Tirzepatide: Dual GIP/GLP-1 Agonist (Mounjaro, Zepbound)
First approved dual incretin — GIP + GLP-1. ~21% mean weight loss at 72 weeks (SURMOUNT-1).
✅ Approved
- Full name
- Tirzepatide
- Class
- Dual GIP/GLP-1 receptor agonist
- Half-life
- ~5 days
- Route
- Subcutaneous weekly
- Brand names
- Mounjaro (T2D), Zepbound (obesity) — Eli Lilly
- Regulatory status
- FDA-approved: Mounjaro 2022 (T2D), Zepbound 2023 (obesity), Zepbound December 2024 (moderate-to-severe obstructive sleep apnea with obesity — first OSA drug).
What it is
Tirzepatide is the first approved dual agonist of the GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 receptors. A 39-amino-acid peptide engineered with a C20 fatty-diacid moiety for albumin binding, it carries a biased signalling profile at the GIP receptor and produces weight loss of a magnitude previously only achievable with bariatric surgery. It is the current efficacy benchmark for non-surgical obesity therapy.
How it works
Tirzepatide activates both GIP and GLP-1 receptors in pancreatic islets, enhancing glucose-dependent insulin secretion and suppressing glucagon. The GLP-1 arm also slows gastric emptying and acts on hypothalamic satiety circuits.
The GIP component contributes additional anorexic signalling via the dorsomedial hypothalamus and appears to counter GLP-1–induced nausea while amplifying energy-expenditure effects. Biased agonism at the GIP receptor — recruiting arrestin less aggressively than native GIP — may preserve receptor signalling over chronic dosing.
What the research shows
The SURMOUNT (obesity) and SURPASS (T2D) programmes established tirzepatide as the highest-efficacy incretin therapy to date.
Jastreboff AM et al. (2022) — SURMOUNT-1: tirzepatide in obesity
Jastreboff A.M. et al., NEJM 2022;387:205–216. 👥 Human studies
2,539 adults without diabetes randomised to 5, 10, or 15 mg weekly tirzepatide or placebo for 72 weeks. Primary endpoint: percent weight change.
Mean weight loss: −15.0% (5 mg), −19.5% (10 mg), −20.9% (15 mg) versus −3.1% placebo. 57% achieved ≥20% weight loss at the 15 mg dose — unprecedented for a pharmacological intervention.
Limitations: GI adverse events 44–85% dose-dependent. Participants with diabetes were excluded; effects in T2D (SURMOUNT-2) were smaller at ~15%.
Rosenstock J et al. (2021) — SURPASS-1: tirzepatide monotherapy in T2D
Rosenstock J. et al., Lancet 2021;398:143–155. 👥 Human studies
478 drug-naive T2D patients randomised to tirzepatide 5/10/15 mg or placebo for 40 weeks.
HbA1c reduction: −1.87% (5 mg) to −2.07% (15 mg) vs +0.04% placebo. Weight loss 7–9.5 kg. More than half achieved HbA1c <5.7% at the top dose.
Limitations: Short-duration monotherapy trial; long-term durability of glycaemic effect beyond one year remains under study.
Safety and limitations
GI effects (nausea 17–39%, diarrhoea 13–22%, vomiting 6–17%) dominate and are dose-dependent. Rare risks mirror the GLP-1 class: pancreatitis, gallbladder disease, and rodent thyroid C-cell tumours (black-box warning). Contraindicated in personal/family MTC history. Muscle loss during rapid weight reduction is a known concern.
SURMOUNT-4 showed weight regain after discontinuation analogous to semaglutide: sustained therapy is required. Real-world tolerance is the main limiting factor.
Sources
- Jastreboff A.M. et al. SURMOUNT-1. NEJM 2022;387:205–216. PubMed
- Rosenstock J. et al. SURPASS-1. Lancet 2021;398:143–155. PubMed
- Garvey W.T. et al. SURMOUNT-2 (T2D). Lancet 2023;402:613–626. PubMed
- Aronne L.J. et al. SURMOUNT-4 (maintenance). JAMA 2024;331:38–48. PubMed
- Frías J.P. et al. SURPASS-2 (vs semaglutide). NEJM 2021;385:503–515. PubMed