Tirzepatide for Type 2 Diabetes: The SURPASS Trials

Why the SURPASS Trials Matter

The SURPASS program is the Phase 3 clinical trial series that established tirzepatide as a treatment for Type 2 diabetes. Five trials, each with a different design and comparator, tested the compound across a wide range of patient populations and treatment backgrounds. Together, they enrolled more than 6,000 participants and generated the evidence base that led to FDA approval of Mounjaro in May 2022.

What makes the program worth examining closely is not just the size of the HbA1c reductions, which are the largest ever reported for an injectable incretin therapy. It is the consistency. Tirzepatide outperformed its comparator in every trial, at every dose, on both the primary glycemic endpoint and the secondary weight endpoint. That kind of sweep does not happen often in diabetes research.

SURPASS-1: Monotherapy Against Placebo

SURPASS-1 was the simplest design: tirzepatide alone, without background medication, versus placebo. This is the cleanest way to isolate the drug’s effect. Participants had a mean baseline HbA1c of 7.9% and had not been previously treated with injectable diabetes medications.

At 40 weeks, tirzepatide at 5mg, 10mg, and 15mg reduced HbA1c by 1.87%, 1.89%, and 2.07%, respectively. Placebo reduced it by 0.04%. The percentage of participants reaching HbA1c below 7% was 87%, 92%, and 88% across the three dose arms. Placebo achieved 19%.

The weight loss numbers are notable for a diabetes trial. Participants lost 7.0 kg to 9.5 kg, compared to 0.7 kg on placebo. Most diabetes drugs are weight-neutral or cause weight gain. Tirzepatide’s dual metabolic effect was apparent from the very first trial.

SURPASS-2: The Head-to-Head Against Semaglutide

This is the trial that drew the most attention. SURPASS-2 is the only completed study that directly compared tirzepatide against semaglutide, the most successful GLP-1 agonist on the market. Both drugs were given to adults with Type 2 diabetes already on metformin, for 40 weeks.

All three tirzepatide doses beat semaglutide 1mg on both HbA1c reduction and weight loss. The differences were statistically significant for every comparison.

In SURPASS-2, even the lowest tirzepatide dose produced greater HbA1c reduction than semaglutide at the highest tested dose. The separation widened at 10mg and 15mg.

SURPASS-3: Against Insulin Degludec

SURPASS-3 moved the comparison to a different drug class entirely: basal insulin. Participants were adults with Type 2 diabetes on metformin, with or without an SGLT2 inhibitor, who were randomized to tirzepatide or insulin degludec (a long-acting insulin analog). The trial ran for 52 weeks.

Tirzepatide at 15mg reduced HbA1c by 2.37%, compared to 1.34% for insulin degludec. More striking was the weight differential: participants on tirzepatide 15mg lost 12.9 kg, while those on insulin gained 2.3 kg. That is a 15 kg difference in body weight over one year.

The insulin comparison is clinically significant. For decades, basal insulin has been the standard escalation step when oral medications and first-line injectables fail to achieve glycemic targets. SURPASS-3 suggests that tirzepatide could serve as a more effective alternative at that decision point, with the added benefit of weight reduction rather than weight gain.

SURPASS-4: High Cardiovascular Risk Population

SURPASS-4 enrolled the most medically complex patients in the program. All 2,002 participants had Type 2 diabetes and established atherosclerotic cardiovascular disease, or were at high risk for it. They were on one to three oral diabetes medications and were randomized to tirzepatide or insulin glargine for 52 weeks.

This trial produced the largest HbA1c reduction in the program. Tirzepatide at 15mg reduced HbA1c by 2.58%, compared to 1.44% for insulin glargine. Weight loss was 11.7 kg versus a gain of 1.9 kg with insulin.

SURPASS-5: Add-on to Insulin Glargine

SURPASS-5 addressed a different clinical scenario: patients who were already on basal insulin but still not reaching their glycemic targets. All 475 participants were receiving insulin glargine, and tirzepatide or placebo was added on top of it.

Tirzepatide 15mg added to insulin glargine reduced HbA1c by 2.40%, compared to 0.93% for placebo added to insulin. The percentage reaching HbA1c below 7% was 97% for tirzepatide 15mg versus 34% for placebo. Weight loss was 10.5 kg versus a gain of 0.8 kg.

That 97% target attainment figure is extraordinary. In most diabetes trials, hitting 60% or 70% is considered a strong result. Reaching nearly universal target attainment suggests that for patients already on insulin, adding tirzepatide addresses the residual glycemic gap that insulin alone leaves behind.

Side Effects Across the Program

The adverse event profile was consistent across all five SURPASS trials. Gastrointestinal symptoms dominated, as expected with incretin-based therapies. Nausea, diarrhea, and decreased appetite were the most frequently reported events.

The incidence of hypoglycemia was low when tirzepatide was used without insulin or sulfonylureas. In SURPASS-5, where all participants were on background insulin, hypoglycemia rates were higher but still manageable with dose adjustment. No episodes of severe hypoglycemia were reported at rates that differed meaningfully between tirzepatide and comparator arms.

What the Data Means for the Diabetes Landscape

The SURPASS program established several things simultaneously. Tirzepatide is the most effective injectable incretin therapy tested for HbA1c reduction. It produces weight loss that is unusual in magnitude for a diabetes treatment. And the dual GIP/GLP-1 mechanism translates into clinical results that exceed what the best GLP-1-only agonist delivers.

For a full breakdown of how tirzepatide’s dual mechanism works at the molecular level, see our tirzepatide research profile.

The next generation of multi-receptor agonists is already in clinical testing. Retatrutide, a triple agonist targeting GIP, GLP-1, and glucagon receptors, showed 24.2% weight loss in its Phase 2 trial. Whether adding a third receptor target improves on tirzepatide’s already strong glycemic results is one of the central questions in metabolic research right now.

The data from SURPASS 1 through 5 is publicly available, published in the New England Journal of Medicine and The Lancet. The numbers speak for themselves. What remains to be seen is the long-term cardiovascular outcomes data from SURPASS-CVOT, which will determine whether tirzepatide’s metabolic benefits translate into reduced cardiovascular events over time.

This article is for educational and informational purposes only. It is not intended as medical advice and should not be used to diagnose, treat, or prevent any condition. Always consult with a qualified healthcare professional before making health-related decisions. Clinical trial data referenced here is sourced from peer-reviewed publications and may not reflect the most current findings.