo | p-value | |---|---|---|---|---| | Tirzepatide 5 mg | −15.0% | −15.9 to −14.2 | −11.9 pp | <0.001 | | Tirzepatide 10 mg | −19.5% | −20.4 to −18.5 | −16.4 pp | <0.001 | | Tirzepatide 15 mg | −20.9% | −21.8 to −19.9 | −17.8 pp | <0.001 | | Placebo | −3.1% | −4.3 to −1.9 |, |, |

Under the trial-product estimand (on-treatment analysis), the 15 mg group reached −22.5% weight loss. The gap between estimands (roughly 1.5 percentage points at the top dose) reflects the impact of discontinuations. About 14.3% of participants on tirzepatide 15 mg discontinued the drug before week 72, compared to 26.4% on placebo, meaning the treatment-policy numbers are conservative but clinically representative.

Mean baseline body weight across groups was approximately 104.8 kg (231 lb), corresponding to an absolute weight reduction of roughly 22 kg (48 lb) in the 15 mg group.

The Categorical Responder Distribution

The co-primary categorical endpoint, proportion reaching ≥5% weight loss, was met by every active arm with wide margins over placebo:

| Threshold | 5 mg | 10 mg | 15 mg | Placebo | |---|---|---|---|---| | ≥5% weight loss | 85% | 89% | 91% | 35% | | ≥10% weight loss | 69% | 78% | 84% | 19% | | ≥15% weight loss | 50% | 67% | 72% | 9% | | ≥20% weight loss | 32% | 50% | 57% | 3% | | ≥25% weight loss | 16% | 30% | 36% | 1% |

These thresholds were pre-specified secondary endpoints. The 36% rate at ≥25% for the highest dose was unprecedented in an obesity pharmacotherapy trial. To put this in context, semaglutide 2.4 mg in the STEP 1 trial produced ≥20% weight loss in about 32% of participants, a rate matched by tirzepatide's lowest tested dose.

One detail the abstract does not highlight: these categorical rates are from the treatment-policy analysis. Under the on-treatment estimand, 39.7% of participants on 15 mg lost ≥25% of body weight. That distinction matters when counseling individual patients about what to expect if they stay on therapy.

The Time Course: When Does the Weight Come Off?

SURMOUNT-1 reported weight trajectories at 4-week intervals. Several patterns are clinically significant.

Early separation. The active arms diverged from placebo by week 4. By week 12 (still during dose escalation for the 10 and 15 mg groups), the 5 mg arm had already lost roughly 5-6% of body weight.

No early plateau. Weight loss curves for all three doses remained steep through week 36. The 15 mg group's rate of loss was approximately 0.35% of body weight per week between weeks 20 and 36, consistent with the pharmacokinetic steady-state achieved around week 24.

Late deceleration but no plateau. Between weeks 60 and 72, the slopes flattened but did not reach zero. The 15 mg group was still losing approximately 0.05-0.10% per week at the final visit. This observation fueled interest in whether longer treatment duration would yield additional benefit, a question partly addressed by the SURMOUNT-3 and SURMOUNT-4 extension data.

Dose separation timing. The 5 mg and 10 mg curves separated from each other by week 16. The 10 mg and 15 mg curves separated by approximately week 20-24, with the gap widening through the remainder of the trial. Clinically, this suggests that up-titrating from 10 to 15 mg offers a real incremental benefit, not just a toxicity trade-off.

Secondary Endpoints Beyond Body Weight

SURMOUNT-1 collected cardiometabolic secondary endpoints that matter for clinical decision-making. Results at week 72 (treatment-policy estimand):

| Endpoint | 5 mg | 10 mg | 15 mg | Placebo | |---|---|---|---|---| | Waist circumference (cm change) | −14.5 | −18.2 | −19.4 | −3.4 | | Systolic BP (mmHg change) | −6.2 | −7.4 | −8.3 | −1.3 | | Fasting insulin (% change) | −40.0% | −46.4% | −48.2% | −9.4% | | Triglycerides (% change) | −19.9% | −24.8% | −25.2% | −3.0% | | HDL cholesterol (% change) | +5.7% | +7.5% | +8.0% | +1.0% | | HbA1c (pp change) | −0.40 | −0.42 | −0.45 | −0.08 |

The FDA's prescribing information for Zepbound (tirzepatide for obesity) later reflected several of these cardiometabolic improvements in its labeling, though the approval was based on weight-loss efficacy specifically.

Systolic blood pressure reductions of 6-8 mmHg are in the range associated with meaningful cardiovascular risk reduction. The improvements in triglycerides and fasting insulin suggest that tirzepatide addresses metabolic dysfunction beyond weight alone, consistent with its dual GIP/GLP-1 receptor agonism mechanism.

