Tirzepatide
Mounjaro · Zepbound · LY3298176 · GIP/GLP-1 dual agonist
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Tirzepatide is a synthetic 39-amino acid peptide functioning as a dual agonist at both the glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor. FDA-approved as Mounjaro (2022, type 2 diabetes) and Zepbound (2023, chronic weight management). The dual receptor engagement distinguishes tirzepatide from pure GLP-1 agonists: GIP receptor activation contributes additional adipose tissue lipolysis, improved insulin secretion sensitivity, and possibly superior tolerability at equivalent efficacy doses. GLP-1 receptor activation drives appetite suppression, delayed gastric emptying, and incremental insulin release. The synergy between both pathways is the mechanistic basis for tirzepatide's superior weight loss outcomes versus semaglutide in head-to-head RCT data.
SURMOUNT-5 (NEJM, May 2025) was the first head-to-head RCT comparing tirzepatide vs semaglutide for obesity. 751 adults with BMI ≥30 or ≥27 with comorbidity, no T2D. Tirzepatide produced 20.2% mean weight loss vs 13.7% for semaglutide — a statistically significant superiority. 31.6% vs 16.1% achieved ≥25% body weight loss. The SURMOUNT-1 (2022) and SURMOUNT-2 (2023) trials established tirzepatide vs placebo efficacy. Mechanism advantage over pure GLP-1 agonists: dual receptor engagement amplifies fat-specific lipolysis via GIP receptor signalling in adipose tissue.
TirzepatideAlso known as: Mounjaro, Zepbound, LY3298176, GIP/GLP-1 dual agonist
Mechanism of Action
Tirzepatide is a synthetic 39-amino acid peptide functioning as a dual agonist at both the glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor. FDA-approved as Mounjaro (2022, type 2 diabetes) and Zepbound (2023, chronic weight management). The dual receptor engagement distinguishes tirzepatide from pure GLP-1 agonists: GIP receptor activation contributes additional adipose tissue lipolysis, improved insulin secretion sensitivity, and possibly superior tolerability at equivalent efficacy doses. GLP-1 receptor activation drives appetite suppression, delayed gastric emptying, and incremental insulin release. The synergy between both pathways is the mechanistic basis for tirzepatide's superior weight loss outcomes versus semaglutide in head-to-head RCT data.
Reported Research Benefits
- SURMOUNT-5 (NEJM, 2025): 20.2% mean body weight loss vs 13.7% for semaglutide at 72 weeks — a 47% relative superiority
- 31.6% of tirzepatide patients achieved ≥25% body weight loss vs 16.1% on semaglutide (SURMOUNT-5)
- HbA1c reduction of 2.1% at maximum dose in SURPASS-5 type 2 diabetes trial
- Cardiovascular risk reduction under investigation in ongoing SELECT-equivalent trials
- Dual GIP+GLP-1 mechanism drives greater adipose lipolysis than GLP-1 monotherapy
- FDA-approved for chronic weight management (Zepbound) and type 2 diabetes (Mounjaro)
Dosing Protocol & Reconstitution
FDA-Approved Dosing Protocol
Tirzepatide is FDA-approved as Mounjaro (type 2 diabetes) and Zepbound (chronic weight management). Dosing follows a strict slow-titration protocol to minimise gastrointestinal adverse events.
Titration Schedule (FDA-Approved)
| Weeks | Weekly Dose | Purpose |
|---|---|---|
| 1–4 | 2.5 mg | Initiation — tolerability assessment, no expected efficacy |
| 5–8 | 5 mg | First efficacy dose |
| 9–12 | 7.5 mg | Incremental increase if tolerated |
| 13–16 | 10 mg | Incremental increase if tolerated |
| 17–20 | 12.5 mg | Incremental increase if tolerated |
| 21+ | 15 mg | Maximum approved dose |
Titration principle: do not increase to the next dose if the current dose is causing significant GI adverse effects (nausea, vomiting, diarrhea). Extending a dose level by 4 weeks is acceptable and common practice.
