Tirzepatide

Tirzepatide is a once-weekly subcutaneous injection developed by Eli Lilly and Company. It is a synthetic 39-amino-acid peptide that functions as a dual agonist of two incretin hormone receptors: the glucagon-like peptide-1 (GLP-1) receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor. This dual mechanism distinguishes it from earlier GLP-1-only agonists such as semaglutide (Ozempic/Wegovy).

Tirzepatide received FDA approval in May 2022 under the brand name Mounjaro for the treatment of type 2 diabetes in adults. In November 2023, the FDA approved tirzepatide under the brand name Zepbound specifically for chronic weight management in adults with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with at least one weight-related condition such as hypertension, dyslipidemia, type 2 diabetes, obstructive sleep apnea, or cardiovascular disease.

Tirzepatide is the most widely prescribed injectable weight-management medication in the United States as of 2026, with broad coverage by commercial insurance and a dedicated patient savings program from Eli Lilly. Its approval for both type 2 diabetes (Mounjaro) and obesity (Zepbound) makes it unique among FDA-approved GLP-1 class drugs in having separate labeled indications and separate branded products for each indication.

Important context: Tirzepatide is FDA-approved for the specific indications described above. Use for any other purpose — including off-label use in individuals who do not meet the labeled BMI/comorbidity criteria — is not FDA-sanctioned and should only be considered under physician supervision after a careful benefit-risk discussion.

Tirzepatide's metabolic effects arise from simultaneous activation of two distinct incretin hormone receptors:

• GLP-1 receptor agonism: GLP-1 (glucagon-like peptide-1) is secreted by intestinal L-cells in response to food intake. GLP-1 receptor activation stimulates glucose-dependent insulin secretion from pancreatic beta cells, suppresses glucagon release from alpha cells, slows gastric emptying, and activates hypothalamic satiety pathways — reducing appetite and total caloric intake. This is the primary mechanism shared with semaglutide.
• GIP receptor agonism: GIP (glucose-dependent insulinotropic polypeptide) is secreted by intestinal K-cells. GIP receptor activation amplifies glucose-stimulated insulin secretion and exerts favorable effects on adipose tissue metabolism. The combination of GLP-1 and GIP agonism produces synergistic appetite suppression and metabolic effects beyond what either receptor pathway achieves alone — which is the proposed reason tirzepatide produces greater mean weight loss than semaglutide in head-to-head comparisons.

Tirzepatide's GLP-1 activity is approximately equivalent to that of semaglutide at therapeutic doses, while its GIP agonism adds a complementary metabolic layer. This dual-receptor architecture is associated with greater reductions in fasting glucose, HbA1c, body weight, waist circumference, and triglycerides compared to GLP-1 monotherapy in head-to-head trials.

Tirzepatide has one of the most robust clinical development programs of any obesity pharmacotherapy, with Phase 3 data across multiple large randomized controlled trials.

SURMOUNT Program (Obesity)

• SURMOUNT-1 (NEJM 2022): 2,539 adults with obesity or overweight plus a comorbidity were randomized to tirzepatide 5 mg, 10 mg, or 15 mg weekly, or placebo, over 72 weeks alongside reduced-calorie diet and increased physical activity. Mean weight loss at 72 weeks: 15.0% (5 mg), 19.5% (10 mg), 20.9% (15 mg) versus 3.1% placebo. At the highest dose, 57.4% of participants achieved ≥20% body weight reduction. PMID: 35658024.
• SURMOUNT-2 (Lancet 2023): 938 adults with type 2 diabetes and obesity achieved mean weight loss of 13.4% (10 mg) and 15.7% (15 mg) at 72 weeks — remarkable in a population where diabetes limits GLP-1 drug weight-loss magnitude.
• SURMOUNT-3 (Nat Med 2023): Evaluated intensive lifestyle intervention as lead-in before tirzepatide initiation; participants achieved 18.4% additional body weight reduction during tirzepatide treatment (15 mg), on top of 6.9% lost during lifestyle lead-in — total ~25%.
• SURMOUNT-4 (JAMA 2023): Randomized withdrawal trial demonstrating that discontinuing tirzepatide leads to significant weight regain — underscoring that chronic obesity pharmacotherapy requires ongoing treatment to maintain results.

SURPASS Program (Type 2 Diabetes)

• SURPASS-2 (NEJM 2021): Tirzepatide 5 mg, 10 mg, and 15 mg were superior to semaglutide 1 mg in HbA1c reduction (−2.01% to −2.30% vs −1.86%) and weight loss (−6.2 kg to −11.2 kg vs −5.7 kg) over 40 weeks. This is the only published head-to-head RCT between the two drugs.
• SURPASS-CVOT (Lancet 2025): The cardiovascular outcomes trial demonstrated that tirzepatide significantly reduced the composite risk of major adverse cardiovascular events (MACE) versus placebo in patients with type 2 diabetes and established cardiovascular disease. This extended the cardiovascular benefit previously shown for GLP-1 agonists to the dual GLP-1/GIP class.

The safety profile of tirzepatide is consistent with the GLP-1 receptor agonist drug class, with some modifications attributable to its GIP component.

Common Adverse Effects

• Gastrointestinal (most common): Nausea (18–22%), diarrhea (17–20%), vomiting (9–11%), constipation (11–14%), and abdominal discomfort. These effects are dose-dependent and most prevalent during dose escalation, typically resolving over time. Managing the dose escalation rate reduces GI event frequency.
• Decreased appetite: Clinically intentional but can progress to insufficient caloric intake or disordered eating patterns in susceptible individuals. Protein intake monitoring is advised.
• Injection site reactions: Mild redness, bruising, or discomfort at the injection site.

