Mounjaro vs Ozempic: Key Differences Explained (UK 2026)

Mounjaro and Ozempic are two of the most talked-about injectable medications in the UK, but they are not the same drug and they are not approved for the same primary purpose. Mounjaro (tirzepatide) is manufactured by Eli Lilly and is a dual GIP/GLP-1 receptor agonist. It is licensed in the UK for type 2 diabetes and is widely prescribed privately for weight management. Ozempic (semaglutide) is manufactured by Novo Nordisk and is a GLP-1 receptor agonist. It is primarily licensed for type 2 diabetes, not weight loss. The weight loss version of semaglutide is called Wegovy, which is the same active ingredient at a higher dose (2.4mg vs Ozempic's maximum of 2mg). Despite this distinction, Ozempic is frequently used off-label for weight loss in the UK, which is why the comparison with Mounjaro comes up so often. Both medications are injected once weekly using pre-filled pen devices.

The most important scientific difference between Mounjaro and Ozempic lies in their mechanism of action. Ozempic targets one hormone pathway: GLP-1 (glucagon-like peptide-1). GLP-1 is a gut hormone that slows gastric emptying, suppresses appetite, and improves blood sugar control. By mimicking GLP-1, Ozempic makes you feel fuller for longer and reduces hunger signals. Mounjaro targets two hormone pathways simultaneously: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). GIP is another gut hormone that works alongside GLP-1 to further enhance insulin sensitivity, improve fat metabolism, and amplify appetite suppression. This dual mechanism is often referred to as a "twincretin" approach. The dual action of Mounjaro is believed to be the primary reason it has shown greater average weight loss in clinical trials compared to single-action GLP-1 medications. By activating two complementary pathways, tirzepatide produces a more comprehensive metabolic effect than targeting GLP-1 alone.

Head-to-head and cross-trial comparisons consistently show Mounjaro producing greater average weight loss than Ozempic. In the SURPASS-2 trial, which directly compared tirzepatide to semaglutide 1mg in people with type 2 diabetes, tirzepatide achieved significantly greater weight loss at all dose levels. Participants on tirzepatide 15mg lost an average of 12.4kg compared to 6.2kg for semaglutide 1mg over 40 weeks. Looking at the broader trial programmes:

• Mounjaro (SURMOUNT-1): 15mg dose achieved 20.9% average body weight loss over 72 weeks in people without diabetes
• Ozempic (STEP-1, using Wegovy 2.4mg): Achieved 14.9% average body weight loss over 68 weeks in people without diabetes

While these are not perfectly comparable trials (different participants, slightly different durations), the consistent pattern across multiple studies is that tirzepatide produces approximately 5-7 percentage points more weight loss on average than semaglutide at their respective maximum doses. It is crucial to note that these are averages. Individual responses vary significantly, and some people respond better to semaglutide than tirzepatide, and vice versa. There is no guaranteed way to predict which medication will work better for a specific individual.

Both Mounjaro and Ozempic share a similar side effect profile because they both activate the GLP-1 receptor. The most common side effects for both medications are gastrointestinal:

• Nausea: Reported by 15-30% of users for both medications. Tends to be worst during initial weeks and after dose increases.
• Diarrhoea: Affects approximately 10-20% of users for both.
• Constipation: Reported by 10-15% of users for both.
• Vomiting: Less common than nausea but possible with both medications.
• Decreased appetite: Common with both, and partly the intended therapeutic effect.

In clinical trials, the overall rates of gastrointestinal side effects were broadly similar between tirzepatide and semaglutide, though some analyses suggest slightly higher rates of certain side effects (particularly diarrhoea) with tirzepatide at higher doses. Both medications carry similar rare but serious risk warnings, including pancreatitis, gallbladder problems, and the theoretical risk of thyroid tumours based on animal studies. For most users of either medication, side effects are mild to moderate and improve within 2-4 weeks of starting or increasing a dose. For practical tips on managing side effects, see our side effects guide.

Price is often a deciding factor for UK users choosing between these medications. As of March 2026, approximate monthly costs through private prescription are:

• Mounjaro (tirzepatide): £130 - £320 per month depending on dose and pharmacy
• Ozempic (semaglutide): £150 - £280 per month depending on dose and pharmacy

Prices for both medications have been decreasing over the past year as more pharmacies enter the market and competition increases. For Mounjaro specifically, our price comparison table tracks live prices across 60+ GPhC-registered pharmacies. When comparing costs, consider the total treatment cost rather than just the monthly price at your current dose. Mounjaro has six dose levels (2.5mg to 15mg) while Ozempic has fewer dose options (0.25mg, 0.5mg, 1mg, and off-label 2mg). The titration journey and maintenance dose will determine your ongoing monthly expense. Use our cost calculator to estimate total treatment costs for Mounjaro.

The decision between Mounjaro and Ozempic should be made with a qualified prescriber, not based solely on online research. However, here are the key factors to discuss with your prescriber. Mounjaro may be preferred if: you want the medication with the strongest average weight loss data, you have type 2 diabetes and want dual metabolic benefits, you have tried semaglutide previously without sufficient results, or the cost is comparable or favourable at your required dose. Ozempic may be preferred if: you have responded well to semaglutide before, your prescriber recommends it based on your specific health profile, it is more readily available or more affordable for your situation, or you prefer the established longer track record of semaglutide. It is worth noting that Ozempic is technically licensed for type 2 diabetes, not weight management. If your primary goal is weight loss, your prescriber may recommend Mounjaro or Wegovy (the weight-management-licensed version of semaglutide) as more appropriate options. Ultimately, both medications are effective, evidence-based treatments. The best choice depends on your individual health profile, treatment history, tolerance, budget, and prescriber recommendation. What matters most is choosing a treatment you can adhere to consistently over the long term, as sustained use is required for sustained results.