The efficacy leader among approved drugs: dual GIP/GLP-1 agonism delivering −20.9% at 15 mg in SURMOUNT-1 and beating semaglutide head-to-head in SURMOUNT-5 (−20.2 vs −13.7) — sold as Mounjaro for diabetes and Zepbound for weight and sleep apnea.
Deeper data: the trial index holds every figure above with sources; the head-to-head file, the OSA indication.
Tirzepatide is the most effective weight-loss medication with full FDA approval as of our last verification: a once-weekly injectable that activates both the GLP-1 receptor and the GIP receptor — the “dual agonist” design that separated it from everything before it.
Mechanism, in plain terms
GLP-1 receptor activation slows gastric emptying, amplifies satiety signaling in the brain, and improves insulin response after meals. Tirzepatide adds GIP receptor activity on top; the precise contribution of GIP is still being worked out academically, but the empirical result is not subtle — in trials, tirzepatide produces greater average weight loss than any GLP-1-only agent. The molecule’s half-life is roughly five days, which is what makes once-weekly dosing work and why washout takes weeks, not days.
The evidence base
In SURMOUNT-1 (New England Journal of Medicine, 2022), adults with obesity and without diabetes lost an average of about 15% of body weight at the 5 mg dose and 20.9% at 15 mg over 72 weeks, against roughly 3% on placebo. In the head-to-head SURMOUNT-5 trial, tirzepatide beat semaglutide 2.4 mg directly — 20.2% versus 13.7% — the comparison we unpack in our head-to-head explainer. The SURPASS program established its type 2 diabetes efficacy, SURMOUNT-4 documented what happens when you stop (covered here), and SURMOUNT-OSA produced the first drug approval for obstructive sleep apnea (explainer).
Approved products and dosing
Two brands carry the molecule: Mounjaro (type 2 diabetes, approved 2022) and Zepbound (chronic weight management, 2023; moderate-to-severe obstructive sleep apnea in adults with obesity, December 2024). The labeled ladder starts at 2.5 mg weekly — a tolerability dose, not a treatment dose — then steps through 5, 7.5, 10, 12.5, and 15 mg, typically no faster than every four weeks. There is no FDA-approved oral tirzepatide in any form; anything sold as a tirzepatide tablet or drop sits outside the approved evidence base (full analysis).
Safety essentials
The label carries a boxed warning about thyroid C-cell tumors seen in rodents (human relevance unknown; contraindicated with personal or family history of medullary thyroid carcinoma or MEN 2). The dominant real-world burden is gastrointestinal — nausea, constipation, diarrhea, vomiting, concentrated during dose escalation. Two tirzepatide-specific flags deserve more attention than they get: the labeled interaction with oral hormonal contraceptives (switch to non-oral or add a barrier method for four weeks after initiation and after each dose increase — detailed in our pregnancy and fertility explainer) and the standard class cautions around gallbladder disease, pancreatitis, and pre-anesthesia disclosure (surgery guide). Base rates from the trials are tabulated in the side-effects file.
The 2026 cost landscape
Brand list prices sit near four figures a month before insurance, with manufacturer self-pay channels (LillyDirect vials) and savings cards lowering that substantially for eligible buyers — verify current terms directly. Compounded tirzepatide occupies a contested legal lane since the FDA declared the shortage resolved; our legal-status explainer maps it. The only compounded figures we currently publish as verified are in the ledger, dated and sourced. Whatever the channel, price the maintenance dose, not the starting dose — the arithmetic is in the true-cost file.
References
Zepbound and Mounjaro prescribing information, Eli Lilly — pi.lilly.com. SURMOUNT-1, NEJM 2022; SURMOUNT-5; SURMOUNT-4, JAMA 2024; SURMOUNT-OSA, NEJM 2024 — full citations in the linked explainers.