Zepbound Clinical Trial Results: What SURMOUNT-1 Through 5 Tell Us

When Eli Lilly's tirzepatide received FDA approval for chronic weight management in November 2023, the brand name Zepbound entered a landscape that semaglutide had largely defined. The molecule was not new — tirzepatide had been approved for type 2 diabetes as Mounjaro in May 2022 — but the dedicated obesity indication required its own evidence base, and Lilly's SURMOUNT trial program is the document that built it.

Five trials run, five published, a body of data that has shifted the clinical conversation about what weight loss pharmacology can deliver. Some of the findings are the headline numbers everyone has heard. Some are quieter, and arguably more clinically useful.

SURMOUNT-1: the trial that defined the category

SURMOUNT-1, published in the New England Journal of Medicine in July 2022 with Ania Jastreboff of Yale as first author, randomised 2,539 adults with obesity (BMI ≥30, or ≥27 with comorbidities) without type 2 diabetes to tirzepatide 5mg, 10mg, 15mg, or placebo for 72 weeks. The primary endpoints were percent change in body weight and the proportion achieving at least 5% weight loss.

The results were unprecedented in pharmacological weight loss research. Mean weight loss at 72 weeks was 15.0% on the 5mg dose, 19.5% on 10mg, and 20.9% on 15mg, compared with 3.1% on placebo. The proportion achieving at least 5% loss exceeded 85% across all tirzepatide doses. The proportion achieving at least 20% loss reached 57% on the highest dose — a figure previously associated only with bariatric surgery outcomes.

Beyond the headline numbers, SURMOUNT-1 documented improvements in waist circumference, blood pressure, fasting insulin, lipids, and the SF-36 physical functioning score. Gastrointestinal adverse events — nausea, diarrhoea, constipation, vomiting — were the most common side effects, predominantly mild to moderate, and clustered during the dose-escalation period. Discontinuation due to adverse events ranged from 4.3% to 7.1% across tirzepatide arms versus 2.6% for placebo.

What the response distribution actually looked like

The 20.9% mean conceals considerable individual variation. Roughly a third of patients on the highest dose lost more than 25% of body weight. A smaller proportion lost less than 5%. The distribution matters for clinical expectation-setting — the average is a useful number, but the individual response range is what patients actually live with.

SURMOUNT-2: tirzepatide in patients with type 2 diabetes

SURMOUNT-2, published in The Lancet in July 2023 with Garvey as first author, addressed an important clinical question: does tirzepatide produce similar weight loss in patients who also have type 2 diabetes? The diabetes population has historically responded less robustly to weight loss interventions, partly due to insulin therapy, partly due to the metabolic milieu of the disease.

SURMOUNT-2 enrolled 938 adults with obesity and type 2 diabetes, randomised to tirzepatide 10mg, 15mg, or placebo for 72 weeks. Mean weight loss was 12.8% on 10mg and 14.7% on 15mg, versus 3.2% on placebo. The figures are lower than SURMOUNT-1's, but the proportion achieving at least 5% loss still reached 79–83%. HbA1c reductions of 2.0–2.1% accompanied the weight loss.

The clinical translation is meaningful. Patients with type 2 diabetes and obesity — a population that often finds previous weight loss tools poorly effective — can expect roughly two-thirds to three-quarters of the weight loss response observed in the non-diabetes population.

SURMOUNT-3: when intensive lifestyle intervention isn't enough

SURMOUNT-3, published in Nature Medicine in October 2023, enrolled a population the previous trials had excluded: adults who had completed a 12-week intensive lifestyle intervention and had achieved at least 5% weight loss, then were randomised to tirzepatide or placebo for an additional 72 weeks.

The question was whether tirzepatide could extend weight loss beyond what behavioural intervention had already produced. The answer was a clear yes. Patients on tirzepatide lost an additional 18.4% of body weight beyond their lifestyle-phase loss, for a total weight loss of approximately 26.6%. Placebo patients regained on average 2.5% during the same window.

SURMOUNT-3 is the trial that most directly speaks to clinical reality. Most patients arriving at a weight loss medication consultation have already attempted lifestyle interventions, often repeatedly. The trial demonstrates that tirzepatide is not simply a substitute for lifestyle change — it adds substantial additional benefit on top of it.

SURMOUNT-4: the discontinuation question

SURMOUNT-4, published in JAMA in December 2023 with Louis Aronne of Weill Cornell as first author, addressed the most controversial question surrounding GLP-1 obesity medications: what happens when treatment stops?

The protocol enrolled 670 adults who completed 36 weeks of open-label tirzepatide (achieving a mean weight loss of 20.9%), then randomised them to either continue tirzepatide or switch to placebo for another 52 weeks. The continuation arm lost an additional 5.5% during the second phase, ending at a mean total loss of 25.3%. The placebo arm regained 14.0% during the same period — losing roughly half of their initial achievement.

The interpretation in the medical community settled relatively quickly. Tirzepatide treatment for obesity behaves like treatment for any other chronic disease: it works while taken, and the underlying pathophysiology reasserts itself when it stops. The framing has shifted accordingly from "weight loss drug" to "chronic disease management."

