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Mounjaro vs Wegovy: What the Head-to-Head Trial Shows

Mounjaro (tirzepatide) and Wegovy (semaglutide) are the two main weight loss injections in the UK. Here is how they differ, what the first head-to-head trial found, and the safety points to know before you start.

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Comparison card of Mounjaro (tirzepatide) and Wegovy (semaglutide), showing 20.2% and 13.7% average weight loss at 72 weeks in the SURMOUNT-5 head-to-head trial

Mounjaro and Wegovy are the two weight loss injections most people in the UK ask about. Both are once-weekly, prescription-only medicines, and both are licensed for weight management in adults with obesity, or who are overweight with a weight-related health condition.

The short answer to “which works better?” now comes from a direct comparison. In the SURMOUNT-5 trial, people on Mounjaro lost more weight on average than people on Wegovy. But the average is not the whole story, and the right choice depends on your health, how you tolerate each medicine, and cost.

This guide explains the differences. It is general information, not medical advice: a prescriber has to assess you before either medicine is issued.

Mounjaro and Wegovy at a Glance

Mounjaro Wegovy
Active ingredient Tirzepatide Semaglutide
How it acts On two gut-hormone receptors: GLP-1 and GIP On one: GLP-1
How it is taken Weekly injection Weekly injection (a daily pill also exists)
Dose steps 2.5, 5, 7.5, 10, 12.5 and 15mg 0.25, 0.5, 1, 1.7 and 2.4mg, plus a higher 7.2mg dose
Average weight loss, head-to-head (72 weeks) 20.2% 13.7%
Other licensed uses Type 2 diabetes Lowering the risk of heart attack and stroke

How Each One Works

Both medicines copy hormones your gut releases after a meal. GLP-1 (glucagon-like peptide-1) tells the brain you are full, slows how fast the stomach empties, and helps the pancreas release insulin when blood sugar rises. Your own GLP-1 breaks down within minutes. The versions in these medicines are built to last about a week, which is why one injection covers seven days.

  • Wegovy contains semaglutide, which acts on the GLP-1 receptor only.
  • Mounjaro contains tirzepatide, which acts on the GLP-1 receptor and on the receptor for a second gut hormone, GIP (glucose-dependent insulinotropic polypeptide). GIP also helps control blood sugar, and the combination is thought to explain why tirzepatide produces more weight loss on average.

Both are peptides: short chains of amino acids, made to resist the enzymes that break the natural hormones down.

What the Head-to-Head Trial Found

Until 2025, the two medicines could only be compared across separate trials with different people in them. SURMOUNT-5 (NEJM, 2025) was the first trial to put them side by side. It enrolled 751 adults with obesity, or overweight plus a weight-related condition, and without type 2 diabetes. Each person took the highest dose of one medicine they could tolerate, for 72 weeks.

  • Average weight loss: 20.2% of body weight on tirzepatide, 13.7% on semaglutide.
  • Waist size: down 18.4cm on average on tirzepatide, 13.0cm on semaglutide.
  • Bigger losses: people on tirzepatide were more likely to lose at least 10%, 15%, 20% and 25% of their body weight.
  • Side effects: mostly gastrointestinal, mild to moderate, and mainly during the dose increases, in both groups.

The trial was funded by Eli Lilly, which makes Mounjaro. Averages also hide a wide spread: some people lose more on semaglutide than the tirzepatide average, and some lose little on either.

How the numbers compare with the original trials

Each medicine’s own placebo-controlled trial gives a similar picture:

  • SURMOUNT-1 (NEJM, 2022): tirzepatide 15mg led to 20.9% average weight loss at 72 weeks, compared with 3.1% on placebo.
  • STEP 1 (NEJM, 2021): semaglutide 2.4mg led to 14.9% average weight loss at 68 weeks, compared with 2.4% on placebo.
  • STEP UP (Lancet Diabetes & Endocrinology, 2025): a higher 7.2mg dose of semaglutide led to 18.7% average weight loss at 72 weeks, compared with 15.6% on 2.4mg. That narrows the gap with tirzepatide, but it has not been tested directly against it. Gastrointestinal side effects were more common at 7.2mg (71% of people, against 61% on 2.4mg).

You may see higher figures quoted elsewhere, such as 22.5% for tirzepatide. Those count only people who stayed on treatment. The figures above count everyone who started, which is closer to what a new patient can expect.

Doses: Start Low, Go Slow

The two medicines use different molecules at different strengths, so a milligram of one is not equal to a milligram of the other.

  • Mounjaro starts at 2.5mg a week and can rise in 2.5mg steps, usually every 4 weeks, up to a maximum of 15mg.
  • Wegovy starts at 0.25mg a week and rises through 0.5mg, 1mg and 1.7mg to a 2.4mg maintenance dose over about 16 weeks. The 7.2mg dose is a further step for some people.

