Mounjaro vs Wegovy: comparing the two leading weight loss injections

Understanding the fundamental differences between these two medicines helps explain their varying clinical profiles.

Active ingredients and mechanism:

• [Mounjaro](/en/mounjaro/) contains tirzepatide, a dual GIP/GLP-1 receptor agonist. It activates both the glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptors
• [Wegovy](/en/wegovy/) contains semaglutide, a GLP-1 receptor agonist only. It targets the GLP-1 receptor without affecting GIP

Dosing schedules:

• Both are given as once-weekly subcutaneous injections
• Mounjaro is titrated from 2.5 mg to a maximum of 15 mg, increasing every 4 weeks
• Wegovy is titrated from 0.25 mg to a maintenance dose of 2.4 mg over 16 weeks

Licensed indications in the UK:

• Mounjaro is licensed for type 2 diabetes and for weight management in adults with a BMI of 30 or above (or 27 with weight-related comorbidities)
• Wegovy is licensed for weight management under the same BMI criteria
• Semaglutide is also available as Ozempic (lower doses) specifically for type 2 diabetes

Manufacturer:

• Mounjaro is manufactured by Eli Lilly
• Wegovy is manufactured by Novo Nordisk

Comparing weight loss outcomes between Mounjaro and Wegovy requires looking at data from their respective pivotal trials, as well as emerging head-to-head evidence.

Mounjaro (SURMOUNT-1 trial):

• Population: adults with obesity or overweight (without diabetes)
• Duration: 72 weeks
• Results at 15 mg: 22.5% mean body weight reduction (placebo-subtracted: approximately 20%)
• Proportion losing more than 20% body weight: approximately 57%

Wegovy (STEP-1 trial):

• Population: adults with obesity or overweight (without diabetes)
• Duration: 68 weeks
• Results at 2.4 mg: 14.9% mean body weight reduction (placebo-subtracted: approximately 12.4%)
• Proportion losing more than 20% body weight: approximately 32%

Important context:

These are cross-trial comparisons, which have inherent limitations due to differences in study populations, duration and methodology.

However, the magnitude of the difference is substantial and consistent across multiple analyses.

Emerging head-to-head data:

The SURMOUNT-5 trial directly compared tirzepatide 15 mg with semaglutide 2.4 mg for weight management.

Preliminary results showed tirzepatide produced statistically superior weight loss over 72 weeks, confirming the trend seen in indirect comparisons.

Both medicines produce clinically meaningful weight loss that exceeds older treatments such as orlistat or liraglutide.

Both medicines share a similar gastrointestinal side effect profile, which is expected given their overlapping mechanism at the GLP-1 receptor.

Common side effects shared by both:

• Nausea: the most frequently reported effect for both. Approximately 24 to 29% with Mounjaro and 44% with Wegovy at maintenance doses
• Diarrhoea: approximately 17% with Mounjaro, 30% with Wegovy
• Vomiting: approximately 9 to 12% with Mounjaro, 24% with Wegovy
• Constipation: approximately 11% with Mounjaro, 24% with Wegovy
• Injection site reactions: mild and transient with both

Tolerability differences:

Although direct comparison of adverse event rates across trials requires caution, Mounjaro appears to have a somewhat more favourable gastrointestinal tolerability profile.

The discontinuation rate due to adverse events was approximately 4 to 7% for Mounjaro versus 7% for Wegovy.

Serious side effects:

Both carry warnings regarding:

• Pancreatitis (rare but serious)
• Gallbladder events (increased with rapid weight loss)
• Potential thyroid C-cell effects (based on animal studies; clinical significance uncertain)
• Acute kidney injury secondary to dehydration

Mental health considerations:

The MHRA and EMA are monitoring reports of suicidal ideation with GLP-1 agonists. No causal link has been established, but patients should report any changes in mood to their prescriber.

Availability and cost are practical factors that influence which treatment patients may be offered.

NHS availability:

• Wegovy received NICE approval for weight management in March 2023 and has been available through NHS specialist weight management services
• Mounjaro (tirzepatide) received NICE approval for weight management in late 2024. It is available for type 2 diabetes and is being introduced through NHS obesity services
• Both medicines have experienced supply constraints due to high global demand

Private prescription costs:

• Private costs vary but both medicines are expensive, typically ranging from 150 to 300 pounds per month depending on the dose and supplier
• Neither medicine is available over the counter
• Some online prescribing services offer both, subject to clinical assessment

NHS prescribing criteria:

• BMI of 30 or above (or 27 with weight-related comorbidities such as type 2 diabetes, hypertension or obstructive sleep apnoea)
• Engagement with a specialist weight management service
• Treatment is reviewed at regular intervals, and continued prescribing depends on achieving a minimum weight loss target (usually 5% of body weight by a defined timepoint)

Switching between treatments:

Some patients may be switched from one medicine to the other due to supply issues, tolerability or inadequate response.

There is currently no standardised switching protocol, but clinicians generally restart the titration schedule of the new medicine to minimise gastrointestinal side effects.

The choice between Mounjaro and Wegovy depends on individual clinical circumstances, availability and personal preference.

Factors that may favour Mounjaro:

• Greater average weight loss in clinical trials
• Dual mechanism may offer additional metabolic benefits
• Potentially better gastrointestinal tolerability based on trial data
• Licensed for both type 2 diabetes and weight management (useful if both conditions are present)

Factors that may favour Wegovy:

• Longer track record with established safety data from the SELECT cardiovascular outcomes trial
• Proven cardiovascular benefit: semaglutide 2.4 mg reduced the risk of major adverse cardiovascular events by 20% in the SELECT trial
• More extensive NHS prescribing experience
• May suit patients who prefer a medicine with a longer history of real-world use

What your prescriber will consider:

• Your BMI and weight loss goals
• Whether you have type 2 diabetes or other metabolic conditions
• Your cardiovascular risk profile
• Previous experience with GLP-1 agonists
• Current medicine availability and supply
• Cost considerations (particularly relevant for private prescriptions)

The bottom line:

Both Mounjaro and Wegovy represent major advances in pharmacological weight management.

Whichever is prescribed, combining treatment with dietary improvements and increased physical activity will optimise long-term outcomes.

Discuss the options with your prescriber, who can tailor the choice to your specific situation.