Tirzepatide: The SURMOUNT Trials and What Dual Incretin Agonism Shows

What tirzepatide is

Tirzepatide is a 39 amino acid synthetic peptide engineered to activate both the GIP receptor and the GLP-1 receptor simultaneously. The molecule is built on a GIP backbone with amino acid substitutions that preserve GLP-1 receptor binding, and a C20 fatty diacid chain that allows it to bind reversibly to serum albumin. That albumin tethering extends the plasma half-life to approximately five days, making once weekly subcutaneous injection practical.

Eli Lilly developed tirzepatide under the research designation LY3298176. The FDA approved it in May 2022 as Mounjaro for adults with type 2 diabetes, and in November 2023 as Zepbound for chronic weight management in adults with a BMI of 30 or above, or 27 or above with at least one weight-related comorbidity. The two brand names represent the same active compound at the same dose range; they differ in their approved indications and labeling.

The compound belongs to a class researchers sometimes call twincretins, reflecting the two-incretin-hormone concept. It is the first drug in that class to receive FDA approval, predating other dual agonist candidates that remain in clinical development as of 2026.

The dual incretin mechanism

Incretin hormones are peptides secreted by the gut wall in response to nutrients arriving in the small intestine. Their primary physiological role is to amplify insulin secretion from pancreatic beta cells in anticipation of rising blood glucose. Two incretins dominate this system: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). Both are released within minutes of eating, and both are inactivated rapidly by the enzyme DPP-4.

GLP-1 receptor agonists like semaglutide exploit one half of that system. Tirzepatide activates both receptors in a single molecule. The GLP-1 receptor is expressed on pancreatic beta cells, hypothalamic neurons involved in appetite regulation, the vagus nerve, and the gastrointestinal tract. The GIP receptor is also present on beta cells and on adipose tissue, where it appears to influence fat storage and energy metabolism.

• •GLP-1 receptor activation: glucose-dependent insulin secretion, glucagon suppression, gastric emptying delay, hypothalamic satiety signaling
• •GIP receptor activation: additional incretin-mediated insulin secretion, possible effects on adipose tissue energy partitioning
• •Combined activation appears to produce larger reductions in HbA1c and body weight than selective GLP-1 receptor agonism at matched comparator doses in trial settings
• •Gastric emptying is slowed, reducing post-meal glucose excursions and blunting appetite through gut stretch receptors
• •Both receptors are expressed in the brain, and central effects on food reward circuitry are an active area of research

The SURPASS program: type 2 diabetes trials

Eli Lilly ran the SURPASS program to evaluate tirzepatide in adults with type 2 diabetes. The program included five major trials comparing tirzepatide against placebo and against active comparators including semaglutide, insulin degludec, and insulin glargine. Results from the SURPASS trials were reported primarily in The Lancet and The New England Journal of Medicine between 2021 and 2022 and formed the evidentiary basis for Mounjaro's FDA approval.

Across the SURPASS program, tirzepatide consistently achieved HbA1c reductions greater than 2 percentage points in most dose groups, which is substantially larger than the reductions typically seen with older second-line diabetes medications. The weight effects were also notable for a glucose-lowering agent: most participants lost rather than gained weight, and body weight reductions in some groups approached those seen in dedicated obesity trials.

The SURMOUNT program: obesity and weight management trials

The SURMOUNT program evaluated tirzepatide as a treatment for obesity and overweight in adults with and without type 2 diabetes. These trials were the foundation for Zepbound's FDA approval in November 2023 and represented some of the largest weight reductions ever reported in randomized controlled trials of pharmacological weight management.

SURMOUNT-1, published in NEJM in July 2022, enrolled 2,539 adults without type 2 diabetes who had a BMI of 30 or above, or 27 or above with at least one weight-related comorbidity. Participants were randomized to tirzepatide 5mg, 10mg, or 15mg weekly or to placebo, all with lifestyle intervention. At 72 weeks, mean body weight reductions were 15.0 percent in the 5mg group, 19.5 percent in the 10mg group, and 20.9 percent in the 15mg group, compared with 3.1 percent with placebo. More than half of participants at the 10mg and 15mg doses achieved at least 20 percent body weight reduction.

