Tirzepatide

What it is

Tirzepatide is a 39-amino-acid synthetic peptide developed by Eli Lilly that functions as a dual agonist at both the GLP-1 receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor. It was the first agent in this novel “twincretin” class to receive FDA approval — Mounjaro for type 2 diabetes (May 2022) and Zepbound for chronic weight management (November 2023).

The molecule contains a C20 fatty diacid moiety attached via a linker, similar to semaglutide’s albumin-binding modification, providing a half-life of approximately 5 days and enabling once-weekly subcutaneous dosing. Tirzepatide’s amino acid sequence is based on native GIP rather than GLP-1, but it has been modified to bind both receptors with high affinity. Notably, it shows greater agonism at GIP receptors than at GLP-1 receptors at clinically relevant concentrations — a profile distinct from native GIP, which is a much weaker GLP-1 receptor agonist.

Mechanism of action

Tirzepatide activates both GIP and GLP-1 receptors. GLP-1 receptor activation produces the same effects as semaglutide: glucose-dependent insulin secretion, glucagon suppression, slowed gastric emptying, and central appetite reduction. GIP receptor activation contributes additional effects:

• Adipose tissue: GIP enhances insulin sensitivity in adipocytes and may improve lipid storage capacity, potentially reducing ectopic fat deposition (in liver and muscle) during weight gain.
• Bone: GIP receptors are expressed on osteoblasts; GIP signaling supports bone formation; effects of tirzepatide on bone health have not been established.
• Insulin sensitivity: the GIP component has been studied for possible effects on peripheral insulin sensitivity.
• Nausea modulation: research has examined whether GIP receptor activation affects GLP-1-related nausea; this has not been established clinically.

Research findings

SURPASS program (type 2 diabetes): Five SURPASS trials enrolled approximately 6,200 patients comparing tirzepatide to placebo, semaglutide 1 mg, insulin degludec, and insulin glargine. SURPASS-2 (2021, NEJM) directly compared tirzepatide 5/10/15 mg to semaglutide 1 mg over 40 weeks. All tirzepatide doses produced superior HbA1c reduction (-2.01 to -2.30% vs. -1.86%) and superior weight loss (-7.6 to -11.2 kg vs. -5.7 kg). The 15 mg dose produced nearly twice the weight loss of semaglutide 1 mg.

SURMOUNT-1 (chronic weight management): Published 2022 in NEJM. 2,539 adults without diabetes randomized to tirzepatide 5/10/15 mg or placebo for 72 weeks. Mean weight loss was 15.0%, 19.5%, and 20.9% across the three doses, vs. 3.1% with placebo. Individual results vary. Approximately 50% of patients on the 10 and 15 mg doses lost ≥20% of body weight. Individual results vary.

SURMOUNT-2 (T2DM): Demonstrated 12.8–14.7% weight loss with tirzepatide 10/15 mg in patients with type 2 diabetes.

SURMOUNT-OSA (obstructive sleep apnea): Tirzepatide significantly reduced apnea-hypopnea index alongside weight loss, leading to FDA approval of Zepbound for OSA in patients with obesity (December 2024).

SURPASS-CVOT (cardiovascular outcomes): compared tirzepatide with dulaglutide in adults with type 2 diabetes and established cardiovascular disease; results were reported in 2025. Refer to current FDA prescribing information for approved indications.

Side effects and contraindications

Side effect profile parallels semaglutide but is often more pronounced during titration: nausea, decreased appetite (which is the goal but can become uncomfortable), early satiety, occasional vomiting, fatigue, and constipation. Slow titration, adequate hydration, low-fat meals, and protein-forward eating mitigate most symptoms.

Same serious risks as semaglutide: pancreatitis, gallbladder events (somewhat higher rates than semaglutide in trials, perhaps reflecting more rapid weight loss), and theoretical medullary thyroid cancer concerns from rodent data. Diabetic retinopathy progression with rapid glycemic improvement is a consideration in patients with established eye disease.

Contraindicated in: pregnancy and breastfeeding, MEN-2, personal/family history of medullary thyroid cancer, history of pancreatitis, severe gastroparesis.

What we don’t yet know

Long-term cardiovascular outcomes continue to be studied. Direct comparison of tirzepatide vs. semaglutide for cardiovascular benefit is unstudied. The optimal balance between aggressive titration (faster weight loss but more side effects) and slower titration is not well characterized. Effects on bone density during 20%+ weight loss remain an area of active study. As with semaglutide, weight regain after discontinuation is the rule rather than the exception, but the long-term metabolic trajectory in patients who discontinue is still being mapped.