Tirzepatide

Tirzepatide is a dual GIP / GLP-1 receptor agonist developed by Eli Lilly — a single peptide molecule that activates both the GLP-1 and GIP (glucose-dependent insulinotropic polypeptide) receptors. The dual-agonist design was a bet that activating GIP alongside GLP-1 would produce additive metabolic benefit. That bet paid off: tirzepatide produces the largest mean weight loss of any FDA-approved obesity drug to date. It's branded as Mounjaro (T2D, 2022) and Zepbound (chronic weight management, 2023).

Tirzepatide is a 39-amino-acid peptide designed to bind both the GLP-1 receptor and the GIP receptor with high affinity. GLP-1 activation produces the same set of effects as semaglutide — pancreatic beta-cell insulin secretion, gastric emptying delay, central appetite suppression. GIP activation's contribution to weight loss in humans is still debated; in animal models, GIP receptor agonism alone doesn't reliably produce weight loss, but added to GLP-1 it does. One hypothesis: GIP agonism modulates adipose-tissue metabolism directly; another: the two signals converge in hypothalamic circuits with nonlinear additive effects. The molecule is engineered with a C20 fatty di-acid chain enabling albumin binding and a weekly dosing half-life.

Tirzepatide's T2D program ran through the SURPASS trial series (SURPASS-1 through -5, 2018-2022), showing HbA1c reductions larger than any GLP-1 agonist or basal insulin comparator. The obesity program was SURMOUNT. SURMOUNT-1 (NEJM 2022) enrolled adults with BMI ≥30 or ≥27 with weight-related comorbidity; 15 mg weekly produced 20.9% mean weight loss at 72 weeks versus 3.1% on placebo. SURMOUNT-2 extended efficacy to patients with T2D. SURMOUNT-5 (NEJM 2025) was the first head-to-head RCT vs semaglutide; tirzepatide 15 mg produced 20.2% vs Wegovy 2.4 mg's 13.7% at 72 weeks. In 2024, SURMOUNT-OSA demonstrated efficacy in obstructive sleep apnea, leading to FDA approval for that indication.

FDA approved: Mounjaro for T2D (May 2022), Zepbound for chronic weight management (Nov 2023), Zepbound for moderate-to-severe OSA in adults with obesity (Dec 2024). Ongoing: SURMOUNT-MMO (cardiovascular outcomes), SUMMIT (HFpEF), ongoing work on NAFLD/MASH and chronic kidney disease. Phase 3 readouts for each pending 2025-2027.

FDA declared the tirzepatide shortage resolved Dec 19, 2024. 503A compounders lost discretion Feb 18, 2025; 503B by Mar 19, 2025. Compounded tirzepatide is no longer broadly legal in the US. Lilly has launched direct-to-patient programs (LillyDirect) to improve cash-pay access for patients without coverage. See our FDA timeline for the full history. Full regulatory timeline →

Same muscle-loss and rebound concerns as semaglutide, magnified in proportion to the larger weight loss. The GIP receptor's role remains an open scientific question: tirzepatide clearly works, but mechanistic studies disagree on whether the GIP component is the dominant added ingredient or whether it modulates tolerability, reduces the effective semaglutide-equivalent dose, or acts through tissue-specific pathways not fully characterized. Long-term (>2 year) safety data is still accumulating.

• SURMOUNT-1 trial (NEJM, 2022) →
• SURMOUNT-5 head-to-head (NEJM, 2025) →
• SURMOUNT-OSA (NEJM, 2024) →
• FDA Zepbound prescribing information →