Tirzepatide and semaglutide are currently the two most prescribed injectable weight loss treatments in the UK private market. Both are GLP-1-based medications, but they differ in their mechanism, efficacy, dosing schedule, and cost. This guide provides a comprehensive, evidence-based comparison.
Semaglutide (Wegovy, Ozempic) is a GLP-1 receptor agonist — it mimics the glucagon-like peptide-1 hormone, which suppresses appetite, slows gastric emptying, and improves insulin sensitivity. It was the first injectable treatment to achieve widespread use for weight management in the UK.
Tirzepatide (Mounjaro) is a dual GIP and GLP-1 receptor agonist — it activates both the glucose-dependent insulinotropic polypeptide (GIP) and GLP-1 pathways simultaneously. This dual mechanism appears to produce synergistic effects on appetite suppression and metabolic regulation, resulting in significantly greater weight loss in clinical trials.
| Drug | Trial | Duration | Average Weight Loss | ≥20% Loss |
|---|---|---|---|---|
| Tirzepatide 15mg (Mounjaro) | SURMOUNT-1 | 72 weeks | 22.5% | 57% |
| Tirzepatide 10mg (Mounjaro) | SURMOUNT-1 | 72 weeks | 19.5% | 49% |
| Semaglutide 2.4mg (Wegovy) | STEP 1 | 68 weeks | 14.9% | 32% |
| Semaglutide 1mg (Ozempic) | SUSTAIN 6 | 104 weeks | ~4.3% | N/A |
| Liraglutide 3mg (Saxenda) | SCALE | 56 weeks | 8.4% | 14% |
Direct head-to-head trials (SURMOUNT-5) confirm tirzepatide produces ~47% more weight loss than semaglutide 2.4mg (Wegovy) over 72 weeks.
| Feature | Tirzepatide (Mounjaro) | Semaglutide (Wegovy) |
|---|---|---|
| Injection frequency | Once weekly | Once weekly |
| Starting dose | 2.5mg | 0.25mg |
| Maintenance dose | 5–15mg | 2.4mg |
| Titration period | ~20 weeks to 15mg | ~16 weeks to 2.4mg |
| Pen type | Single-use autoinjector | Single-use autoinjector |
| Storage | Refrigerated | Refrigerated |
Both drugs share a similar GI side effect profile, particularly during dose escalation. Nausea, vomiting, and diarrhoea are the most commonly reported effects and typically improve after the first few weeks at each dose level.
| Treatment | Starting Dose Price | Maintenance Dose Price | Discount Codes |
|---|---|---|---|
| Mounjaro (tirzepatide) | From £139/month (2.5mg) | From £219/month (15mg) | Yes |
| Wegovy (semaglutide) | From £149/month (0.25mg) | From £249/month (2.4mg) | Yes |
| Ozempic (semaglutide) | From £89/month (0.5mg) | From £119/month (2mg) | Yes |
If maximum weight loss is your primary goal and you can tolerate the GI side effects, tirzepatide (Mounjaro) is currently the most effective option available in the UK based on clinical evidence. It produces roughly 50% more weight loss than semaglutide (Wegovy) in head-to-head trials.
Semaglutide (Wegovy) remains an excellent option — particularly for people who have previously used semaglutide for diabetes (Ozempic) and tolerated it well, or for those where Mounjaro is temporarily unavailable.