Mounjaro is the brand name for tirzepatide developed by Eli Lilly. It belongs to a newer class than Ozempic — instead of activating a single receptor, it engages two: GLP-1 and GIP. The dual mechanism is what unlocks the larger weight-loss endpoint observed in published trials. This article maps the mechanism, the SURMOUNT trial data, and why two receptors beat one.
What tirzepatide is structurally
Tirzepatide is a synthetic 39 amino-acid peptide with a C20 fatty-diacid sidechain that binds serum albumin and extends plasma half-life to approximately 5 days. The sequence is designed to activate both the GLP-1 receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor — with a slight bias toward GIP at physiological concentrations.
Why GLP-1 + GIP beats GLP-1 alone
GLP-1 and GIP are the two main incretin hormones. They act in complementary ways:
| Receptor | Primary effects (research) |
|---|---|
| GLP-1R | Appetite suppression, gastric slowing, glucose-dependent insulin release |
| GIPR | Insulin sensitisation, adipose tissue lipid handling, possible nausea attenuation |
Activating both produces additive metabolic effects. GIP receptor activation also appears to reduce some of the gastrointestinal side effects typical of pure GLP-1 agonism, particularly nausea.
SURMOUNT trial data — the headline numbers
SURMOUNT-1 (Jastreboff et al., NEJM 2022) is the most-cited phase 3 result. Adults with obesity received weekly tirzepatide at 5, 10, or 15 mg or placebo for 72 weeks. Mean weight loss at the highest dose: 22.5% versus 2.4% on placebo. The 5 mg dose still produced 15.0% loss. SURMOUNT-3 and SURMOUNT-4 extended these findings into different cohorts with consistent results.
How that compares with Ozempic
The STEP 1 trial of semaglutide showed approximately 14.9% loss at 68 weeks. The SURMOUNT-1 trial of tirzepatide showed 22.5% loss at 72 weeks. Direct head-to-head data is now also available from SURPASS-2 (in diabetes) showing tirzepatide outperformed semaglutide on both glycaemic and weight endpoints.
Pharmacokinetic profile
- Plasma half-life ~5 days — supports weekly dosing
- Time to peak concentration: 8–72 hours after subcutaneous administration
- Steady state reached after approximately 4 weeks
- Primarily renal clearance
Side effect profile in trials
Most-reported adverse events match the incretin class: nausea (28% at 15 mg in SURMOUNT-1), diarrhoea, decreased appetite, vomiting, constipation. Most are mild-to-moderate and decrease with continued exposure. The dual-receptor activation appears to produce lower nausea rates than equivalent semaglutide doses in some research models.
Mounjaro vs Zepbound — same molecule, different label
Same naming logic as semaglutide brands. Tirzepatide is sold as Mounjaro for type 2 diabetes and as Zepbound for chronic weight management. Same molecule, same dose range, different indication on the box. See our detailed brand breakdown.
Research handling notes
Tirzepatide is supplied lyophilised. Reconstitution in bacteriostatic water at 5–10 mg/mL is standard for stability studies. The 39 amino-acid sequence is more sensitive to repeated freeze-thaw than shorter peptides — single freeze-thaw is acceptable, multiple cycles degrade integrity.
Chempeptides supplies HPLC-verified tirzepatide with batch-specific CoA — see the research catalogue for current availability.
Research use only. The data summarised above is from published clinical research literature.