Slow increases give the gut time to adjust, which cuts nausea and vomiting. Not everyone needs the top dose: if weight loss is steady on a lower one, your prescriber may keep you there.

Side Effects Compared

The common side effects are the same for both, and mostly affect the gut:

  • Nausea, the most common, especially in the first weeks and after each dose increase
  • Diarrhoea or constipation
  • Vomiting
  • Indigestion, bloating and tiredness early on

In SURMOUNT-5, fewer people stopped tirzepatide because of gut side effects than stopped semaglutide (2.7% against 5.6%). Tolerance still varies a lot between people.

The UK regulator, the MHRA, highlights some rarer risks in its guidance on GLP-1 medicines:

  • Pancreatitis: rare, but it has been reported with both, sometimes seriously. Severe stomach pain that spreads to the back and does not go away needs urgent medical help.
  • An eye condition called NAION: very rarely linked to semaglutide. A sudden change in eyesight needs urgent attention. The MHRA is reviewing whether other GLP-1 medicines carry the same risk.
  • Dehydration: heavy vomiting or diarrhoea can lead to severe dehydration.

Contraception and Pregnancy

This is one of the clearest practical differences for women:

  • Mounjaro and the pill: the MHRA advises anyone taking an oral contraceptive to add a barrier method, such as condoms, for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. Tirzepatide may make the pill less effective. Switching to a coil or implant avoids the issue.
  • Before trying for a baby: stop semaglutide at least 2 months beforehand, and tirzepatide at least 1 month beforehand.
  • During pregnancy: neither should be used.

Cost in the UK

Both are available on the NHS for people who meet strict criteria, through specialist weight management services, and Mounjaro is being rolled out gradually through GP practices. Eligibility and waiting times vary by area, so most people pay privately.

Private prices vary by provider and dose. Starting doses of the two cost broadly similar amounts. At higher doses Mounjaro is usually the more expensive of the two, after its UK list price rose sharply in September 2025. Compare the price at your likely maintenance dose, not just the first month.

Which One Might Suit You?

Only a prescriber can decide, but these are the points that usually come up:

Mounjaro is often considered when:

  • The priority is the largest average weight loss
  • Someone has type 2 diabetes or insulin resistance
  • Weight loss has stalled on Wegovy
  • Semaglutide caused side effects that were hard to manage

Wegovy is often considered when:

  • Someone has heart disease, since semaglutide is licensed to lower the risk of heart attack and stroke. In the SELECT trial (NEJM, 2023), it cut major cardiovascular events by 20% in people with heart disease and overweight or obesity.
  • Cost at maintenance doses matters
  • Tirzepatide caused side effects that were hard to manage
  • Someone prefers a medicine with a longer track record, or would rather take a daily pill than an injection

People switch in both directions. The MHRA advises talking to a healthcare professional before switching, because the doses do not convert one to one.

Get Them Only on Prescription

Both medicines are prescription-only in the UK. The MHRA warns against buying GLP-1 medicines from beauty salons, social media sellers, or anywhere that skips a consultation. It also warns that genuine products come as pre-filled pens or tablets: powder in vials that must be mixed before injecting is not an authorised medicine. You can check that an online pharmacy is registered on the General Pharmaceutical Council register.

If you are in the UK and want a clinician-led programme, SheMed prescribes both Mounjaro and Wegovy for women, with ongoing clinical support. Our code NOW20OFF takes 20% off your first month; see our SheMed listing for details.

Frequently Asked Questions

Is Mounjaro stronger than Wegovy?

On average, yes. In the SURMOUNT-5 head-to-head trial, tirzepatide (Mounjaro) led to 20.2% average weight loss at 72 weeks, against 13.7% for semaglutide (Wegovy). Individual results vary widely.

Does Wegovy 7.2mg match Mounjaro?

It comes closer. In the STEP UP trial, 7.2mg led to 18.7% average weight loss at 72 weeks. It has not been compared directly with tirzepatide.

Which has fewer side effects?

Both mainly cause gut side effects. In SURMOUNT-5, slightly fewer people stopped tirzepatide because of them than stopped semaglutide, but everyone reacts differently.

Can I switch from Wegovy to Mounjaro?

Yes, many people do, but only with a prescriber’s advice. The doses do not convert one to one, so you usually restart on a set dose.

Does Mounjaro affect the contraceptive pill?

It can. The MHRA advises adding a barrier method for 4 weeks after starting and after each dose increase, or switching to a non-oral method.

Do I need a prescription?

Yes. Both are prescription-only medicines in the UK, and a healthcare professional must assess you first.

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