Tirzepatide

Tirzepatide is a synthetic 39-amino-acid peptide developed by Eli Lilly. It activates two incretin receptors at once, GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). It is administered as a once-weekly subcutaneous injection and is sold under the brand names Mounjaro (type 2 diabetes) and Zepbound (obesity and OSA).

The molecule was designed to combine the appetite and glucose effects of GLP-1 agonism with the metabolic effects of GIP receptor activation, which historically had been considered redundant or counterproductive. Phase 1 and Phase 2 work suggested otherwise. The current published evidence base now includes more than 10,000 randomized patients across the SURPASS (diabetes) and SURMOUNT (obesity) programs combined.

Tirzepatide received its first FDA approval in May 2022 for adults with type 2 diabetes, under the brand name Mounjaro. The obesity indication followed in November 2023 as Zepbound. In December 2024, the FDA approved Zepbound as the first drug for moderate-to-severe obstructive sleep apnea in adults with obesity. A cardiovascular risk reduction label was added in 2025 based on the SUMMIT trial in heart failure with preserved ejection fraction.

The SURMOUNT Trial Program

Five completed Phase 3 obesity trials anchor the tirzepatide weight-loss evidence base.

SURMOUNT-1 (Jastreboff et al., NEJM 2022). 2,539 adults with obesity or overweight with at least one weight-related complication, excluding diabetes. Mean body-weight reduction was 16.0 percent at 5 mg, 21.4 percent at 10 mg, and 22.5 percent at 15 mg versus 2.4 percent for placebo over 72 weeks. About nine of every ten participants on tirzepatide lost at least 5 percent of body weight.

SURMOUNT-2 (Garvey et al., Lancet, 2023). Adults with obesity and type 2 diabetes. Tirzepatide 15 mg produced 14.7 percent weight loss versus 3.2 percent for placebo.

SURMOUNT-3. Adults who had completed a 12-week intensive lifestyle program. Tirzepatide produced an additional 18.4 percent loss after the lifestyle phase, versus 2.5 percent regain on placebo.

SURMOUNT-4 (Aronne et al., 2023). A withdrawal trial. 670 patients who had completed a 36-week open-label lead-in were randomized to continue tirzepatide or switch to placebo. The placebo group regained an average of 14 percent of their lead-in body weight. The drug works while you take it.

A three-year extension analysis (Jastreboff et al., NEJM 2025) reported a 94 percent reduction in type 2 diabetes onset in patients with obesity and prediabetes at baseline.

SURMOUNT-5: Head-to-Head Against Semaglutide

SURMOUNT-5 (Aronne et al., New England Journal of Medicine, 2025) enrolled 751 adults with obesity, randomized 1:1 to tirzepatide (maximum tolerated dose) or semaglutide 2.4 mg (the Wegovy obesity dose) for 72 weeks. Mean weight reduction was 20.2 percent with tirzepatide versus 13.7 percent with semaglutide. The 6.5-percentage-point difference was statistically significant. Gastrointestinal adverse events were comparable between groups.

OSA and Cardiovascular Outcomes

The SURMOUNT-OSA program tested tirzepatide in adults with moderate-to-severe obstructive sleep apnea and obesity. After 52 weeks, tirzepatide alone produced 18 percent mean weight loss and reduced apnea-hypopnea index by roughly 25 events per hour. The FDA approval for OSA followed in December 2024.

The SUMMIT trial tested tirzepatide in patients with heart failure with preserved ejection fraction and obesity. Tirzepatide reduced the composite of HF events and improved exercise capacity. The cardiovascular risk reduction label addition followed in 2025.

Tirzepatide binds both the GIP receptor and the GLP-1 receptor as a single molecule. The two receptors sit on different cell populations and drive partially overlapping but distinct downstream effects.

GLP-1 receptor activation in the pancreas increases glucose-dependent insulin secretion. The same receptor in the hypothalamus and brainstem reduces appetite and slows gastric emptying. These two effects together account for most of the glycemic and weight-loss benefit seen with selective GLP-1 agonists like semaglutide and liraglutide.

