Tirzepatide

Tirzepatide is the most potent approved weight-loss medicine in the incretin class. It is a prescription medicine approved for type 2 diabetes (Mounjaro) and chronic weight management (Zepbound).

The mechanism extends beyond single GLP-1 agonism. Tirzepatide activates both GIP and GLP-1 receptors on a single synthetic peptide. Dual incretin agonism stimulates glucose-dependent insulin secretion, suppresses glucagon, slows gastric emptying, and reduces appetite via hypothalamic pathways. The GIP component is proposed to add metabolic and adipose-tissue effects beyond GLP-1 agonism alone.

The SURMOUNT-1 trial (N=2,539, 72 weeks) established weight-loss efficacy: 16.0% (5 mg), 21.4% (10 mg), and 22.5% (15 mg) versus 2.4% with placebo. At least 5% body weight reduction was achieved by 89% (5 mg) and 96% (10 mg and 15 mg) versus 28% placebo. These are the largest mean weight-loss percentages for any approved medicine in the incretin class.

Body composition data is more detailed than for semaglutide. SURMOUNT-1 reported approximately three-times-greater percent reduction in fat mass than lean mass: 33.9% fat mass reduction versus 10.9% lean mass reduction. A DXA substudy (N=160, 124 on tirzepatide) and the SURPASS-3 MRI substudy measured fat and lean mass changes in adults with and without type 2 diabetes. The 3:1 fat-to-lean loss ratio suggests a body-composition profile that partially spares muscle relative to total weight lost, though muscle loss still occurs.

No dedicated cardiovascular outcomes trial equivalent to semaglutide's SELECT has been completed. SURPASS-2 (N=1,879, 40 weeks) compared tirzepatide to semaglutide 1 mg on glycemic and weight endpoints in type 2 diabetes: tirzepatide 5 mg reduced HbA1c by 2.01 percentage points. But this was not a cardiovascular outcomes trial.

Safety is consistent with the GLP-1 class. Gastrointestinal adverse events (nausea, diarrhea, vomiting, constipation) are the most common side effects. Tirzepatide carries the class boxed warning regarding thyroid C-cell tumors seen in rodent studies and is contraindicated with personal or family history of MTC or MEN 2.

Dosing follows a stepwise escalation: starting at 2.5 mg weekly, increasing at 4-week intervals through 5 mg, 7.5 mg, 10 mg, 12.5 mg, to a maintenance dose of 5, 10, or 15 mg once weekly.

For longevity, the GIP component may confer additional metabolic effects on adipose tissue and lipid handling beyond GLP-1 agonism alone. General GLP-1-class anti-inflammatory and NASH resolution benefits are mechanistically relevant. The larger relative fat-mass loss versus lean-mass loss ratio is a favorable signal for body composition compared to simpler GLP-1 agonists, though sarcopenia remains a concern in absolute terms given the large total weight lost.