Mounjaro vs Wegovy: Which is Better for Weight Loss?

Mounjaro (tirzepatide) and Wegovy (semaglutide) are the two most prominent prescription weight loss injections available in the UK. Both are once-weekly injectable medications that have shown significant results in clinical trials. However, they differ in their mechanism of action, efficacy data, dosing schedules, and cost. This guide compares both medications to help you have an informed discussion with your prescriber about which option may be more suitable for you. It is important to emphasise that the choice between these medications should always be made in consultation with a qualified prescriber who can assess your individual health profile.

The fundamental difference between Mounjaro and Wegovy is how they work at a molecular level. Wegovy (semaglutide) is a GLP-1 receptor agonist. It mimics the GLP-1 gut hormone, which slows gastric emptying, reduces appetite, and improves blood sugar regulation. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. It mimics both GIP and GLP-1, targeting two hormonal pathways simultaneously. GIP (glucose-dependent insulinotropic polypeptide) is thought to enhance the effects of GLP-1, improve fat metabolism, and contribute to greater overall appetite suppression. This dual action is believed to be the primary reason Mounjaro has shown greater average weight loss in clinical trials compared to GLP-1-only medications.

Both medications have been extensively studied in large-scale clinical trials. For Mounjaro, the SURMOUNT programme is the key evidence base. In SURMOUNT-1, participants without diabetes taking the 15mg dose lost an average of 22.5% of their body weight over 72 weeks. The 10mg group lost approximately 21.4%, and the 5mg group lost around 15%. For Wegovy, the STEP programme provides the primary evidence. In STEP-1, participants taking semaglutide 2.4mg lost an average of 14.9% of their body weight over 68 weeks, compared to 2.4% for placebo. While direct head-to-head trials between tirzepatide and semaglutide for weight management have been limited, the SURPASS-2 trial (in type 2 diabetes patients) showed tirzepatide was superior to semaglutide 1mg for both weight loss and blood sugar control. The available evidence consistently suggests that tirzepatide produces greater average weight loss, though individual responses to both medications can vary significantly.

Both Mounjaro and Wegovy share similar side effect profiles, largely because they both act on the GLP-1 pathway. The most common side effects for both medications are gastrointestinal: nausea, diarrhoea, constipation, vomiting, and decreased appetite. In clinical trials, these side effects were reported at broadly similar rates for both medications, though direct comparisons are difficult due to differences in trial design and populations. For both medications, side effects tend to be most pronounced during the initial weeks and after dose increases, and generally improve over time. Serious side effects are rare for both medications but can include pancreatitis, gallbladder problems, and allergic reactions. Both carry warnings about potential thyroid tumour risk based on animal studies, and neither should be used by people with a personal or family history of medullary thyroid carcinoma or MEN 2. Your prescriber will discuss the full risk profile with you during your consultation.

The two medications have different dose ranges and titration schedules:

• Mounjaro: Available in six doses (2.5mg, 5mg, 7.5mg, 10mg, 12.5mg, 15mg). Starts at 2.5mg with dose increases every 4 weeks. Maximum dose is 15mg.
• Wegovy: Available in five doses (0.25mg, 0.5mg, 1mg, 1.7mg, 2.4mg). Starts at 0.25mg with dose increases every 4 weeks. Maximum dose is 2.4mg.

Both medications are injected once weekly on the same day each week. Mounjaro uses the KwikPen auto-injector, while Wegovy uses the FlexTouch pen. Both are pre-filled, single-use devices designed for self-injection. The titration period to reach the maximum dose is approximately 20 weeks for Mounjaro (five dose increases) and 16 weeks for Wegovy (four dose increases), though this can vary based on individual tolerance.

Both Mounjaro and Wegovy are available privately in the UK, and prices vary between pharmacies. As a general comparison:

• Mounjaro: Approximately £130 - £320 per month depending on dose and pharmacy.
• Wegovy: Approximately £150 - £300 per month depending on dose and pharmacy.

Prices for both medications have been trending downward as more pharmacies stock them and competition increases. When comparing costs, consider the total treatment cost over several months rather than just the monthly price at the starting dose, as both medications require titration to higher (and typically more expensive) doses. Also consider what is included in the price: consultation fees, delivery, needles, and ongoing prescriber support. Our price comparison table focuses on Mounjaro pricing, but the same principles of comparing total cost apply to Wegovy.

The decision between Mounjaro and Wegovy should be made with your prescriber, not based on internet research alone. Factors that may influence the choice include your medical history, any previous experience with GLP-1 medications, your tolerance for potential side effects, cost considerations, and your prescriber's clinical judgement. If you have tried semaglutide (Wegovy or Ozempic) previously and did not achieve sufficient results, your prescriber may consider switching you to tirzepatide. Conversely, if cost is a primary concern, whichever medication is cheaper at your required dose may be the practical choice. Some people respond better to one medication than the other, and there is no reliable way to predict this in advance. The most important thing is to work with a qualified prescriber who can monitor your progress, adjust your treatment, and ensure your safety throughout the process. Both medications are effective, evidence-based treatments for weight management when used appropriately.