What Is Tirzepatide?
A factual reference · Last reviewed by the OPTML clinical team on April 20, 2026
Tirzepatide is a first-in-class dual GIP and GLP-1 receptor agonist, a synthetic peptide that activates two gut-hormone receptors simultaneously. It is FDA-approved under the brand names Mounjaro for type 2 diabetes and Zepbound for chronic weight management. In the SURMOUNT-1 trial, tirzepatide 15 mg produced an average 20.9% body weight reduction at 72 weeks, the largest weight loss ever reported in a GLP-1-class trial (Jastreboff AM et al., NEJM 2022).
Drug Class and Mechanism
Tirzepatide is the first drug in the dual incretin receptor agonist class. It binds and activates both:
- GIP receptor (glucose-dependent insulinotropic polypeptide), improves insulin secretion, increases adipose glucose uptake, and appears to improve the lipid profile.
- GLP-1 receptor (glucagon-like peptide-1), enhances insulin secretion, suppresses glucagon, delays gastric emptying, and reduces appetite.
Activating both receptors produces greater weight loss and glycemic improvement than activating GLP-1 alone, the physiological basis for tirzepatide's superior efficacy versus semaglutide.
Approved Uses
| Brand | Indication | Form |
|---|---|---|
| Mounjaro | Type 2 diabetes (adults) | Subcutaneous injection, once-weekly, 2.5-15 mg |
| Zepbound | Chronic weight management (BMI ≥30, or ≥27 with comorbidity) | Subcutaneous injection, once-weekly, 2.5-15 mg |
Clinical Trial Outcomes
SURMOUNT-1 (Weight Management)
72-week RCT of 2,539 adults with obesity (BMI ≥30) without diabetes. Results at highest dose (15 mg):
- 20.9% average body weight reduction vs. 3.1% on placebo.
- 91% of participants lost ≥5% of body weight.
- 57% lost ≥20% of body weight.
- Average waist circumference reduction: ~18.9 cm (7.4 inches).
SURMOUNT-5 (Head-to-Head vs. Semaglutide)
A 72-week direct comparison found tirzepatide produced ~20.2% weight loss vs. 13.7% for semaglutide, a ~47% relative improvement with tirzepatide.
SURPASS Trials (Type 2 Diabetes)
Across the SURPASS program, tirzepatide reduced HbA1c by 2.0-2.6 percentage points, the strongest glycemic effect seen with any injectable non-insulin diabetes therapy.
Dosing and Administration
Tirzepatide is administered as a once-weekly subcutaneous injection. Titration is essential to minimize GI side effects:
- Start: 2.5 mg weekly × 4 weeks (non-therapeutic starting dose).
- Escalate: 5 mg, then 7.5, 10, 12.5, 15 mg in 4-week intervals as tolerated.
- Maintenance: any of 5, 10, 15 mg based on individual tolerance and response.
Side Effects
Most common adverse events in SURMOUNT-1 (15 mg arm):
- Nausea (39%)
- Diarrhea (22%)
- Constipation (17%)
- Vomiting (13%)
- Decreased appetite (11%)
Like semaglutide, tirzepatide carries a boxed warning for thyroid C-cell tumors. Contraindicated in patients with personal/family history of medullary thyroid carcinoma or MEN 2.
Compounded Tirzepatide
Compounded tirzepatide refers to tirzepatide prepared by state-licensed 503A compounding pharmacies using pharmaceutical-grade active ingredient. The molecule is identical to the branded drug; compounded formulations are not FDA-approved finished products. FDA regulatory status has evolved with Mounjaro/Zepbound shortage determinations.
References
- Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022;387(3):205-216.
- Aronne LJ, Sattar N, Horn DB, et al. Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial. JAMA. 2024;331(1):38-48.
- Frias JP, Davies MJ, Rosenstock J, et al. Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes. N Engl J Med. 2021;385(6):503-515.
- Eli Lilly. Mounjaro (tirzepatide) prescribing information. FDA label.
- Eli Lilly. Zepbound (tirzepatide) prescribing information. FDA label.