Tirzepatide

Tirzepatide is a synthetic dual agonist of the glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor — the first agent in its class to target both receptors in a single molecule. This dual mechanism distinguishes it pharmacologically from all GLP-1 receptor agonists; some researchers refer to the class as "twincretins" to reflect that tirzepatide activates GIP signaling in addition to GLP-1 signaling. GIP is an incretin hormone secreted in the upper small intestine in response to fat and carbohydrate intake; GIP receptor activation augments glucose-stimulated insulin secretion, reduces glucagon secretion in a glucose-dependent manner, and appears to modulate adipose tissue metabolism and energy expenditure through mechanisms still under investigation. The combined GIP and GLP-1 effect produces glucose lowering and weight reduction effects that, in clinical trials, have exceeded those observed with GLP-1 mono-agonists.¹²

Tirzepatide has an elimination half-life of approximately 5 days, enabling once-weekly subcutaneous injection. It is manufactured by Eli Lilly and sold under two brand names: Mounjaro (for type 2 diabetes) and Zepbound (for weight management and obstructive sleep apnea).¹²

Mounjaro (tirzepatide injection):

• Adjunct to diet and exercise to improve glycemic control in adults with type 2 diabetes mellitus.¹

Zepbound (tirzepatide injection):

• Chronic weight management in adults with an initial BMI of ≥30 kg/m² (obesity), or ≥27 kg/m² (overweight) with at least one weight-related comorbid condition, as an adjunct to a reduced-calorie diet and increased physical activity.
• Treatment of moderate-to-severe obstructive sleep apnea in adults with obesity (indication added December 2024).²

Off-label use: Mounjaro is frequently prescribed off-label for weight management in patients without a diabetes diagnosis; Zepbound is the approved label for that use.

All tirzepatide products use a structured dose-escalation schedule.¹²

Starting dose is 2.5 mg subcutaneously once weekly for 4 weeks. The dose is then increased in 2.5 mg increments at approximately 4-week intervals based on response and tolerability. Available doses are 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg. The 2.5 mg dose is for treatment initiation only and is not approved as a maintenance dose.

For type 2 diabetes (Mounjaro), the dose is titrated to the lowest effective maintenance dose based on glycemic response. For weight management (Zepbound), the target maintenance dose is typically 10 mg or 15 mg once weekly.

Injection is subcutaneous in the abdomen, thigh, or upper arm; injection site should be rotated.

Real-world practice: Same as semaglutide — the 4-week escalation is a minimum. Many patients benefit from 6–8 weeks at each step. GI side effects are the primary reason for a slower pace. The 2.5mg starting dose is for initiation only and is not an effective maintenance dose.

Subcutaneous injection.

Unopened pens or vials: refrigerate at 36°F to 46°F (2°C to 8°C); do not freeze. After first use: store at room temperature below 86°F (30°C) or refrigerated; use within 21 days for vials and 21 days for pens.¹ Keep away from direct heat and light.

Boxed warning: Tirzepatide causes dose-dependent and duration-dependent thyroid C-cell tumors in rats. It is unknown whether tirzepatide causes thyroid C-cell tumors including medullary thyroid carcinoma (MTC) in humans. Tirzepatide is contraindicated in patients with a personal or family history of MTC or MEN 2.¹

Key warnings and considerations from the FDA labels:¹²

• Pancreatitis: Cases of acute pancreatitis have been reported. Discontinue if suspected; do not restart if confirmed.
• Gastrointestinal effects: Nausea, diarrhea, vomiting, and constipation are the most common adverse effects, predominantly during dose escalation. Tolerating the escalation schedule reduces discontinuation rates.
• Hypoglycemia: Low risk with tirzepatide alone; substantially elevated risk when combined with insulin or sulfonylureas. Dose reduction of the insulin or secretagogue is often needed at initiation.
• Oral contraceptives: Tirzepatide delays gastric emptying and may reduce absorption of orally administered medications. Females using oral contraceptives should switch to a non-oral method or add a barrier method for 4 weeks after each dose initiation or escalation.¹
• Gallbladder disease: Rapid weight loss of any kind increases the risk of cholelithiasis (gallstones) and cholecystitis. Cases have been reported in tirzepatide clinical trials.
• Heart rate increase: Modest mean increases in resting heart rate (approximately 2–4 bpm) have been observed in clinical trials.
• Pregnancy: May cause fetal harm. Discontinue when pregnancy is recognized; females should switch to a non-tirzepatide-based approach at least 4 weeks before attempting conception (manufacturer recommendation varies — consult prescribing information).¹
• Compounded tirzepatide: As of late 2024–early 2025, following removal of tirzepatide from the FDA drug shortage list, the FDA has taken regulatory action against compounding of tirzepatide, indicating that compounded versions are not FDA-approved alternatives. Individuals using compounded tirzepatide should be aware of this regulatory status.