Tirzepatide (Mounjaro, Zepbound)
Tirzepatide (LY3298176)
A first-in-class dual GIP and GLP-1 receptor agonist approved by the FDA for type 2 diabetes (Mounjaro) and chronic weight management (Zepbound). Administered once weekly via subcutaneous injection. Demonstrated superior weight loss vs semaglutide in head-to-head trials.
Last updated: 2026-03-12
Safety Summary
GI adverse events are the primary dose-limiting toxicities and most common reason for discontinuation (~10% at 15 mg). Incidence is dose-dependent: pooled GI AE rates ~39% at 5 mg, ~46% at 10 mg, ~49% at 15 mg. GI symptoms typically improve with time and slow titration. Severe GI events can lead to dehydration and acute kidney injury. Thyroid C-cell tumor boxed warning (rodent finding, human relevance unknown). No evidence of clinically meaningful tolerance/tachyphylaxis through 104 weeks of follow-up. Weight regain occurs upon discontinuation (~5.63 kg average in obesity populations). Anti-tirzepatide antibodies detected in some patients but with minimal impact on PK/efficacy per Phase 3 analysis.
Known Side Effects
Very common (12-35.5% across doses)
Very common (12-22%)
Common (2-16.3%)
Common
Common
Common
Common
Common
Common
Uncommon (RR ~1.97 in meta-analysis)
Rare
Common when combined (~22.6% in some trial arms)
Rare
Rare
Who Should NOT Use This
Rodent C-cell tumor finding; GLP-1/GIP class effect. Human relevance unknown but warrants contraindication.
Patients with MEN 2 are at increased risk for MTC.
Previous anaphylaxis or angioedema to the drug or its excipients.
Animal reproductive studies showed fetal effects. Use non-oral contraceptive methods during titration (delayed gastric emptying may reduce oral contraceptive absorption).
Overlapping mechanisms; increased AE risk.
Increased hypoglycemia risk; reduce concomitant secretagogue/insulin dose.
Tirzepatide delays gastric emptying.
Discontinue if pancreatitis suspected.
Delayed gastric emptying may alter absorption of oral drugs including oral contraceptives, warfarin, digoxin.
Talk to Your Doctor
Before considering Tirzepatide (Mounjaro, Zepbound), discuss it with your healthcare provider. Ask about potential interactions with your current medications, whether it is appropriate for your health conditions, and what monitoring may be needed.
Sources: [1-6]
Evidence Assessment
FDA-approved drug with multiple indications. Mounjaro (NDA 215866, approved May 13, 2022) for T2DM; Zepbound (NDA 217806, approved Nov 8, 2023) for chronic weight management; OSA indication approved Dec 2024. Supported by extensive Phase 3 programs: SURPASS (5+ trials in T2DM, >6000 participants), SURMOUNT (4+ trials in obesity, >5000 participants). Head-to-head superiority vs semaglutide demonstrated. Also authorized by EMA, TGA, Health Canada.
1Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1)NCT04184622PMID 35658024
Jastreboff AM, Aronne LJ, Ahmad NN, et al. - New England Journal of Medicine (2022) - Phase 3 RCT - 2539
Mean weight change at 72 weeks: 5mg -15.0%, 10mg -19.5%, 15mg -20.9% vs placebo -3.1%. 85-91% of tirzepatide groups achieved -¥5% weight loss vs 35% placebo.
Limitations: 72-week duration; predominantly US/European population. Industry-sponsored (Eli Lilly).
2Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes (SURPASS-2)NCT03987919PMID 34170647
FrÃas JP, Davies MJ, Rosenstock J, et al. - New England Journal of Medicine (2021) - Phase 3 RCT (active comparator) - 1879
At 40 weeks, tirzepatide (5, 10, and 15 mg) was superior to semaglutide (1 mg) in reducing HbA1c (mean change: -2.01 to -2.30% vs -1.86%) and body weight (-7.6 to -11.2 kg vs -5.7 kg).
Limitations: Semaglutide comparator dose (1.0 mg) was not the maximum approved weight-loss dose (2.4 mg). Open-label design. Industry-sponsored.
3Tirzepatide vs Semaglutide in Adults with Obesity without Diabetes (head-to-head)NCT05822830
NEJM 2025 - New England Journal of Medicine (2025) - Phase 3b RCT (active comparator)
Tirzepatide demonstrated superior weight loss versus semaglutide 2.4 mg with higher proportions achieving -¥10%, -¥15%, and -¥20% thresholds.
Limitations: Specific sample size not extracted from available raw data.
4Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity (SURMOUNT-4)NCT04660643PMID 38079201
Aronne LJ, Sattar N, Horn DB, et al. - JAMA (2024) - Phase 3 Randomized Withdrawal Trial - 670
After 36-week tirzepatide lead-in, participants randomized to continue lost additional 5.5% body weight; those switched to placebo regained 14.0% over 52 weeks. Demonstrates continued treatment required for maintenance.
Limitations: Lead-in design; limited to prior responders. Industry-sponsored.
5Tirzepatide after intensive lifestyle intervention in adults with overweight or obesity (SURMOUNT-3)
Wadden TA et al. - Nature Medicine (2023) - Phase 3 RCT - 579
Additional weight change from randomization: tirzepatide -18.4% vs placebo +2.5%; treatment difference -20.8 pp (95% CI -23.2 to -18.5, P<0.001).
Limitations: Post-intensive-lifestyle-intervention population.
6Tirzepatide in Adults With Type 1 Diabetes: A Phase 2 Randomized Placebo-Controlled Clinical Trial
Snaith AG et al. - Diabetes Care (2026) - Phase 2 RCT - 24
Weight change: tirzepatide -10.3 kg vs placebo -0.7 kg (difference -8.7 kg, P<0.0001). Insulin dose reduced ~35%. First RCT of tirzepatide in T1DM.
Limitations: Very small sample (n=24); short duration (12 weeks).