What Is Tirzepatide and Where Does It Sit in the GLP Class?

Tirzepatide is a synthetic peptide that acts as a dual agonist at two receptors: the glucagon-like peptide-1 (GLP-1) receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor. That dual mechanism separates it from single-agonist compounds like semaglutide. Eli Lilly developed the molecule under the internal designation LY3298176 before it reached the branded drug stage.

In the GLP-class hierarchy, tirzepatide is one of the most clinically advanced compounds. It has completed large-scale Phase 3 programs, cleared FDA review twice under two separate indications, and has active trials running in additional disease areas as of 2025. The research-chemical market also carries tirzepatide peptide powder, but those products carry no FDA approval and are not interchangeable with the licensed drugs.

Mechanistically, the GIP agonism is thought to amplify the insulin-sensitizing and appetite-suppressing effects seen with GLP-1 alone. A 2022 paper in Cell Metabolism (PMID 35662412) examined receptor-level signaling differences between single and dual agonism in preclinical models, though the clinical weight-loss data comes from human RCTs, not that mechanistic work.

Regulatory Milestones: Mounjaro and Zepbound

FDA approved Mounjaro (tirzepatide injection, Eli Lilly) on May 13, 2022, for adults with type 2 diabetes as an adjunct to diet and exercise. That approval followed the SURPASS Phase 3 program. Mounjaro is a prescription drug; the approval does not extend to any other tirzepatide product.

FDA approved Zepbound (tirzepatide injection, Eli Lilly) on November 8, 2023, for chronic weight management in adults with obesity (BMI 30 or above) or overweight (BMI 27 or above) with at least one weight-related condition. This was a separate NDA review based primarily on the SURMOUNT trial program. Zepbound is also a prescription drug, and again the approval is brand-specific.

In June 2024, FDA added a new indication to Zepbound: reduction of obstructive sleep apnea (OSA) severity in adults with obesity. That decision followed the SURMOUNT-OSA trial readout. It marked one of the first FDA approvals of a GLP-class drug for a non-metabolic primary indication, which is a notable regulatory milestone for the broader compound class.

Research-chemical tirzepatide sold through peptide vendors is not covered by any of these approvals. The FDA has taken enforcement actions against compounders and vendors selling unapproved versions of GLP-class peptides; the agency's shortage-related compounding policies for tirzepatide were updated in 2024 and are subject to ongoing regulatory activity.

SURPASS and SURMOUNT: Key Trial Readouts with Numbers

The SURPASS program comprised five Phase 3 RCTs in type 2 diabetes. SURPASS-2, published in the New England Journal of Medicine in 2021 (PMID 34170647), enrolled 1,879 participants and compared tirzepatide at three doses against semaglutide 1 mg. At 40 weeks, the highest tirzepatide group achieved a mean HbA1c reduction of 2.46 percentage points versus 1.86 for semaglutide. Body weight reductions in that group averaged 11.2 kg versus 5.7 kg for semaglutide.

The SURMOUNT-1 trial, published in the New England Journal of Medicine in 2022 (PMID 35658024), enrolled 2,539 adults with obesity or overweight without diabetes. At 72 weeks, participants in the highest-dose group lost a mean of 22.5% of body weight versus 2.4% in the placebo group. That 22.5% figure became one of the most-cited numbers in GLP-class coverage and was a primary driver of the Zepbound approval.

SURMOUNT-OSA, the sleep apnea trial, enrolled 469 participants across two cohorts (those using CPAP and those not). Results published in 2024 showed tirzepatide reduced the apnea-hypopnea index (AHI) by approximately 25 to 30 events per hour more than placebo depending on cohort, alongside significant weight loss. Those numbers supported the June 2024 OSA indication for Zepbound.

SURPASS-CVOT, the cardiovascular outcomes trial, reported results in 2024. The trial enrolled over 13,000 adults with type 2 diabetes and established cardiovascular disease. Tirzepatide showed a statistically significant reduction in major adverse cardiovascular events (MACE) compared to placebo, a finding that mirrors the cardiovascular outcome data seen with other GLP-class drugs and strengthens the compound's long-term clinical profile.

What Active Trials Are Running in 2025?

As of mid-2025, Eli Lilly has multiple active tirzepatide trials registered on ClinicalTrials.gov. SURMOUNT-5 is a head-to-head comparison of tirzepatide against semaglutide 2.4 mg (Wegovy) in adults with obesity, with a primary completion date in 2025. That trial is closely watched because it is the first large RCT to directly compare the two leading GLP-class weight drugs head-to-head in a weight-management population.

SUMMIT is a Phase 3 trial examining tirzepatide in heart failure with preserved ejection fraction (HFpEF) in patients with obesity. Interim results presented in 2024 showed improvements in the Kansas City Cardiomyopathy Questionnaire score and six-minute walk distance, and Lilly has indicated plans to seek an additional FDA indication based on that data.

Additional trials are examining tirzepatide in non-alcoholic steatohepatitis (NASH, now more commonly called MASH), chronic kidney disease, and as a potential perioperative weight-loss tool. Most of these are in Phase 2 or early Phase 3 as of 2025. Results from the MASH program are expected in 2025 to 2026.

Researchers outside Lilly have also registered investigator-initiated studies examining tirzepatide in polycystic ovary syndrome, lipodystrophy, and other metabolic conditions. Those trials are generally smaller, and their results will carry less regulatory weight than Lilly's sponsored Phase 3 programs.

What Changed Most Recently?

The most significant recent development is the SURPASS-CVOT cardiovascular outcomes readout in 2024, which confirmed a MACE benefit. That positions tirzepatide alongside semaglutide as a GLP-class compound with demonstrated cardiovascular event reduction in a high-risk population, not just metabolic marker improvement.

The OSA indication for Zepbound, granted June 2024, is the other headline change. It expanded the approved use case for tirzepatide beyond metabolic disease and set a precedent for GLP-class drugs in sleep medicine. Clinicians and payers are still working through coverage and prescribing patterns for that indication.

On the supply and access side, FDA removed tirzepatide from its drug shortage list in late 2024, which triggered changes to compounding policies. Pharmacies that had been compounding tirzepatide under shortage exemptions faced new compliance deadlines. That regulatory shift affects the broader peptide and compounding market, though it does not change the status of research-chemical tirzepatide, which remains unapproved regardless of shortage status.

SURMOUNT-5 head-to-head data against semaglutide is the most anticipated near-term readout. Preliminary data presented at conferences in early 2025 suggested tirzepatide produced greater mean weight loss than semaglutide 2.4 mg over 72 weeks, but the full peer-reviewed publication had not been released as of this tracker update.