SURPASS-2 Trial: A Plain-English Overview of What It Established

At a glance

Detail	Value
Participants	1,879 adults with type 2 diabetes on metformin
Intervention	Tirzepatide 5 mg, 10 mg, or 15 mg subcutaneous weekly
Comparator	Semaglutide 1 mg subcutaneous weekly
Duration	40 weeks
Primary endpoint	Change from baseline in A1C
Key result	All tirzepatide doses were superior to semaglutide 1 mg on A1C reduction (−2.01% to −2.30% vs −1.86%; p<0.001 for noninferiority and superiority)

What question did SURPASS-2 ask?

Before this trial, tirzepatide had only been tested against placebo or older diabetes drugs. Clinicians already had strong evidence that semaglutide 1 mg (marketed as Ozempic) was among the most effective GLP-1 receptor agonists for lowering blood sugar and body weight. The obvious next question: could a drug that targets two incretin receptors (GIP and GLP-1) actually beat a drug that targets one?

SURPASS-2 was designed to answer that directly. The primary publication describes a randomized, open-label, phase 3 trial with a noninferiority-and-superiority design. The investigators first tested whether tirzepatide was "at least as good" as semaglutide, and if that bar was cleared, they tested whether it was genuinely better.

Who was enrolled?

The trial recruited 1,879 adults across 128 sites in nine countries. Eligible participants had type 2 diabetes with an A1C between 7.0% and 10.5%, were already taking metformin at a stable dose of at least 1 to 500 mg/day, and had a BMI of 25 kg/m² or higher. The average baseline A1C was approximately 8.28%, and the average BMI was about 34.2 kg/m². Roughly 53% of participants were male. People using any injectable diabetes therapy, including insulin, were excluded.

This is an important detail. The study population was relatively early in its treatment course: on metformin alone, not yet needing insulin. That makes the results most directly applicable to a common clinical decision point, the moment when a clinician considers adding a second agent to metformin, a scenario described in the ADA Standards of Care.

What were participants given?

Participants were randomized 1:1:1:1 into four arms:

• Tirzepatide 5 mg weekly (n = 470)
• Tirzepatide 10 mg weekly (n = 469)
• Tirzepatide 15 mg weekly (n = 470)
• Semaglutide 1 mg weekly (n = 470)

Both drugs were titrated upward over the first several weeks to reduce gastrointestinal side effects. Tirzepatide started at 2.5 mg for four weeks, then increased by 2.5 mg every four weeks until the assigned dose was reached. Semaglutide followed its FDA-approved label titration: 0.25 mg for four weeks, 0.5 mg for four weeks, then 1 mg.

The trial was open-label, meaning both patients and investigators knew which drug was being given. This is a standard design choice when two injectable medications have different devices and titration schedules, but it does introduce the possibility of expectation bias, a limitation the authors acknowledged.

What was measured?

The primary endpoint was change in A1C from baseline to week 40. Key secondary endpoints included change in body weight, the proportion of participants reaching an A1C below 7.0% (a common clinical target), and the proportion reaching an A1C below 5.7% (the threshold for a reading in the normal, non-diabetic range).

The study used two estimand frameworks for analysis. The "treatment policy" estimand counted all randomized participants regardless of whether they stayed on the drug. The "efficacy" estimand focused on effects while participants were actually taking the medication. Both matter: the first reflects real-world effectiveness; the second isolates the drug's pharmacological effect.

What did they find?

A1C reductions

All three tirzepatide doses beat semaglutide 1 mg on the primary endpoint. Under the treatment-policy estimand:

Each comparison met criteria for both noninferiority and superiority (p<0.001 for all). The 10 mg and 15 mg doses drove roughly twice the incremental benefit over semaglutide as the 5 mg dose did.

Target achievement

The proportion of participants who reached an A1C below 7.0% was 82% (5 mg), 86% (10 mg), and 86% (15 mg) versus 79% for semaglutide. More strikingly, 27% to 46% of tirzepatide-treated participants achieved an A1C below 5.7%, compared with 19% of those on semaglutide. Getting nearly half of a type 2 diabetes cohort into the normal glycemic range was unprecedented in a phase 3 trial at that time.

Weight loss

Body weight reductions followed a clear dose-response pattern for tirzepatide:

The 10 mg and 15 mg tirzepatide arms produced statistically significantly more weight loss than semaglutide 1 mg. The 5 mg arm showed a numerically larger reduction but did not meet the threshold for statistical significance for superiority on weight.

Safety and side effects

Gastrointestinal adverse events were the most common issue in every arm. Nausea affected 17% to 22% of tirzepatide patients versus 18% of semaglutide patients. Diarrhea was reported in 13% to 16% versus 12%. Vomiting occurred in 6% to 10% versus 8%. Most GI events were mild to moderate and clustered during dose escalation.

Hypoglycemia was infrequent across all arms, consistent with the mechanisms of both drugs, which stimulate insulin secretion only when blood glucose is elevated. Clinically significant hypoglycemia (blood glucose <54 mg/dL) occurred in fewer than 1% of participants in any group.

Five participants on tirzepatide and one on semaglutide had adjudicated acute pancreatitis events, though the overall incidence was low and consistent with background rates in type 2 diabetes populations. No new safety signals emerged beyond what had been seen in earlier SURPASS trials.

Limitations worth knowing

Several caveats temper how broadly these results should be applied.

Open-label design. Participants knew their treatment assignment. This can influence subjective symptom reporting and even adherence behavior. A double-blind design would have been more rigorous, though practically difficult given the different injection devices.

