Mounjaro vs Wegovy: comparing the two leading weight loss injections

These two medicines differ in a few key ways. Knowing how helps explain why they work differently.

Active ingredients and how they work:

• [Mounjaro](/en-gb/mounjaro/) contains tirzepatide. It is a dual GIP/GLP-1 receptor agonist, which means it switches on two gut hormone signals at once. These are the glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptors
• [Wegovy](/en-gb/wegovy/) contains semaglutide. It is a GLP-1 receptor agonist, so it switches on the GLP-1 signal only. It does not affect GIP

Dosing schedules:

• You take both as a once-weekly injection under the skin
• Mounjaro starts at 2.5 mg and builds up to a maximum of 15 mg. The dose rises every 4 weeks
• Wegovy starts at 0.25 mg and builds up to a maintenance dose of 2.4 mg over 16 weeks

What they are licensed for in the UK:

• Mounjaro is licensed for type 2 diabetes. It is also licensed for weight management in adults with a BMI of 30 or above. Adults with a BMI of 27 can use it too, if they have weight-related health problems
• Wegovy is licensed for weight management under the same BMI rules
• Semaglutide also comes as Ozempic at lower doses, just for type 2 diabetes

Manufacturer:

• Eli Lilly makes Mounjaro
• Novo Nordisk makes Wegovy

To compare weight loss, we look at the main trials for each medicine. We also look at new trials that test them side by side.

Mounjaro (SURMOUNT-1 trial):

• Who took part: adults with obesity or overweight, but without diabetes
• How long: 72 weeks
• Results at 15 mg: 22.5% mean body weight reduction. After taking out the placebo effect, this was about 20%
• Share who lost more than 20% of their body weight: about 57%

Wegovy (STEP-1 trial):

• Who took part: adults with obesity or overweight, but without diabetes
• How long: 68 weeks
• Results at 2.4 mg: 14.9% mean body weight reduction. After taking out the placebo effect, this was about 12.4%
• Share who lost more than 20% of their body weight: about 32%

Important context:

These are comparisons across separate trials. That has limits, because the groups of people, the trial length and the methods all differed. Even so, the gap in weight loss is large.

It also shows up again and again across many analyses.

New head-to-head data:

The SURMOUNT-5 trial tested tirzepatide 15 mg directly against semaglutide 2.4 mg for weight management.

Early results showed tirzepatide led to more weight loss over 72 weeks, and the difference was statistically significant. This backs up what the indirect comparisons found.

Both medicines lead to real, useful weight loss. Both beat older treatments such as orlistat or liraglutide.

Both medicines have a similar set of gut side effects. This makes sense, as they both act on the GLP-1 receptor.

Common side effects shared by both:

• Nausea (feeling sick): the most common effect for both. About 24 to 29% with Mounjaro and 44% with Wegovy at maintenance doses
• Diarrhoea: about 17% with Mounjaro, 30% with Wegovy
• Vomiting (being sick): about 9 to 12% with Mounjaro, 24% with Wegovy
• Constipation: about 11% with Mounjaro, 24% with Wegovy
• Injection site reactions: mild and short-lived with both

Tolerability differences:

We should be careful when we compare side effect rates across separate trials. Still, Mounjaro seems to be a bit easier on the gut.

The share of people who stopped because of side effects was about 4 to 7% for Mounjaro and 7% for Wegovy.

Serious side effects:

Both carry warnings about:

• Pancreatitis, which means swelling of the pancreas. This is rare but serious
• Gallbladder problems, which rise with fast weight loss
• Possible effects on thyroid C-cells. This is based on animal studies, and what it means for people is not clear
• Acute kidney injury caused by losing too much fluid

Mental health considerations:

The MHRA and EMA are watching reports of suicidal thoughts with GLP-1 agonists. No clear link has been proven. Even so, tell your prescriber about any change in your mood.

Cost and supply are real-world factors. They affect which treatment you may be offered.

NHS availability:

• NICE approved Wegovy for weight management in March 2023. Since then it has been available through NHS specialist weight management services
• NICE approved Mounjaro (tirzepatide) for weight management in late 2024. It is available for type 2 diabetes and is now coming into NHS obesity services
• Both medicines have had supply shortages, because demand worldwide is high

Private prescription costs:

• Private costs vary, but both medicines are dear. They usually cost from 150 to 300 pounds per month, depending on the dose and the supplier
• Neither medicine is sold over the counter
• Some online prescribing services offer both, after a clinical assessment

NHS prescribing criteria:

• A BMI of 30 or above. A BMI of 27 also counts if you have weight-related health problems, such as type 2 diabetes, high blood pressure or sleep apnoea (when breathing stops and starts during sleep)
• You take part in a specialist weight management service
• Your treatment is reviewed at set times. To keep getting it, you need to hit a minimum weight loss target. This is usually 5% of your body weight by a set point

Switching between treatments:

Some people are switched from one medicine to the other. This may be due to supply problems, side effects or a poor response. There is no set switching plan yet.

As a rule, clinicians restart the dose build-up of the new medicine. This helps keep gut side effects low.

The right choice depends on your own health, what is in stock and what you prefer.

Reasons that may favour Mounjaro:

• More weight loss on average in trials
• Its dual action may bring extra benefits for how the body handles energy
• It may be a bit easier on the gut, based on trial data
• It is licensed for both type 2 diabetes and weight management, which helps if you have both

Reasons that may favour Wegovy:

• A longer track record, with safety data from the SELECT heart outcomes trial
• A proven heart benefit. In the SELECT trial, semaglutide 2.4 mg cut the risk of major heart events by 20%
• More NHS prescribing experience
• It may suit you if you want a medicine with a longer history of everyday use

What your prescriber will think about:

• Your BMI and your weight loss goals
• Whether you have type 2 diabetes or other conditions linked to how your body handles energy
• Your risk of heart problems
• Whether you have taken GLP-1 agonists before
• What is in stock and available now
• The cost, which matters most for private prescriptions

The bottom line:

Both Mounjaro and Wegovy are major steps forward in weight loss medicine. Whichever you take, you will get the best long-term results by also eating better and moving more.

Talk the options through with your prescriber, who can match the choice to your situation.