GIP receptor activation is the newer half of the story. GIP was long viewed as the blunted incretin in type 2 diabetes, with reduced insulinotropic effect in hyperglycemic patients. Tirzepatide's clinical effect challenged that view. Adding GIP signaling appears to improve insulin sensitivity in adipose tissue, modulate energy expenditure, and reduce the gastrointestinal side effects that typically limit GLP-1 dosing. The dual-receptor profile is the central reason tirzepatide outperforms semaglutide in direct comparison.

The half-life is approximately five days, supporting once-weekly subcutaneous dosing. The dose escalation schedule starts at 2.5 mg weekly and increases by 2.5 mg every four weeks, with maintenance doses of 5, 10, or 15 mg. Slow titration is what keeps gastrointestinal adverse events at tolerable rates in most patients.

In adults with obesity and no diabetes, tirzepatide produced 22.5 percent mean body-weight reduction at the 15 mg dose over 72 weeks in SURMOUNT-1. Fat-mass reduction outpaced lean-mass reduction by roughly three to one in the highest-dose arm (33.9 percent fat versus 10.9 percent lean).

In adults with obesity and type 2 diabetes, mean weight loss reached 14.7 percent on the 15 mg dose in SURMOUNT-2. The lower effect size reflects a pattern seen with most weight-loss interventions in diabetic populations.

The SUMMIT trial documented reduced heart-failure events and improved exercise capacity in patients with HFpEF and obesity. The SURMOUNT-OSA program reduced apnea-hypopnea index by roughly 25 events per hour at 52 weeks. Three-year extension data reported a 94 percent reduction in type 2 diabetes onset in patients with prediabetes and obesity.

Once-weekly subcutaneous injection. Dose escalation starts at 2.5 mg weekly and increases by 2.5 mg every four weeks. Maintenance doses are 5, 10, or 15 mg weekly. Slow titration is what keeps gastrointestinal adverse events at tolerable rates.

Mounjaro and Zepbound are the same molecule at the same doses; the label differs. Mounjaro is approved for type 2 diabetes. Zepbound is approved for chronic weight management, obstructive sleep apnea in adults with obesity, and cardiovascular risk reduction in patients with HFpEF and obesity.

The FDA removed tirzepatide from its shortage list in October 2024, ending the compounding exception. Compounded tirzepatide is no longer legally available in the US compounding system.

The dominant adverse-event category is gastrointestinal. Across SURMOUNT trials, nausea, diarrhea, constipation, and vomiting occurred more frequently with tirzepatide than placebo. Most events were mild to moderate, concentrated during the 20-week dose-escalation phase, and resolved with continued treatment. Treatment discontinuation due to adverse events ranged from 4.3 percent at 5 mg to 7.1 percent at 15 mg in SURMOUNT-1, versus 2.6 percent for placebo.

Less common but clinically important adverse events include acute pancreatitis (signal under monitoring across the GLP-1 class), gallbladder events at modestly elevated rates, and acute kidney injury in patients who develop severe dehydration from vomiting. Diabetic retinopathy worsening has been observed with GLP-1 class drugs in patients with established retinopathy, particularly during periods of rapid glycemic improvement. Labeling requires baseline retinopathy screening in diabetic patients.

Like all incretin agonists, tirzepatide carries a boxed warning for thyroid C-cell tumors, based on rodent data with a different GLP-1 analog. Patients with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2 (MEN2) should not use the drug.

Direct head-to-head data exists for tirzepatide versus semaglutide. SURMOUNT-5 reported 20.2 percent weight loss with tirzepatide versus 13.7 percent with semaglutide over 72 weeks at labeled doses.

Retatrutide, Lilly's triple agonist (GIP/GLP-1/glucagon), posted 28.7 percent mean weight loss in the TRIUMPH-4 Phase 3 readout in December 2025. Retatrutide is investigational and not approved; no obesity head-to-head against tirzepatide has been completed.

Tirzepatide is on the WADA Prohibited List under section S4 (Hormone and Metabolic Modulators).