# Tirzepatide Weight Loss: The Trial Evidence

> Tirzepatide weight loss trial evidence: SURMOUNT-1 (-20.9% at 72 weeks), SURMOUNT-5 head-to-head versus semaglutide (-20.2% vs -13.7%), and body composition data from the DXA substudy.

## The short version

Tirzepatide weight loss data come from the SURMOUNT phase 3 clinical trials — the largest and most rigorous weight-management studies conducted on any medicine. In adults with obesity who did not have type 2 diabetes, tirzepatide at the highest approved dose of 15 mg reduced body weight by an average of 20.9 percent over 72 weeks, compared to 3.1 percent in the placebo group. In a direct comparison against semaglutide, tirzepatide reduced weight by 20.2 percent versus 13.7 percent. These are trial averages — individual results vary. This page summarises what the published evidence shows, with every quantitative finding cited.

## Tirzepatide weight loss — the SURMOUNT-1 results

SURMOUNT-1 was a 72-week phase 3 double-blind randomised controlled trial of 2,539 adults with obesity (BMI ≥30, or ≥27 with at least one weight-related complication) without type 2 diabetes. Participants received once-weekly tirzepatide at 5, 10, or 15 mg, or placebo, with a 20-week dose-escalation period.

Mean weight changes at week 72 were -15.0% at 5 mg, -19.5% at 10 mg, and -20.9% at 15 mg, versus -3.1% with placebo [4]. The proportion of participants achieving at least 5%, 10%, 15%, and 20% weight loss was substantially higher across all tirzepatide doses than with placebo. At the 15 mg dose, approximately one in three participants achieved at least 25% total body weight loss.

In an extended follow-up of the SURMOUNT-1 cohort with prediabetes at baseline, tirzepatide produced sustained weight reduction and reduced progression to type 2 diabetes [35].

An additional SURMOUNT-3 trial enrolled 579 participants who had already achieved at least 5% weight reduction through a 12-week intensive lifestyle intervention. Adding tirzepatide thereafter produced an additional -18.4% weight change at 72 weeks, versus +2.5% with placebo — a treatment difference of -20.8 percentage points [42].

Tirzepatide results in terms of body composition were characterised in a post hoc DXA substudy of SURMOUNT-1. Approximately 75% of the weight lost was fat mass and approximately 25% was lean mass — proportions consistent with typical weight-loss patterns and similar across most subgroup analyses [14].

## Tirzepatide results — SURMOUNT-5: head-to-head versus semaglutide

SURMOUNT-5 was a phase 3b open-label head-to-head trial in 751 adults with obesity but without type 2 diabetes. Participants were randomised to the maximum tolerated dose of tirzepatide (10 or 15 mg) or the maximum tolerated dose of semaglutide (1.7 or 2.4 mg) once weekly for 72 weeks.

The least-squares mean weight change at week 72 was -20.2% with tirzepatide versus -13.7% with semaglutide (P<0.001) [5]. Tirzepatide also produced greater reductions in waist circumference and higher proportions reaching weight-loss thresholds of ≥10%, ≥15%, ≥20%, and ≥25%.

A network meta-analysis of 31 randomised controlled trials (more than 35,000 patients) placed tirzepatide 15 mg in the top three across weight-related outcomes, with the highest efficacy for achieving at least 15% weight loss versus placebo (risk ratio 10.24, 95% CI 6.42–16.34) among all agents examined [43].

## Cardiometabolic effects of tirzepatide weight loss

A post hoc analysis of SURMOUNT-1 found that improvements in cardiometabolic risk factors scaled with the degree of tirzepatide weight loss. In participants losing at least 35% of body weight, mean changes included systolic blood pressure -14.2 mmHg, diastolic blood pressure -9.2 mmHg, waist circumference -32.4 cm, and HOMA-IR (a measure of insulin resistance) -59.7% [44]. Insulin resistance and HbA1c began to improve even with modest weight reductions, while lipid improvements were seen primarily after more than 10% weight reduction.

Weight regain after stopping tirzepatide is documented. SURMOUNT-4 found that participants switched to placebo regained weight while those continuing on tirzepatide kept losing [22]. The regain was associated with worsening cardiometabolic risk factors [23], consistent with the evidence that the metabolic benefits depend on continued treatment.

A systematic review and meta-analysis of 6 randomised controlled trials (4,531 participants) found tirzepatide 10 and 15 mg significantly improved patient-reported physical function versus placebo on both the SF-36 and IWQOL-Lite-CT instruments [45].

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A reading room for the published tirzepatide trial record — not a clinic, not a prescriber.