Retatrutide vs Tirzepatide: Complete Comparison
How does retatrutide (the upcoming triple agonist) compare to tirzepatide (Mounjaro)? This guide compares their mechanisms, trial results, availability, and what the differences mean for you.
- • ~24% weight loss (Phase 2)
- • Phase 3 trials ongoing
- • UK availability: 2027+ (estimated)
- • ~21% weight loss (Phase 3)
- • MHRA approved, NICE recommended
- • Available NHS & privately
The Key Difference: Glucagon
Both medications target GLP-1 and GIP receptors. The crucial difference is that retatrutide also targets the glucagon receptor - hence "triple agonist" vs "dual agonist."
Reduces appetite, slows gastric emptying, improves blood sugar. The primary driver of weight loss.
Enhances insulin secretion, may improve fat metabolism. Amplifies GLP-1 effects.
May increase energy expenditure (calorie burning). This is the additional component in retatrutide.
Why glucagon matters: While GLP-1 and GIP primarily reduce how much you eat (energy intake), glucagon may increase how many calories your body burns (energy expenditure). This dual approach of reducing intake AND increasing expenditure could explain retatrutide's higher weight loss in trials. However, this is theoretical and needs confirmation in Phase 3 trials.
Detailed Comparison
| Category | Retatrutide | Tirzepatide |
|---|---|---|
| Generic Name | Retatrutide (LY3437943) | Tirzepatide |
| Brand Name | None (not approved) | Mounjaro, Zepbound |
| Manufacturer | Eli Lilly | Eli Lilly |
| Mechanism | Triple agonist (GLP-1 + GIP + Glucagon) | Dual agonist (GLP-1 + GIP) |
| Dosing Frequency | Weekly (expected) | Weekly |
| Weight Loss (Trials) | ~24% (Phase 2, preliminary) | ~21% (Phase 3, confirmed) |
| Trial Programme | TRIUMPH (ongoing) | SURMOUNT (completed) |
| Development Phase | Phase 3 (ongoing) | Approved |
| UK MHRA Status | Not submitted | Approved |
| UK NHS Availability | No | Yes (with criteria) |
| UK Private Availability | No | Yes |
| Expected UK Launch | 2027+ (speculative) | Available now |
Clinical Trial Results Compared
- ~24% weight loss at highest dose (12mg)
- 48-week trial duration
- ~2% HbA1c reduction (diabetes patients)
- 338 participants in Phase 2 obesity trial
Important: Phase 2 data only. Results may differ in larger Phase 3 trials. Not yet peer-reviewed long-term.
- ~21% weight loss at highest dose (15mg)
- 72-week trial duration
- ~2.1% HbA1c reduction (SURPASS trials)
- 2,539 participants in SURMOUNT-1
Robust data: Phase 3 completed with large cohort. Results led to regulatory approval. Long-term data available.
Why You Can't Directly Compare the Numbers
While retatrutide's ~24% appears higher than tirzepatide's ~21%, direct comparison is not valid because: (1) different trial phases with different sample sizes, (2) different trial durations, (3) different patient populations, (4) no head-to-head comparison trials. Only a direct comparison study could definitively show which is more effective.
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UK Availability: The Practical Difference
- • Not approved anywhere in the world
- • Cannot be legally prescribed
- • No legitimate purchase source
- • Clinical trial only (if eligible)
- • UK availability: 2027+ at earliest
- • MHRA approved for T2D and weight loss
- • NICE recommended for NHS use
- • Available through NHS (with criteria)
- • Available through private prescription
- • GP can prescribe in many areas (2025)
The Bottom Line
If you're considering treatment now, tirzepatide (Mounjaro) is your option. Retatrutide may eventually offer slightly better results, but it could be years away with no guaranteed approval. Many patients achieve excellent results with tirzepatide. Consider starting treatment rather than waiting for an unapproved medication.
Side Effect Comparison
Both medications share similar gastrointestinal side effects due to their GLP-1 activity. Retatrutide may have additional effects from its glucagon component.
- • Nausea (especially when starting)
- • Diarrhoea
- • Vomiting
- • Constipation
- • Abdominal pain
- • Reduced appetite
- • Increased heart rate (glucagon effect)
- • Full profile still being established
- • Long-term effects unknown
Note: Phase 3 trials will provide more complete safety data.
What Should You Do?
Consider Tirzepatide (Mounjaro) Now If:
- • You want to start weight loss treatment now
- • You meet NHS eligibility criteria
- • You prefer a medication with established safety data
- • You don't want to wait years for an alternative
Consider Waiting/Monitoring Retatrutide If:
- • You're not in urgent need of treatment
- • You've tried other medications without success
- • You want to participate in clinical trials
- • You understand it may never be approved
Frequently Asked Questions
Related Guides
References & Sources
- Jastreboff AM, et al. Retatrutide Phase 2 trial. NEJM 2023.
- Jastreboff AM, et al. SURMOUNT-1: Tirzepatide for obesity. NEJM 2022.
- MHRA. Mounjaro (tirzepatide) approval documentation.
- NICE Technology Appraisal: Tirzepatide for weight management.
Medical Disclaimer
This comparison is for educational purposes only and is not medical advice. Retatrutide is an investigational medication not approved for use. Tirzepatide requires a prescription and medical assessment. Consult with a qualified healthcare professional before starting any weight loss medication.
Last updated: December 2025
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