Administration
- Route: subcutaneous injection
- Frequency: once weekly (same day each week, any time of day, with or without food)
- Injection sites: abdomen, thigh, or upper arm — rotate sites weekly
- Form: single-dose autoinjector pen (Mounjaro, Zepbound) or compounded vial (where still permitted — compounded tirzepatide availability is now restricted following the FDA shortage-list removal in October 2024)
Dose Selection for Target Indication
Type 2 diabetes: SURPASS trial data supports 5 mg, 10 mg, and 15 mg weekly as efficacious maintenance doses. Dose selection balances HbA1c reduction against GI tolerability.
Weight management: SURMOUNT-1 data showed mean body weight reduction of −15.0%, −19.5%, and −20.9% at 5 mg, 10 mg, and 15 mg respectively over 72 weeks — clearly dose-dependent. SURMOUNT-5 (NEJM, 2025) directly compared tirzepatide 15 mg vs semaglutide 2.4 mg over 72 weeks: −20.2% vs −13.7% weight loss, favouring tirzepatide.
Contraindications
- Personal or family history of medullary thyroid carcinoma (MTC) — absolute contraindication
- Multiple endocrine neoplasia syndrome type 2 (MEN 2) — absolute contraindication
- History of pancreatitis — caution; weigh risk-benefit
- Severe gastroparesis — avoid; tirzepatide delays gastric emptying further
- Pregnancy — not recommended; discontinue if planning pregnancy
- Type 1 diabetes and diabetic ketoacidosis — not indicated
Common Adverse Events (>5%)
- Nausea (most common, typically worst during titration)
- Diarrhea and vomiting
- Constipation
- Decreased appetite
- Abdominal pain and dyspepsia
- Injection-site reactions (mild)
Most GI adverse events are dose-dependent and improve with time or dose-level extension. The slow titration schedule exists specifically to minimise them.
Research Notes
SURMOUNT-5 (NEJM, May 2025) was the first head-to-head RCT comparing tirzepatide vs semaglutide for obesity. 751 adults with BMI ≥30 or ≥27 with comorbidity, no T2D. Tirzepatide produced 20.2% mean weight loss vs 13.7% for semaglutide — a statistically significant superiority. 31.6% vs 16.1% achieved ≥25% body weight loss. The SURMOUNT-1 (2022) and SURMOUNT-2 (2023) trials established tirzepatide vs placebo efficacy. Mechanism advantage over pure GLP-1 agonists: dual receptor engagement amplifies fat-specific lipolysis via GIP receptor signalling in adipose tissue.
Research Summary
SURMOUNT-5 (NEJM 2025, n=751): tirzepatide 20.2% vs semaglutide 13.7% weight loss at 72 weeks (p<0.001). 47% greater relative weight loss. SURPASS trials (2021–2022) established 2.1% HbA1c reduction at maximum dose. FDA approved Mounjaro (T2D, 2022) and Zepbound (obesity, 2023). Compounded tirzepatide enforcement began April/May 2026 following shortage resolution.
Side Effects & Safety
Most common: nausea (18–31%), diarrhoea (12–17%), vomiting (6–10%), constipation — highest incidence during dose titration, generally attenuates within 4–8 weeks. Serious but rare: pancreatitis, gallbladder disease. Contraindicated in personal or family history of medullary thyroid carcinoma or MEN2 syndrome (class effect with GLP-1 agonists based on rodent carcinogenicity data — clinical significance in humans not established). Injection site reactions: mild, transient.
Stability & Storage
Refrigeration required at 2–8°C. Do not freeze. Single-dose autoinjector pens can be kept at room temperature (up to 30°C) for up to 21 days. Protect from light.
Molecular Data
- Sequence
- Tyr-Aib-Glu-Gly-Thr-Phe-Thr-Ser-Asp-Tyr-Ser-Ile-Aib-Leu-Asp-Lys(C20 fatty diacid)-Ile-Ala-Gln-Lys-Ala-Phe-Val-Gln-Trp-Leu-Ile-Ala-Gly-Gly-Pro-Ser-Ser-Gly-Ala-Pro-Pro-Pro-Ser-NH2 (39 residues; Aib = α-aminoisobutyric acid)
- Molecular Formula
- C225H348N48O68
- Molecular Weight
- 4813.5 Da
- CAS Number
- 2023788-19-2
- Half-Life
- ~5 days (once-weekly dosing)
Primary literature: https://www.nejm.org/doi/full/10.1056/NEJMoa2416394