SURMOUNT-1: how much weight tirzepatide took off in 72 weeks

SURMOUNT-1 gave 2,539 adults with obesity weekly tirzepatide or placebo for 72 weeks. Average weight loss was 15.0% on 5 mg, 19.5% on 10 mg and 20.9% on 15 mg, against 3.1% on placebo [1]. Side effects were mostly gastrointestinal and front-loaded, and between 4% and 7% of people on the drug stopped because of them [1].

Why this study matters

Tirzepatide acts on two gut-hormone receptors, GIP and GLP-1, where semaglutide acts on one. SURMOUNT-1 tested it as an obesity treatment in people without diabetes at a point when its effects in that group were unknown [1], and it's one of the trials behind the FDA's approval of tirzepatide for chronic weight management as Zepbound in November 2023 [3].

The “20% weight loss” figure you'll see everywhere comes from this paper. It's real, but it belongs to the highest dose, and the paper reports it two different ways.

What they did

Everyone started on 2.5 mg, with the dose rising by 2.5 mg every four weeks until they reached their assigned dose, so the 15 mg group took 20 weeks to get there [1]. All groups had counselling aimed at a 500-calorie daily deficit and at least 150 minutes of activity a week.

The average participant was 44.9 years old, weighed 104.8 kg and had a BMI of 38.0. Two-thirds (67.5%) were women, 70.6% were White, and 40.6% had prediabetes [1]. Only 5.5% had a BMI in the 27 to 30 range [1], so this is really a trial of people with obesity, not overweight.

What they found

The main analysis counted every randomised participant, whether or not they kept taking their injections (the “treatment-regimen estimand”). A second analysis estimated what would happen if everyone took the drug as intended. Both are in the paper; the first is the more conservative [1].

Compared with placebo, the difference was 11.9, 16.4 and 17.8 percentage points for the three doses (p below 0.001 for each) [1]. In kilograms, the as-intended estimate at 15 mg works out to 23.6 kg [1].

Waist size, blood pressure, fasting insulin and lipids all improved more on tirzepatide [1]. Of the participants with prediabetes, 95.3% on tirzepatide had normal blood sugar at 72 weeks, against 61.9% on placebo [1].

A subgroup had DXA scans, and 160 of the 255 had usable scans at both ends of the trial. In them, fat mass fell 33.9% on tirzepatide against 8.2% on placebo, and the ratio of fat to lean mass dropped from 0.93 to 0.70 [1]. That suggests most of the weight lost was fat, though lean mass fell too.

Side effects

Between 78.9% and 81.8% of people on tirzepatide reported at least one adverse event, compared with 72.0% on placebo [1]. Nausea, diarrhoea and constipation led the list; the authors describe them as mostly mild to moderate and concentrated in the dose-escalation period.

Stopping because of side effects happened in 4.3%, 7.1% and 6.2% of the 5, 10 and 15 mg groups, and 2.6% on placebo [1]. Serious adverse events hit 6.3% of participants overall and were similar across groups, with roughly a fifth of them related to Covid-19, which ran through the trial period [1]. There were 11 deaths, spread across all four arms including four on placebo. Four adjudicated cases of pancreatitis were spread evenly across the groups, including placebo. Cholecystitis was more frequent on tirzepatide, but at 0.6% or less in any group [1].

Caveats

• Sponsor-run. Eli Lilly, which makes tirzepatide, designed the trial, monitored the sites and did the data analysis [1].
• Placebo dropouts. 86.0% of participants finished the 72 weeks, but completion was 88.4% to 89.8% on tirzepatide against 77.0% on placebo [1]. The statistical methods account for this, but it's a reminder that people who weren't losing weight tended to leave.
• Motivated volunteers with fairly healthy numbers. The authors note participants may have been more committed to weight management than most people with obesity, and that baseline blood pressure and lipids were relatively normal, which leaves less room to show improvement [1].
• No hard outcomes. Weight, waist and blood markers are not heart attacks, strokes or deaths. SURMOUNT-1 wasn't built to measure those.

The three-year follow-up

Participants who had prediabetes at the start (1,032 people) stayed on their assigned treatment for 176 weeks, a little over three years [2]. Weight loss held up: −12.3%, −18.7% and −19.7% on the three doses, against −1.3% on placebo. Type 2 diabetes was diagnosed in 1.3% of the tirzepatide groups and 13.3% of the placebo group (hazard ratio 0.07) [2]. After 17 weeks off treatment, those figures became 2.4% and 13.7%, so some of the protection started to fade once the drug stopped [2]. Lilly funded this analysis as well.

What is still open

• Whether the weight loss reduces heart attacks, strokes and deaths in people without diabetes; SURMOUNT-1 was not built to measure those, so that answer has to come from a dedicated outcomes trial.
• How weight behaves after stopping over the longer term, since the 17-week off-drug window in the three-year analysis covered only people with prediabetes.
• How it performs in people with a BMI between 27 and 30, who were barely represented.
• The long-term effect on muscle and bone, given DXA data from 160 people.

What this means

For someone carrying obesity and prediabetes, the three-year data moves tirzepatide from a weight-loss drug to something closer to preventive medicine. SURMOUNT-1 took 20.9% off at the top dose [1]; the follow-up then showed type 2 diabetes appearing in 1.3% of tirzepatide participants against 13.3% on placebo across 176 weeks [2]. That diabetes signal, not the headline weight number, is the strongest reason to treat. Our position is that for a person with obesity and prediabetes, tirzepatide belongs ahead of watch-and-wait, and the weight loss is the mechanism rather than the goal.

The 20.9% figure belongs to the 15 mg dose and to the as-intended analysis, which reached 22.5%; the 5 mg group — where many people actually settle — lost 15.0% [1]. We would set expectations at the dose a person reaches, not the dose on the label. Side effects follow the GLP-1 pattern: nausea, diarrhoea and constipation, concentrated in the escalation weeks, with 4.3% to 7.1% stopping because of them [1]. That is a titration and counselling problem. A patient who quits at week six because nobody explained the first month was failed by the plan, not by the drug.

The three-year analysis is blunt about the alternative to staying on treatment: after 17 weeks off, the diabetes rates in the two groups nearly converged, 2.4% against 13.7% [2]. Obesity behaves like a chronic condition here, and the honest frame for a patient is long-term therapy, not a fixed course that ends at a target weight. The missing outcomes trial does not change that case, and it should not be used as a reason to wait. Tirzepatide cleared the FDA for chronic weight management as Zepbound in November 2023 [3], an approval built on weight change rather than on heart attacks and strokes, so the mortality question is an addendum to a decision a patient can already make. Where a single fact decides it, we would rather see someone treated for a diabetes risk we can measure today than held back for a mortality answer that has not arrived.

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