Tirzepatide · 3 min read
SURMOUNT-1 through -5 in plain language — the 20.9% headline, the diabetes gap, the maintenance-withdrawal experiment, and the head-to-head that ended the semaglutide debate.
SURMOUNT-1 through -5 in plain language — the 20.9% headline, the diabetes gap, the maintenance-withdrawal experiment, and the head-to-head that ended the semaglutide debate.
Tirzepatide's obesity evidence base is a five-trial program called SURMOUNT, and each trial answered a different question. Read together, they explain both the enthusiasm and the fine print.
The foundation trial (NEJM, 2022): 2,539 adults with obesity or overweight plus a comorbidity, without diabetes, randomized to 5, 10, or 15 mg weekly or placebo for 72 weeks. Average body-weight reductions: 15.0%, 19.5%, and 20.9% against 3.1% for placebo — and 91% of the 15 mg arm lost at least 5%, with over a third losing 25% or more. Those are bariatric-surgery-adjacent numbers from a weekly injection, and they created the modern market. A three-year extension in participants with prediabetes later showed a 94% reduction in progression to type 2 diabetes.
Same design in type 2 diabetes (Lancet, 2023): average losses of 12.8% and 14.7% at 10 and 15 mg. Meaningfully lower than SURMOUNT-1 — weight loss is consistently harder with diabetes, across every drug in the class — and still the strongest diabetes-population result on record at the time.
SURMOUNT-3 asked whether tirzepatide adds to intensive lifestyle intervention: after a 12-week diet-and-exercise lead-in, participants randomized to tirzepatide lost an additional 18.4% on average while the placebo group regained. SURMOUNT-4 asked the harder question — what happens when you stop. Everyone took tirzepatide for 36 weeks (average loss 20.9%), then half were switched to placebo: over the following year the placebo group regained about 14% of body weight while those continuing lost a further 5.5%. That single trial is the honest core of every "is this forever?" conversation, covered in depth in stopping GLP-1s and weight regain.
The comparison everyone wanted (NEJM, 2025): tirzepatide versus semaglutide 2.4 mg, open-label, 751 adults, 72 weeks. Tirzepatide 20.2%, semaglutide 13.7% — with higher proportions of tirzepatide patients at every loss threshold. It converted a cross-trial inference into a direct result, and it is the single most load-bearing fact in our tirzepatide-vs-semaglutide analysis.
SURMOUNT was a weight program, not an outcomes program. Cardiovascular outcome evidence arrived separately: SURPASS-CVOT (2025) showed tirzepatide's cardiovascular safety and benefit against an active comparator in type 2 diabetes, while semaglutide's SELECT trial holds the non-diabetic outcomes result for the class. And every SURMOUNT number describes the FDA-approved product at trial doses with trial support — a benchmark compounded programs borrow but did not earn. Trial averages also hide a wide response distribution: some participants lose far more, a meaningful minority far less.
In SURMOUNT-1, averages were 15.0% (5 mg), 19.5% (10 mg), and 20.9% (15 mg) over 72 weeks in people without diabetes; 12.8–14.7% in type 2 diabetes (SURMOUNT-2); and 20.2% versus semaglutide's 13.7% head-to-head (SURMOUNT-5).
SURMOUNT-4 measured it directly: participants switched to placebo after 36 weeks regained about 14% of body weight over the next year, while those continuing lost a further 5.5%.
In the SURMOUNT-1 three-year extension among participants with prediabetes, tirzepatide reduced progression to type 2 diabetes by about 94% versus placebo while on treatment.