Semaglutide and tirzepatide are two of the most studied peptide-based compounds in metabolic medicine research. Both target GLP-1 receptors, but tirzepatide adds GIP (glucose-dependent insulinotropic polypeptide) receptor agonism, creating a dual mechanism. This comparison covers trial efficacy, reported adverse events, regulatory 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 |
| Trial cohort range | Lower-dose to highest-dose cohorts (obesity/T2D indications) | Lower-dose to highest-dose cohorts |
Body-Weight Reduction 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 Body-Weight Reduction | Duration |
|---|---|---|---|
| STEP 1 (NEJM, 2021) | Semaglutide, highest-dose cohort | 14.9% | 68 weeks |
| SURMOUNT-1 (NEJM, 2022) | Tirzepatide, highest-dose cohort | 22.5% | 72 weeks |
| SURMOUNT-1 | Tirzepatide, mid-dose cohort | 19.5% | 72 weeks |
| SURMOUNT-1 | Tirzepatide, lowest-dose cohort | 15.0% | 72 weeks |
Even tirzepatide's lowest-dose cohort matched semaglutide's highest-dose cohort for body-weight reduction. The highest-dose tirzepatide cohort produced approximately 50% more body-weight reduction than semaglutide's highest-dose cohort. 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's higher-dose cohort in type 2 diabetes patients. Tirzepatide across its cohorts produced significantly greater HbA1c reductions than semaglutide's higher-dose cohort (Frias et al., NEJM, 2021).
Adverse Events Reported in Published Trials
| Adverse Event | Semaglutide, highest-dose cohort | Tirzepatide, highest-dose cohort |
|---|---|---|
| Nausea | 44% | 31% |
| Diarrhoea | 30% | 23% |
| Vomiting | 24% | 17% |
| Constipation | 24% | 19% |
Trial data suggests tirzepatide has a somewhat better gastrointestinal tolerability profile at equivalent or superior efficacy, possibly because the GIP receptor component counterbalances some of the GLP-1-mediated pathways implicated in nausea.
Lean Mass Research
Trial data indicates both compounds are associated with reductions in lean mass alongside fat mass reduction. Approximately 25-39% of total body-weight reduction across both drug classes has been attributed to lean mass in published data. This is a growing area of research interest, particularly regarding older adults and populations without structured resistance training. Ongoing research into lean-mass preservation during GLP-1 therapy is summarised in the myostatin blocker research overview.
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 |
Comparative Summary
Trial data indicates tirzepatide has a clear efficacy advantage for maximum body-weight reduction based on available evidence. For patients already established on semaglutide with satisfactory trial-consistent outcomes, switching may not be indicated by the evidence. Cost, availability, and prescriber assessment also factor into clinical decision-making.
For the broader context of GLP-1 peptides, including retatrutide and oral formulations, see the research overview.
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.