Tirzepatide Clinical Trials: What SURPASS and SURMOUNT Actually Measured

A drug development programme is not a single study. It is a sequence of trials, each designed to answer a narrower question than the last, and the meaning of any one result depends on which question that trial was built to answer. Tirzepatide's programme splits cleanly in two. The SURPASS trials asked whether the drug lowers blood glucose in people with type 2 diabetes, and how it compares with the standard treatments already used for that. The SURMOUNT trials asked whether it reduces body weight in people with obesity, and later whether it improves specific weight-related conditions.

A third question, the one regulators care about most for any long-term metabolic drug, is whether the medicine changes the rate of serious cardiovascular events such as heart attack, stroke and cardiovascular death. That question requires a much larger and longer trial, because those events are relatively rare and you need many patient-years of follow-up to count enough of them to compare groups reliably. SURPASS-CVOT was built for that purpose.

Origen supplies research material, not medicines, and nothing in this article describes a dosing protocol or a therapeutic recommendation. It summarises published trial evidence for research context only.

HUMAN — PHASE 3 RANDOMISED TRIALS

The SURPASS trials all enrolled adults with type 2 diabetes and all used HbA1c change as the primary endpoint. HbA1c reflects the proportion of haemoglobin in the blood that has sugar attached to it, which accumulates over the lifespan of a red blood cell, so a single HbA1c measurement summarises roughly the previous two to three months of glucose control rather than a single moment. A reduction of one percentage point is generally considered a clinically meaningful change in diabetes care.

What distinguishes the SURPASS trials from one another is mainly the comparator, the treatment tirzepatide was measured against, and the background therapy participants were already taking. Comparing a drug against placebo answers a different question from comparing it against the best existing alternative, and the programme deliberately did both.

In SURPASS-1, which tested tirzepatide against placebo in people not taking other glucose-lowering drugs, HbA1c fell by 1.91, 1.93 and 2.11 percentage points across the three doses. Those are large reductions by the standards of diabetes trials, but the comparison is against placebo, which is the least demanding comparator available. [1]

The more informative comparisons came from the trials that used active comparators. SURPASS-3 measured tirzepatide against insulin degludec and SURPASS-4 against insulin glargine, both long-acting insulins that represent established treatment when oral medicines are no longer sufficient. SURPASS-4 is also notable for enrolling a population selected for increased cardiovascular risk, which makes its safety data more relevant to the patients clinicians worry about most. [3][4]

SURPASS-2 deserves the closest reading of the five, because it is the one trial in the diabetes programme where tirzepatide was compared directly against another modern once-weekly injectable rather than against insulin or placebo. It was open-label, meaning participants and investigators knew which drug was being given. Open-label design is a genuine limitation for subjective outcomes, though HbA1c is measured in a laboratory and is therefore far less vulnerable to expectation than, for example, a symptom questionnaire. [6]

The direct comparison against semaglutide is examined in more detail in Tirzepatide vs semaglutide.

HUMAN — PHASE 3 RANDOMISED TRIALS

The SURMOUNT trials moved the question from blood glucose to body weight. Their primary endpoint was percent change in body weight, which is reported rather than absolute kilograms because a 10 kg loss means something very different for a 90 kg person than for a 140 kg person.

SURMOUNT-3 and SURMOUNT-4 are the two most conceptually interesting trials in the set, because they were designed to answer questions that a simple placebo comparison cannot. SURMOUNT-3 first put everyone through twelve weeks of intensive lifestyle intervention and only then randomised them, so its result describes what the drug adds on top of a serious diet and activity programme rather than on top of nothing. [9]

SURMOUNT-4 used a withdrawal design. All participants received tirzepatide during an open lead-in period, and only those who completed it were then randomised either to continue the drug or to switch to placebo. Those who continued lost a further 5.5% on average, while those switched to placebo regained 14.0%. That contrast is the clearest published evidence that the weight change observed in these trials depends on continued treatment rather than persisting after it stops. [10]

SURMOUNT-OSA extended the programme beyond weight itself to a specific weight-related condition, obstructive sleep apnoea, measured by the apnoea-hypopnoea index, which counts the number of breathing interruptions per hour of sleep. It is the trial that supported the third of tirzepatide's regulatory approvals. [11]

HUMAN — PHASE 3 CARDIOVASCULAR OUTCOMES TRIAL

SURPASS-CVOT, published in the New England Journal of Medicine in December 2025, randomised 13,165 participants with type 2 diabetes and atherosclerotic cardiovascular disease to tirzepatide or dulaglutide, a GLP-1 receptor agonist already shown to reduce cardiovascular events. The hazard ratio for the primary composite outcome was 0.92. The trial met its pre-specified non-inferiority criterion but did not demonstrate superiority. [13]

That distinction matters and is frequently reported carelessly. Non-inferiority means the evidence is consistent with tirzepatide being no worse than the active comparator by more than a pre-agreed margin. Superiority would mean the evidence shows it is actually better. A hazard ratio of 0.92 points in a favourable direction, but the trial's own statistical framework did not license the claim that tirzepatide reduces cardiovascular events more than dulaglutide does.

A cardiovascular outcomes trial against an active comparator is a harder test than one against placebo, because the comparator itself has an established benefit. Results from the two designs are not interchangeable.

The tirzepatide programme is unusually complete by the standards of metabolic peptides, but completeness is not the same as certainty, and several caveats apply across the whole set.

• Two of the most cited comparative trials, SURPASS-2 and SURMOUNT-5, were open-label. Blinding was not feasible given different injection devices and titration schedules, but its absence is a real limitation.
• Most trials ran between 40 and 72 weeks. Obesity and type 2 diabetes are lifelong conditions, and trials of this length cannot describe outcomes over decades.
• Trial populations are selected by eligibility criteria and may not represent everyone who would use the drug in practice, including people with significant comorbidities excluded from enrolment.
• Nearly all of these trials were funded and conducted by the manufacturer. That is normal for phase 3 development and does not invalidate the results, but independent replication remains limited.
• Gastrointestinal adverse events, most commonly nausea, vomiting and diarrhoea, were the most frequently reported side effects across the programme and were generally dose-related.

Tirzepatide holds three official approvals from the United States Food and Drug Administration: as Mounjaro for type 2 diabetes in May 2022, as Zepbound for chronic weight management in November 2023, and as Zepbound for obstructive sleep apnoea in adults with obesity in December 2024. Claims of any further approved indication should be checked against the current regulatory record before being repeated.

For the receptor biology behind these results, see How does tirzepatide work?.