Serious Adverse Effects and Warnings

• Thyroid C-cell tumors: Tirzepatide carries a black box warning for thyroid C-cell tumors, based on rodent carcinogenicity studies with GLP-1 agonists. The relevance to humans is unknown. Tirzepatide is contraindicated in individuals with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2).
• Pancreatitis: Acute pancreatitis has been reported with GLP-1 agonists. Tirzepatide should be discontinued if pancreatitis is suspected and not restarted if confirmed. Contraindicated in individuals with a history of drug-induced pancreatitis.
• Gallbladder disease: Rapid weight loss is associated with gallstone formation and acute cholecystitis. Cases were reported in SURMOUNT trials. Consider gallbladder disease evaluation in patients with abdominal symptoms.
• Hypoglycemia: Risk is low when tirzepatide is used alone. Risk increases significantly when used with insulin or insulin secretagogues (sulfonylureas). Dose reduction of those agents is typically recommended.
• Lean mass loss: Significant weight loss with GLP-1/GIP drugs involves both fat and lean mass reduction. In SURMOUNT trials, approximately 25–40% of total weight lost was lean mass. High protein intake (1.2–1.6 g/kg body weight) and resistance training are widely recommended to mitigate this.
• Acute kidney injury: Dehydration secondary to nausea, vomiting, or diarrhea can precipitate acute kidney injury. Maintain adequate hydration.
• Contraindications: Pregnancy (category C), breastfeeding, personal/family history of MTC or MEN 2, known hypersensitivity to tirzepatide or any excipient.

The following dosing information reflects the FDA-approved prescribing information for tirzepatide. This is for educational reference. Dosing should be managed by a prescribing clinician.

FDA-Approved Dosing Schedule (Mounjaro / Zepbound)

• Starting dose: 2.5 mg subcutaneous injection once weekly for 4 weeks
• Dose escalation: Increase by 2.5 mg every 4 weeks as tolerated
• Maintenance doses: 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg weekly
• Maximum approved dose: 15 mg once weekly

Administration

• Subcutaneous injection in the abdomen, thigh, or upper arm; rotate injection sites each week
• Administer on the same day each week, with or without meals
• If a dose is missed and the next scheduled dose is more than 4 days away, administer the missed dose as soon as possible; if fewer than 4 days remain, skip the missed dose and resume the regular schedule

Compounding Status (2024–2026)

During a documented shortage of FDA-approved tirzepatide (2022–2024), FDA enforcement policies permitted compounding pharmacies to produce tirzepatide copies. As of late 2024 and through 2025, the FDA declared the tirzepatide shortage resolved and issued warning letters to compounders. As of 2026, compounded tirzepatide is not FDA-authorized for commercial distribution. Patients should use only FDA-approved branded Mounjaro or Zepbound from licensed pharmacies.

The most clinically relevant comparison for tirzepatide is against semaglutide (Ozempic/Wegovy), the other leading GLP-1-class obesity and diabetes drug.

In the SURPASS-2 trial — the only published head-to-head RCT — tirzepatide at all doses (5 mg, 10 mg, 15 mg) produced greater HbA1c reduction and weight loss compared to semaglutide 1 mg subcutaneous over 40 weeks. There is no published head-to-head RCT comparing tirzepatide to high-dose semaglutide 2.4 mg (Wegovy) in an obesity-focused trial. Indirect comparisons suggest tirzepatide likely produces greater weight loss, but cross-trial comparisons are limited by differences in patient populations, trial durations, and designs.

• United States: FDA-approved. Mounjaro (tirzepatide, 2.5–15 mg) approved May 2022 for type 2 diabetes. Zepbound (tirzepatide, 2.5–15 mg) approved November 2023 for chronic weight management. Requires a valid prescription from a licensed prescriber. DEA Schedule: not a controlled substance.
• European Union: Mounjaro approved by the EMA in September 2022 for type 2 diabetes. Approval for obesity indication (under a separate brand or labeling update) followed in 2023.
• United Kingdom: Approved by the MHRA for type 2 diabetes (Mounjaro) and obesity.
• Off-label use: Tirzepatide is sometimes prescribed off-label for individuals who do not meet the strict FDA-labeled BMI thresholds or comorbidity criteria. Such use is not FDA-sanctioned but is legally permissible under physician prescribing discretion in the United States.
• Compounding: As of 2026, FDA-approved tirzepatide shortage designations have been resolved and compounding is no longer authorized. Patients should use only branded Mounjaro or Zepbound from licensed pharmacies.

Tirzepatide is FDA-approved for two distinct patient populations:

• Mounjaro (type 2 diabetes): Adults with type 2 diabetes mellitus as an adjunct to diet and exercise to improve glycemic control. Used as monotherapy or in combination with other antidiabetic agents (with the exception of other GLP-1 agonists). Not approved for type 1 diabetes.
• Zepbound (chronic weight management): Adults with an initial BMI of ≥30 kg/m², or ≥27 kg/m² with at least one weight-related comorbidity (hypertension, dyslipidemia, type 2 diabetes mellitus, obstructive sleep apnea, or cardiovascular disease). Indicated as an adjunct to a reduced-calorie diet and increased physical activity.

Individuals with a personal or family history of medullary thyroid carcinoma, Multiple Endocrine Neoplasia syndrome type 2, or known hypersensitivity to tirzepatide are not candidates for treatment. A thorough discussion of risks and benefits with a prescribing clinician is required before initiating therapy.