Tirzepatide produced 15% to 21% average weight loss in 72-week obesity trial

Adults with obesity who did not have diabetes lost an average of 15.0% to 20.9% of their body weight over 72 weeks of weekly tirzepatide injections, compared with 3.1% among those who received a placebo, in the phase 3 SURMOUNT-1 trial ^[1].

How the trial was set up

SURMOUNT-1 randomly assigned 2,539 adults to four equal groups: tirzepatide at 5 mg, 10 mg or 15 mg, or placebo, injected under the skin once a week for 72 weeks. The first 20 weeks were a dose-escalation period ^[1].

To enroll, participants needed a body-mass index of 30 or higher, or 27 or higher along with at least one weight-related complication. People with diabetes were excluded ^[1]. The study was double-blind, and it is registered as NCT04184622 ^[1][2]. At the start, the average weight was 104.8 kg and the average BMI was 38.0; 94.5% of participants had a BMI of 30 or higher ^[1].

The paper describes tirzepatide as a glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1 receptor agonist, and notes that its efficacy and safety in people with obesity were not previously known ^[1]. Eli Lilly supported the trial ^[1].

What the numbers showed

Average weight change at week 72 was -15.0% (95% confidence interval, -15.9 to -14.2) on the 5 mg dose, -19.5% (-20.4 to -18.5) on 10 mg and -20.9% (-21.8 to -19.9) on 15 mg, against -3.1% (-4.3 to -1.9) on placebo. Each comparison with placebo had a P value below 0.001 ^[1]. The main analysis counted participants whether or not they continued treatment ^[1].

The share who lost at least 5% of body weight was 85% on 5 mg, 89% on 10 mg and 91% on 15 mg, compared with 35% on placebo ^[1]. For a loss of at least 20%, the figures were 50% on the 10 mg dose and 57% on 15 mg, compared with 3% on placebo ^[1]. The authors reported improvements in all prespecified cardiometabolic measures and concluded that the three doses gave substantial and sustained weight reductions ^[1].

Tolerability

Gastrointestinal problems were the most common adverse events, and most were mild to moderate, showing up mainly during dose escalation ^[1]. Side effects led participants to stop treatment at rates of 4.3% on 5 mg, 7.1% on 10 mg, 6.2% on 15 mg and 2.6% on placebo ^[1].

Why it matters for patients

These are group averages, and the confidence intervals published alongside them show that individual results ranged widely ^[1]. A person's own outcome could fall well above or below the average.

The trial enrolled only people with obesity or overweight who did not have diabetes, so the results do not describe how tirzepatide performs in people with type 2 diabetes ^[1].

Weight was measured while participants were still taking the drug. Whether weight comes back after treatment stops is not addressed in the trial report, and no data beyond 72 weeks are reported ^[1].

For anyone weighing treatment options, this trial sets a reference point for how much weight loss a weekly injected drug produced over 72 weeks in a phase 3 study ^[1]. The publication and the registry entry do not address whether the drug is approved for weight management, what it costs, or whether insurers cover it ^[1][2].

Gastrointestinal complaints were common but usually mild to moderate, and the discontinuation rates across the three doses ranged from 4.3% to 7.1%, meaning most participants stayed on treatment through the end of the trial ^[1].

No head-to-head comparison with any other weight-loss drug appears in the report ^[1]. Whether the average results hold up in broader patient groups, or how the drug compares with other options in routine practice, has not been reported.