Semaglutide and tirzepatide are the two most important peptide-based drugs in metabolic medicine. Both target GLP-1 receptors, but tirzepatide adds GIP (glucose-dependent insulinotropic polypeptide) receptor agonism, creating a dual mechanism. This comparison covers efficacy, safety, access, and cost in the UK context.
Mechanism Comparison
| Feature | Semaglutide | Tirzepatide |
|---|---|---|
| Drug class | GLP-1 receptor agonist | Dual GIP/GLP-1 receptor agonist |
| Brand names | Wegovy (obesity), Ozempic (T2D) | Mounjaro (T2D), Zepbound (obesity, US) |
| Manufacturer | Novo Nordisk | Eli Lilly |
| Administration | Weekly subcutaneous injection | Weekly subcutaneous injection |
| Maintenance dose | 2.4 mg (obesity) / 1.0 mg (T2D) | 15 mg (max) / 5, 10, 15 mg |
Weight Loss Efficacy
No direct head-to-head trial for weight management has been completed, but cross-trial comparison of the major obesity studies provides useful context:
| Trial | Drug | Average Weight Loss | Duration |
|---|---|---|---|
| STEP 1 (NEJM, 2021) | Semaglutide 2.4 mg | 14.9% | 68 weeks |
| SURMOUNT-1 (NEJM, 2022) | Tirzepatide 15 mg | 22.5% | 72 weeks |
| SURMOUNT-1 | Tirzepatide 10 mg | 19.5% | 72 weeks |
| SURMOUNT-1 | Tirzepatide 5 mg | 15.0% | 72 weeks |
Even the lowest tirzepatide dose (5 mg) matched semaglutide's highest dose (2.4 mg) for weight loss. The 15 mg dose produced approximately 50% more weight loss. These are cross-trial comparisons and carry methodological caveats, but the difference is consistently observed.
Diabetes Control (HbA1c)
The SURPASS-2 trial directly compared tirzepatide with semaglutide 1.0 mg in type 2 diabetes patients. Tirzepatide at all doses (5, 10, 15 mg) produced significantly greater HbA1c reductions than semaglutide 1.0 mg (Frias et al., NEJM, 2021).
Side Effect Comparison
| Side Effect | Semaglutide 2.4 mg | Tirzepatide 15 mg |
|---|---|---|
| Nausea | 44% | 31% |
| Diarrhoea | 30% | 23% |
| Vomiting | 24% | 17% |
| Constipation | 24% | 19% |
Tirzepatide appears to have a somewhat better GI tolerability profile at equivalent or superior efficacy, possibly because the GIP receptor component counterbalances some of the GLP-1-mediated nausea pathways.
Muscle Loss Concerns
Both drugs cause significant lean mass loss alongside fat loss. Approximately 25-39% of total weight lost is lean mass across both drug classes. This is a growing concern, particularly for older adults and those without a structured resistance training programme. Strategies for preventing muscle loss on GLP-1 therapy are becoming a critical part of treatment planning.
UK Availability and Pricing
| Semaglutide (Wegovy) | Tirzepatide (Mounjaro) | |
|---|---|---|
| NHS availability | Yes (NICE TA875, 2023) | Yes (NICE TA, late 2024) |
| Oral form | US launch Jan 2026; UK pending | Phase III trials ongoing |
Which Should You Choose?
For patients focused primarily on maximum weight loss, tirzepatide appears to have a clear efficacy advantage based on available data. For those already established on semaglutide with good results and tolerability, switching may not be necessary. Cost, availability, and prescriber preference also influence the decision.
For the broader context of GLP-1 peptides, including Retatrutide and oral formulations, see our complete GLP-1 guide.
Key References
- Wilding, J.P.H. et al. (2021). "Once-weekly semaglutide in adults with overweight or obesity (STEP 1)." NEJM, 384, 989-1002.
- Jastreboff, A.M. et al. (2022). "Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1)." NEJM, 387, 205-216.
- Frias, J.P. et al. (2021). "Tirzepatide versus semaglutide once weekly in patients with type 2 diabetes (SURPASS-2)." NEJM, 385, 503-515.