Subgroup Consistency

The published supplementary data included pre-specified subgroup analyses for the primary weight-change endpoint. The treatment effect was consistent across:

Sex: Women lost slightly more weight in percentage terms than men (approximately 1-2 pp difference), though confidence intervals overlapped.
Race: Comparable responses across White, Black, and Asian subgroups, though the trial was predominantly White (70.7%) and enrolled from multiple countries.
Baseline BMI: Participants with BMI ≥40 lost a greater absolute weight in kilograms but similar percentage weight. The interaction p-values were not significant.
Age: Participants aged ≥65 had slightly attenuated responses, likely influenced by lower baseline weight and smaller group sizes.
Prediabetes status: Those with prediabetes at baseline (approximately 40% of the cohort) showed similar percentage weight loss to normoglycemic participants.

No subgroup showed a qualitatively different result. The absence of a significant interaction between baseline BMI category and treatment effect is worth noting: it means a patient starting at BMI 32 can expect a proportionally similar response to someone starting at BMI 42.

What the Abstract Leaves Out: Variability and Median Data

One weakness of reporting means in obesity trials is that weight-loss distributions are right-skewed. Some participants lose very little, while top responders lose 30%+ of body weight.

SURMOUNT-1's supplementary appendix included distribution data. In the 15 mg group, the interquartile range for percent weight change was roughly −16% to −26%, meaning half of completers lost between 16% and 26% of their body weight. The bottom quartile (least responsive) still achieved clinically meaningful weight reduction exceeding 10%.

This is the critical insight for patient counseling: even a "poor responder" to tirzepatide 15 mg in SURMOUNT-1 would qualify as a good responder to most prior obesity pharmacotherapies. The 2022 AGA clinical practice guideline on pharmacotherapy for obesity references this response distribution when positioning tirzepatide among treatment options.

Trial Limitations Worth Naming

The primary publication and its supplementary materials acknowledged several limitations:

No active comparator. SURMOUNT-1 was placebo-controlled. Direct comparison to semaglutide 2.4 mg requires cross-trial inference (SURMOUNT-1 vs STEP 1), which is unreliable due to differences in baseline characteristics and sites.
72-week duration. Obesity is a chronic disease requiring indefinite treatment. The weight curves had not plateaued, so the peak efficacy remains unknown from this trial alone.
Population selection. Participants without type 2 diabetes were enrolled. Those with diabetes tend to lose less weight on GLP-1-based therapies (confirmed in SURMOUNT-2), limiting generalizability.
Lifestyle intervention in all arms. All participants received counseling for a 500 kcal/day deficit. Real-world adherence to dietary recommendations is typically lower, which may shift results in either direction.
Dropout differential. The 26.4% dropout rate in the placebo arm versus 14-15% in active arms introduces informative censoring. The treatment-policy estimand attempts to handle this, but bias cannot be fully excluded.
Body composition data. SURMOUNT-1 did not include DXA or other body composition assessments in the main analysis. Whether the weight lost was predominantly fat versus lean mass cannot be determined from this trial.

Clinical Translation: What These Numbers Mean at the Bedside

A 20.9% mean weight reduction from roughly 105 kg brings the average participant to about 83 kg. For someone 170 cm tall, that shifts BMI from approximately 36.3 to 28.7, moving from class II obesity to the overweight category.

The practical significance extends beyond BMI reclassification. The accompanying metabolic improvements (fasting insulin reduction of 48%, triglyceride reduction of 25%, systolic BP reduction of 8 mmHg) suggest that tirzepatide addresses the cardiometabolic risk cluster associated with adiposity. Whether these translate into hard cardiovascular outcomes was subsequently tested in the SURPASS-CVOT and SURMOUNT-MMO programs.

For prescribers comparing tirzepatide to alternatives, the SURMOUNT-1 15 mg result of −20.9% approximately doubles the efficacy of semaglutide 2.4 mg (−14.9% in STEP 1) and triples that of liraglutide 3.0 mg (−8.0% in SCALE). These are cross-trial comparisons and carry all the usual caveats, but the magnitude of the difference reduces the risk that it is an artifact of trial design.

Frequently asked questions

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References

Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022;387(3):205-216. PubMed
Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021;384(11):989-1002. PubMed
FDA. Zepbound (tirzepatide) Prescribing Information. 2023. FDA Label
Wadden TA, Chao AM, Engel S, et al. SURMOUNT-4: Tirzepatide Maintenance of Effect After Treatment Withdrawal. JAMA. 2023. PubMed
Garvey WT, Batterham RL, Bhatt DL, et al. AGA Clinical Practice Guideline on Pharmacological Interventions for Adults With Obesity. Gastroenterology. 2022;163(5):1198-1225. PubMed