SURMOUNT-2, reported in The Lancet in 2023, extended the program to adults with type 2 diabetes and obesity or overweight. Weight reductions at 72 weeks were approximately 12.8 percent with tirzepatide 10mg and 14.7 percent with 15mg versus 3.2 percent with placebo. The smaller weight effects compared with SURMOUNT-1 are consistent with the observation that metabolic adaptation in type 2 diabetes tends to attenuate weight loss responses to GLP-1 class agents.

Later SURMOUNT trials examined additional clinical questions. SURMOUNT-3 evaluated tirzepatide following a period of intensive lifestyle intervention to assess whether prior lifestyle change altered subsequent medication response. SURMOUNT-4 examined weight regain after tirzepatide withdrawal, reporting that participants who switched from tirzepatide to placebo regained a substantial fraction of lost weight over the following 52 weeks, underscoring the maintenance requirement of this treatment class.

• •SURMOUNT-1: mean 20.9 percent weight reduction at the 15mg dose versus 3.1 percent placebo at 72 weeks
• •SURMOUNT-2: mean 14.7 percent weight reduction at the 15mg dose in adults with type 2 diabetes at 72 weeks
• •SURMOUNT-CVOT: ongoing cardiovascular outcomes trial evaluating whether tirzepatide reduces MACE events in adults with obesity and established cardiovascular disease
• •Weight regain after discontinuation was documented in SURMOUNT-4, supporting the view that this class treats rather than cures obesity
• •Across SURMOUNT trials, tirzepatide also produced improvements in cardiometabolic markers including blood pressure, waist circumference, triglycerides, and fasting insulin

Safety profile and prescribing context

The safety profile observed across the SURPASS and SURMOUNT programs is broadly consistent with the GLP-1 receptor agonist class. Gastrointestinal adverse events are the most common, with nausea, diarrhea, vomiting, and constipation reported most frequently during dose escalation. These effects typically diminish after several weeks at a stable dose. The standard approach of gradual monthly dose escalation from 2.5mg to the target dose was adopted specifically to improve tolerability.

The Mounjaro and Zepbound prescribing information carries a boxed warning for thyroid C-cell tumors based on findings in rodent studies at supraphysiological exposures. The clinical relevance of this finding for humans is not established, but the label contraindicates use in people with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2. This warning is shared with the GLP-1 class broadly and is not unique to tirzepatide.

Additional labeled risks include acute pancreatitis, gallbladder disease, hypoglycemia when combined with insulin or sulfonylureas, acute kidney injury, and severe gastrointestinal disease. The GI adverse event burden can cause caloric restriction severe enough to produce dehydration and acute kidney injury in some patients, which is why adequate hydration and careful clinical monitoring are standard in prescribing guidelines.

Tirzepatide versus semaglutide in trial context

SURPASS-2 is the only large randomized head-to-head trial comparing tirzepatide directly with semaglutide as of 2026. That trial used semaglutide 1mg weekly as the comparator, which is the dose approved for type 2 diabetes but not the 2.4mg dose used in the Wegovy obesity indication. Tirzepatide at 10mg and 15mg outperformed semaglutide 1mg on both HbA1c and body weight at 40 weeks.

No randomized head-to-head trial comparing tirzepatide 15mg against semaglutide 2.4mg specifically in obesity had been published by mid-2026. Cross-trial comparisons are methodologically problematic because trial populations, run-in protocols, lifestyle co-interventions, and dropout handling differ. The SURMOUNT-1 results (mean 20.9 percent weight reduction at 15mg) appear larger than the STEP 1 results for semaglutide 2.4mg (mean approximately 14.9 percent at 68 weeks in NEJM 2021), but these differences cannot be interpreted as a head-to-head comparison without a trial designed for that purpose.

Compound reference pages