No semaglutide 2 mg arm. The trial compared tirzepatide to semaglutide 1 mg, the highest approved dose at the time. Novo Nordisk subsequently secured approval for semaglutide 2 mg (Ozempic), which produces greater A1C and weight reductions than the 1 mg dose. A head-to-head comparison at higher semaglutide doses would be more informative, and no such trial has been completed.

40-week duration. Diabetes is a lifelong condition. Forty weeks is long enough to capture near-maximal glycemic effects but too short to evaluate cardiovascular outcomes, durability of weight loss, or long-term safety differences.

Population restrictions. The trial excluded patients on insulin, those with A1C above 10.5%, and those with significant renal impairment. Results may not generalize to more advanced diabetes.

Sponsor involvement. Eli Lilly, the manufacturer of tirzepatide, funded the trial and participated in data collection and analysis, which is standard for phase 3 regulatory studies but worth noting. The full trial data are registered at ClinicalTrials.gov (NCT03987919).

What does this mean for clinical practice?

SURPASS-2 established that tirzepatide, later approved as Mounjaro, can lower A1C and body weight more effectively than semaglutide 1 mg in adults with type 2 diabetes already taking metformin. This result contributed directly to the FDA's approval of tirzepatide for type 2 diabetes in May 2022.

For clinicians, the practical takeaway is dose-dependent. Tirzepatide 5 mg outperformed semaglutide 1 mg on A1C but not convincingly on weight. The 10 mg and 15 mg doses showed clear advantages on both endpoints. Tolerability profiles were broadly similar, though the higher tirzepatide doses carried somewhat more GI burden during titration.

The trial does not answer whether tirzepatide is better than higher-dose semaglutide (2 mg or the 2.4 mg used in the obesity indication). It also does not address cardiovascular outcomes, which require separate dedicated trials. The SURPASS-CVOT trial was designed to fill that gap.

Cost, insurance coverage, and formulary access remain major factors in choosing between these medications. A drug's clinical superiority in a trial does not guarantee it will be the right choice for every patient when real-world prescribing constraints are factored in, a point emphasized in recent ADA consensus recommendations.

Frequently asked questions

›What is the SURPASS-2 trial?

SURPASS-2 is a phase 3 randomized clinical trial that compared three doses of tirzepatide (5, 10, and 15 mg weekly) against semaglutide 1 mg weekly in 1,879 adults with type 2 diabetes who were already on metformin. It ran for 40 weeks and was published in the NEJM in 2021.

›Did tirzepatide beat semaglutide in SURPASS-2?

Yes. All three tirzepatide doses produced statistically superior A1C reductions compared to semaglutide 1 mg. The 10 mg and 15 mg doses also produced significantly greater weight loss.

›How much weight did patients lose in SURPASS-2?

Average weight loss ranged from 7.6 kg (tirzepatide 5 mg) to 11.2 kg (tirzepatide 15 mg), compared with 5.7 kg for semaglutide 1 mg, over 40 weeks.

›Was SURPASS-2 a double-blind trial?

No. It was an open-label trial, meaning participants and investigators knew which medication was being administered. This is a recognized limitation, though it is common in trials comparing injectable medications with different devices.

›Why wasn't semaglutide 2 mg included in SURPASS-2?

At the time the trial was designed and conducted, semaglutide 1 mg was the highest approved dose. The 2 mg dose received FDA approval later, so a direct comparison at that dose was not available.

›What side effects were most common in SURPASS-2?

Nausea, diarrhea, and vomiting were the most frequently reported adverse events across all treatment arms. Most GI symptoms were mild to moderate and occurred primarily during the dose-escalation phase.

›Does SURPASS-2 prove tirzepatide is better for the heart than semaglutide?

No. SURPASS-2 was not designed to measure cardiovascular outcomes. A separate cardiovascular outcomes trial, SURPASS-CVOT, was conducted to assess that question.

›Who should consider tirzepatide based on SURPASS-2 results?

The trial results are most directly relevant to adults with type 2 diabetes on metformin who need a second agent. Treatment decisions should factor in individual glycemic targets, weight goals, tolerability, cost, and insurance coverage, as recommended by the ADA Standards of Care.

›What is the difference between tirzepatide and semaglutide?

Semaglutide activates the GLP-1 receptor only. Tirzepatide activates both the GIP and GLP-1 receptors. This dual mechanism is thought to explain its greater effects on blood sugar and weight, though the relative contribution of GIP agonism is still under investigation.

›Is Mounjaro the same as tirzepatide?

Yes. Mounjaro is the brand name for tirzepatide when prescribed for type 2 diabetes. The same molecule is also approved under the brand name Zepbound for chronic weight management.

References

1. Frías JP, Davies MJ, Rosenstock J, et al. Tirzepatide versus semaglutide once weekly in patients with type 2 diabetes. N Engl J Med. 2021;385(6):503-515. PubMed
2. Ozempic (semaglutide) prescribing information. FDA. Label
3. Mounjaro (tirzepatide) prescribing information. FDA. Label
4. Nicholls SJ, Bhatt DL, Buse JB, et al. Comparison of tirzepatide and dulaglutide on major adverse cardiovascular events in participants with type 2 diabetes: SURPASS-CVOT design and baseline characteristics. Am Heart J. 2024;267:1-11. PubMed
5. ElSayed NA, Aleppo G, Aroda VR, et al. Pharmacologic approaches to glycemic treatment: Standards of Care in Diabetes, 2023. Diabetes Care. 2023;46(Suppl 1):S140-S157. PubMed
6. ClinicalTrials.gov. A study of tirzepatide (LY3298176) versus semaglutide once weekly as add-on therapy to metformin in participants with type 2 diabetes (SURPASS-2). NCT03987919