Mounjaro vs Wegovy, side by side.

At a glance

Both are prescription-only weekly injections licensed in the UK for weight management, used alongside a reduced-calorie diet and more physical activity. Both start at a low dose and step up roughly every four weeks.

How they work

Wegovy contains semaglutide, which acts like GLP-1, a hormone your gut releases after eating. It reduces appetite, slows how quickly the stomach empties and helps you feel full for longer.

Mounjaro contains tirzepatide, which acts on GLP-1 and a second hormone, GIP. Acting on both is thought to explain why average weight loss in trials has been larger.

Where Ozempic fits

Ozempic and Wegovy contain the same medicine, semaglutide, made by the same company. The difference is the licence and the dose. In the UK, Ozempic is licensed only for type 2 diabetes, at doses up to 2 mg a week. Wegovy is licensed for weight management, with a maintenance dose of 2.4 mg a week.

So when people compare Mounjaro with Ozempic for weight loss, the like-for-like comparison is Mounjaro with Wegovy. That is the comparison the head-to-head trial below made.

What the trials found

In their main trials against placebo, people on the highest dose of Mounjaro lost about 20% of their body weight on average over 72 weeks, and people on Wegovy about 15% over 68 weeks. Those trials were run separately, in different people, so the figures aren't a direct comparison.

SURMOUNT-5 compared the two directly in 751 adults with obesity but without diabetes, each on the highest dose they could tolerate (10 or 15 mg of tirzepatide, 1.7 or 2.4 mg of semaglutide). After 72 weeks, average weight loss was 20.2% with tirzepatide and 13.7% with semaglutide. Averages hide wide variation: some people lose much more, some much less, and how well you tolerate a medicine matters as much as the average.

The placebo-controlled figures below are as reported in each medicine's UK product information, which analyses the trials slightly differently from the published papers, so they can differ by a point or so from figures you see elsewhere.

Side effects

The side effects are similar because both act on GLP-1: nausea, diarrhoea, constipation and indigestion, mostly in the first weeks and after each dose increase. Rare but serious risks, including pancreatitis and gallbladder problems, apply to both. Our side effects guide covers what's normal and when to get help.

Cost

Both are priced per pen, and each pen lasts four weeks. The price rises with the dose, so the first months cost less than the later ones. Every dose is priced on the Mounjaro and Wegovy pages, and side by side on our pricing page.

The table assumes the dose goes up every four weeks, the fastest it can, until the highest dose, and stays there. Many people step up more slowly or stay on a lower dose, which costs less, so the totals are the most each could cost rather than a forecast. Mounjaro has one more dose step than Wegovy. A subscription takes £10 off every pen of either.

Day-to-day differences

Both are a pre-filled pen that holds four weekly doses, injected on the same day each week at any time, with or without food. The practical differences are small but worth knowing:

• Missed dose: Mounjaro can be taken up to 4 days late, Wegovy up to 5. After that, skip it and take the next dose on your usual day.
• Out of the fridge once started: up to 30 days for a Mounjaro pen (no more than 30°C), and up to 6 weeks for a Wegovy pen (below 30°C or in the fridge).
• Changing your injection day: possible with either, as long as there are at least 3 days between two doses.

Contraception and pregnancy

This is one of the clearest differences. Mounjaro can make the contraceptive pill less reliable in people who are overweight. Its UK product information advises switching to a method that isn't a pill, or adding a barrier method such as condoms for 4 weeks after starting and for 4 weeks after each dose increase.

Semaglutide, in Wegovy, isn't expected to make the contraceptive pill less effective. Neither medicine should be used in pregnancy: stop Mounjaro at least 1 month, and Wegovy at least 2 months, before trying for a baby.

How the choice is made

There isn't one right answer for everyone. A prescriber decides which medicine, if either, is suitable for you. The things they weigh include:

• Your BMI and any weight-related conditions, such as type 2 diabetes, high blood pressure or sleep apnoea.
• Your medical history, including pancreatitis, gallbladder problems, kidney problems or diabetic eye disease.
• Other medicines you take, including the contraceptive pill, insulin or a sulfonylurea, and any weight-loss medicine you've taken before and how you got on with it.
• Heart health: Wegovy is also licensed to reduce the risk of heart attack and stroke in adults with established cardiovascular disease and a BMI of 27 or more.
• What matters to you, such as cost and how you'd like to fit treatment around your life.

Our online service treats adults aged 18 to 65 with a BMI of 27 to 50. Between 27 and 29.9 it usually recommends Wegovy; from 30, both are options. Who can be treated explains the full criteria.

Switching between them

People do switch, usually because of side effects, cost, or results that have stalled. The doses aren't equivalent, so you don't simply carry your dose across: a prescriber decides the starting dose on the new medicine, and you shouldn't take both. Tell your prescriber about any weight-loss medicine you've taken in the past few months.

Neither medicine's product information gives a way to convert a dose of one into the other. Both are written for starting on the lowest dose (2.5 mg of Mounjaro, 0.25 mg of Wegovy) and stepping up no more often than every four weeks, which is why side effects can return for a while after a switch. Wegovy's leaflet also says not to use it with any other GLP-1 medicine.

What happens when you stop

With both, weight tends to come back after stopping. In SURMOUNT-4, people who had taken tirzepatide for 36 weeks and carried on lost a further 6.7% of their weight over the next year on average, while those switched to placebo regained 14.8%. In STEP 4, people who carried on with semaglutide after 20 weeks lost a further 7.9% over 48 weeks, while those switched to placebo regained 6.9%.

These trials weren't designed to compare the two medicines, so they don't show one is easier to come off than the other. They do show it's worth planning with your prescriber how long to stay on treatment and how to come off it.

If you'd prefer a tablet

The Wegovy Pill contains the same medicine as Wegovy, semaglutide, as a daily tablet taken on an empty stomach. It starts from £138.99 per pack.