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 |
| Trial cohort range | Lower-dose to highest-dose cohorts (obesity/T2D indications) | Lower-dose to highest-dose cohorts |
Body-Weight Reduction Efficacy
One head-to-head trial for weight management has now been published. In SURMOUNT-5 (Aronne et al., 2025), 751 adults with obesity but without type 2 diabetes were randomised to tirzepatide or semaglutide; mean body-weight reduction at 72 weeks was 20.2% with tirzepatide and 13.7% with semaglutide. Cross-trial comparison of the major placebo-controlled obesity studies, STEP 1 (Wilding et al., 2021) and SURMOUNT-1 (Jastreboff et al., 2022), provides useful context. All figures below count every randomised participant, whether or not they stayed on treatment:
| 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 | 20.9% | 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 40% more body-weight reduction than semaglutide's highest-dose cohort. These are cross-trial comparisons and carry methodological caveats, but the direction of the difference was confirmed head to head in SURMOUNT-5.
Diabetes Control (HbA1c)
The SURPASS-2 trial directly compared three tirzepatide doses with semaglutide in 1,879 people with type 2 diabetes. Tirzepatide across its cohorts produced significantly greater HbA1c reductions than semaglutide at 40 weeks: 2.01 to 2.30 percentage points, against 1.86 (Frías et al., NEJM, 2021).
Adverse Events Reported in Published Trials
| Adverse Event | Semaglutide, highest-dose cohort | Tirzepatide, highest-dose cohort |
|---|---|---|
| Nausea | 44% | 31% |
| Diarrhoea | 32% | 23% |
| Vomiting | 25% | 12% |
| Constipation | 23% | 12% |
Semaglutide figures are from the STEP 1 results posted on ClinicalTrials.gov; tirzepatide figures are from Lilly's SURMOUNT-1 publication release. Cross-trial tables like this exaggerate differences: when the two drugs were compared directly in SURMOUNT-5, the posted results show nausea (44% vs 44%), diarrhoea (24% vs 23%) and constipation (27% vs 29%) at almost identical rates on tirzepatide and semaglutide, with vomiting less common on tirzepatide (15% vs 21%). The idea that the GIP receptor component counterbalances some of the GLP-1-mediated pathways implicated in nausea is supported so far by animal studies (Borner et al., 2021), not by trial data in people.
Lean Mass Research
Trial data indicates both compounds are associated with reductions in lean mass alongside fat mass reduction. Approximately 25 to 39% of total body-weight reduction across both drug classes has been attributed to lean mass in published data: a 2026 meta-analysis of 20 randomised trials (Eisa et al., 2026) put the figure at about 35% for semaglutide and 25% for tirzepatide. 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 TA1026, December 2024; phased roll-out) |
| Oral form | US launch Jan 2026; MHRA approved June 2026 (not on the NHS at approval) | None; Lilly's oral option is orforglipron, a different molecule (MHRA authorised August 2026) |
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.
References
- Aronne LJ, et al. (2025). Tirzepatide as Compared with Semaglutide for the Treatment of Obesity. New England Journal of Medicine. PubMed 40353578
- Borner T, et al. (2021). GIP Receptor Agonism Attenuates GLP-1 Receptor Agonist-Induced Nausea and Emesis in Preclinical Models. Diabetes. PubMed 34380697
- Eisa N, et al. (2026). Lean Mass Changes With Incretin Therapy Versus Lifestyle Intervention: A Systematic Review and Meta-Analysis of Randomised Controlled Trials. Diabetes, Obesity and Metabolism. PubMed 41877354
- Frías JP, et al. (2021). Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes. New England Journal of Medicine. PubMed 34170647
- Jastreboff AM, et al. (2022). Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine. PubMed 35658024
- Wilding JPH, et al. (2021). Once-Weekly Semaglutide in Adults with Overweight or Obesity. New England Journal of Medicine. PubMed 33567185