Tirzepatide

Tirzepatide is a 39-amino-acid synthetic peptide engineered from the native glucose-dependent insulinotropic polypeptide (GIP) backbone, with a C20 fatty diacid chain attached via a γ-glutamic-acid / OEG linker to a Lys residue for albumin binding ([Jastreboff et al., *N Engl J Med* 2022, 387:205–216](https://doi.org/10.1056/NEJMoa2206038)). It activates both the GIP and GLP-1 receptors as a full or near-full agonist; the dual incretin engagement is the molecular signature that distinguishes it from semaglutide and the rest of the GLP-1 monoagonist class. Downstream effects span the same three mechanisms as GLP-1: glucose-dependent insulin secretion, slowed gastric emptying, and central appetite suppression. The GIP component contributes additional metabolic effects — including modulation of adipose insulin sensitivity and potential alteration of nausea-pathway signaling — that may explain part of the larger weight-loss magnitude relative to GLP-1 monoagonism, though the contribution of each receptor to clinical effect is still being parsed in the literature.

Tirzepatide is the obesity peptide that broke a longstanding ceiling in pharmacotherapy. The pivotal SURMOUNT-1 trial (Jastreboff et al., *NEJM* 2022) randomized 2,539 adults with overweight or obesity and without diabetes to weekly tirzepatide (5, 10, or 15 mg) or placebo for 72 weeks: mean body-weight changes were -16.0%, -21.4%, and -22.5% on the three doses versus -3.1% on placebo. More than half of participants on the 10-mg and 15-mg arms lost more than 20% of body weight — magnitudes previously seen only with bariatric surgery in non-trial settings. Subsequent SURMOUNT and SURPASS trials extended the result to type 2 diabetes, obesity with established cardiometabolic disease, sleep apnea, and head-to-head comparison against semaglutide (where tirzepatide produced larger mean weight loss). The trade-offs cluster in two places. The first is gastrointestinal side effects: nausea, vomiting, diarrhea, and constipation are common during dose escalation, similar in character to semaglutide but somewhat more pronounced at the higher doses. Discontinuation due to GI adverse events ranged from 2.6% (placebo) to 7.1% (10-mg arm) in SURMOUNT-1. The second is the same body-composition and discontinuation-rebound conversation that follows every GLP-1-class agent. Tirzepatide-induced weight loss includes a meaningful lean-mass component — magnitudes vary across body-composition substudies, but the same protein-and-resistance-training mitigation applies. Discontinuation produces partial weight regain on a similar trajectory to semaglutide. The third honest read is that tirzepatide arrived rapidly. The time between FDA approval (Mounjaro for T2D in May 2022, Zepbound for obesity in November 2023) and widespread off-label adoption is shorter than the long-term safety follow-up window. The class-level safety story for GLP-1-pathway drugs is mostly reassuring across decades of liraglutide, exenatide, and dulaglutide use, but the GIP-pathway component is newer in clinical practice. Read the SURMOUNT and SURPASS trials as strong on efficacy, mature on routine adverse-event characterization, and still maturing on the long-tail safety questions that decades-of-use data eventually answer.