Two medications have transformed weight management in the UK: Wegovy (semaglutide) and Mounjaro (tirzepatide). Both are weekly injections that suppress appetite and promote sustained fat loss — but they work differently and cost differently. Here is how to choose.
How They Work
Wegovy contains semaglutide, a GLP-1 receptor agonist. It mimics the hormone GLP-1, which slows gastric emptying, reduces appetite, and prompts the pancreas to release insulin after meals.
Mounjaro contains tirzepatide, which is a dual GIP/GLP-1 agonist. In addition to all of Wegovy's mechanisms, it also activates GIP (glucose-dependent insulinotropic peptide) receptors — a second hormonal pathway that further amplifies satiety signals and fat-burning processes.
Efficacy: What Does the Clinical Evidence Say?
In the landmark SURMOUNT-1 trial, participants taking the maximum dose of Mounjaro (15mg) lost an average of 20.9% of their body weight over 72 weeks. By comparison, in the STEP-1 trial for Wegovy (2.4mg semaglutide), average weight loss was 14.9% over 68 weeks.
That said, both results are extraordinary compared to diet and exercise alone, and individual results vary considerably.
Availability in the UK
Both are widely available through GPhC-registered UK online pharmacies and private clinics. Wegovy received MHRA approval before Mounjaro and was briefly available through NHS-commissioned weight management services via Specialist Weight Management Services (SWMS). Mounjaro received UK National Institute for Health and Care Excellence (NICE) approval for use in England in 2024.
Key UK providers for both:
- Boots Online Doctor — both Mounjaro and Wegovy
- Voy — both, app-based programme
- Juniper UK — both, with AI coaching bundle
- UK Meds — both, with AI BMI verification
Cost Comparison (UK, 2025)
| Drug | Typical monthly cost (starter) | Max dose monthly | |------|-------------------------------|-----------------| | Wegovy 0.25mg (starter) | ~£140 | ~£260 | | Mounjaro 2.5mg (starter) | ~£130 | ~£280 |
Prices vary by provider. Introductory offers are common — always compare the ongoing monthly cost, not just the first month headline price.
Side Effects
Both medications share similar side effect profiles:
- Nausea and vomiting (most common in early weeks)
- Diarrhoea or constipation
- Fatigue
- Headache
- Injection site reactions
Mounjaro tends to cause slightly more nausea in some patients given its dual mechanism. Both medications are contraindicated in pregnancy, and should be stopped 2 months before planned conception.
Switching Between Mounjaro and Wegovy
Some patients switch medications if they plateau or experience intolerable side effects. Juniper UK is notable for allowing patients to switch from Mounjaro to Wegovy with only a 7-day gap — shorter than many other providers require.
Which Should You Choose?
- Choose Mounjaro if you want maximum weight loss efficacy and can tolerate the slightly higher chance of early nausea.
- Choose Wegovy if you prefer a longer clinical track record (semaglutide has been studied for longer), or if your provider doesn't stock Mounjaro.
- Ask your prescriber — they will weigh your health history, any diabetes-related considerations, and your weight loss goals before recommending one over the other.
Use the GLP-1 Directory to compare UK clinics offering both medications, with verified pricing and patient reviews.