SURMOUNT-5: head-to-head with semaglutide

SURMOUNT-5, published in the New England Journal of Medicine in May 2025, was the trial the field had been waiting for: a direct head-to-head comparison of tirzepatide and semaglutide for obesity in patients without diabetes. The two drugs had been compared informally by reading SURMOUNT-1 against STEP 1, but methodological differences in those trials made the comparison imprecise.

SURMOUNT-5 enrolled 751 adults with obesity, randomised to maximum tolerated dose of tirzepatide (up to 15mg weekly) or semaglutide (up to 2.4mg weekly) for 72 weeks. The results favoured tirzepatide: mean weight loss of 20.2% versus 13.7%. The proportion achieving at least 25% weight loss was 31.6% on tirzepatide versus 16.1% on semaglutide.

The clinical interpretation is nuanced. The drugs are not interchangeable, but neither is one universally superior. Tirzepatide produces greater average weight loss; semaglutide has a longer real-world clinical track record and broader cardiovascular outcomes data. Cost, insurance coverage, side-effect tolerance, and individual response all shape the decision in practice — a comparison covered in more detail in our piece on tirzepatide versus semaglutide.

What "additive to lifestyle" actually means in clinical practice

SURMOUNT-3's design is worth dwelling on briefly. The patients who entered the randomisation phase had already demonstrated they could respond to behavioural intervention — they had lost at least 5% of body weight during the 12-week lifestyle lead-in. They were, in effect, the "responders" to behavioural change. The question being asked was whether pharmacological treatment would do anything for people who had already proven capable of behavioural success. The answer was that it added almost four times what behaviour alone had produced.

This matters for the recurring clinical conversation in which patients are told to "try lifestyle first" before being considered for pharmacotherapy. SURMOUNT-3's data suggests that lifestyle success and pharmacological success are not mutually exclusive or even competing — they are additive, and the patients who do well on one do well on the combination.

Adverse events across the program

The SURMOUNT trials report a consistent adverse event profile dominated by gastrointestinal symptoms. Nausea affects roughly 25–30% of patients on tirzepatide, with most episodes occurring during dose escalation. Diarrhoea, constipation, and vomiting are also common. Serious adverse events were rare and not significantly different from placebo across trials.

Concerns about pancreatitis, gallbladder disease, and thyroid C-cell tumours have received considerable attention. The trial data and post-marketing surveillance to date have not established increased risk for pancreatitis or thyroid cancer in humans, though the medullary thyroid cancer warning persists based on rodent studies. Gallbladder events are modestly elevated, consistent with what would be expected from rapid weight loss in general.

What the SURMOUNT trials didn't measure

It is worth being clear about the gaps. The SURMOUNT program established efficacy and short-to-medium-term safety but does not yet contain the multi-year cardiovascular outcomes data that semaglutide accumulated through the SELECT trial. The SURPASS-CVOT trial for tirzepatide is ongoing and is expected to clarify cardiovascular benefit in 2027. Until that readout, the cardiovascular case for tirzepatide rests on weight-loss-mediated risk factor improvement rather than direct outcome data.

The trials also enrolled populations that skewed younger and healthier than the broader patient base that has since received tirzepatide commercially. Mean age in SURMOUNT-1 was 44.9 years, and patients with significant comorbidities, recent bariatric surgery, or active eating disorders were excluded. Real-world effectiveness in older, sicker, more medically complex populations is being established through post-marketing surveillance rather than the trial program itself.

Putting the SURMOUNT program into clinical context

The cumulative evidence places tirzepatide as the most efficacious pharmacological option currently available for chronic weight management. SURMOUNT-1 established the headline efficacy. SURMOUNT-2 extended it to diabetes. SURMOUNT-3 demonstrated additive benefit beyond lifestyle intervention. SURMOUNT-4 clarified that discontinuation produces substantial regain. SURMOUNT-5 provided the head-to-head data that informed prescribing comparisons.

For patients evaluating treatment, our overview of what GLP-1 medications do and how tirzepatide works specifically are useful starting points. For the practical question of brand naming and access, see how Mounjaro and Zepbound relate.

Key takeaways

• SURMOUNT-1 (Jastreboff, NEJM 2022): 20.9% mean weight loss at 72 weeks on tirzepatide 15mg in patients without diabetes; 57% achieved at least 20% loss.
• SURMOUNT-2 (Garvey, Lancet 2023): 14.7% mean weight loss in patients with obesity and type 2 diabetes; HbA1c reduction of 2.1%.
• SURMOUNT-3 (Wadden, Nature Medicine 2023): tirzepatide produced an additional 18.4% loss beyond what intensive lifestyle intervention had achieved.
• SURMOUNT-4 (Aronne, JAMA 2023): discontinuation produced regain of 14% over 52 weeks, supporting the chronic disease management framing.
• SURMOUNT-5 (Aronne, NEJM 2025): tirzepatide 20.2% versus semaglutide 13.7% in the first head-to-head obesity trial.
• Adverse events are predominantly gastrointestinal and cluster during dose escalation; serious adverse events were rare across the program.