Switching guide
Switching From Mounjaro to Wegovy in the UK: When It Makes Sense and How It Works
Since Eli Lilly raised Mounjaro’s UK list price in September 2025, more people have asked whether Wegovy is now the better buy, and more pharmacies have started suggesting it during consultations. The honest answer is that it depends on why you are switching. This guide sets out what the head-to-head trial found, the situations where a switch is reasonable, how a switch is actually done, and what to expect in the first weeks, without taking a side.
Key facts
- In the only direct comparison, SURMOUNT-5, adults without diabetes lost an average of about 20% of their body weight on tirzepatide (Mounjaro) and about 14% on semaglutide (Wegovy) over 72 weeks. Rates of gut side effects were broadly similar.
- Switching because you want more weight loss therefore has weak logic. Switching for tolerability, cost at your particular dose, availability, the Wegovy 7.2 mg dose, or Wegovy’s cardiovascular licence can all be reasonable.
- There is no official dose-conversion table. Prescribers usually start Wegovy at a low or middle dose rather than the top, so expect some early side effects to return and appetite to be less controlled for a few weeks.
- No washout is needed between the two: the next weekly injection is simply the new medicine.
- Prices for both now vary widely between pharmacies and change often. Which is cheaper depends on your dose and your provider, so compare live prices rather than assume.
What the head-to-head trial found
Until 2025 the two medicines had only been compared across separate trials with different designs. SURMOUNT-5 changed that. It randomised 751 adults with obesity but without diabetes to the maximum tolerated dose of either medicine for 72 weeks. Average weight loss was 20.2% with tirzepatide and 13.7% with semaglutide, and more people on tirzepatide reached each weight-loss milestone. Nausea, diarrhoea and constipation were reported at similar rates in both groups, and discontinuation because of side effects was low in both.
Two caveats matter for a switching decision. Averages hide individuals: some people respond better to semaglutide, and a minority do not respond meaningfully to either. And the trial ran both medicines at full dose, whereas most people in the UK stay on a middle Mounjaro dose, so your own comparison is not the trial’s.
When a switch makes sense
| Reason for switching | Does it hold up? |
|---|---|
| Side effects on Mounjaro you cannot manage, even after slowing dose steps | Reasonable. Some people tolerate one medicine better than the other, and the only way to know is to try. |
| Wegovy is cheaper at your dose from your pharmacy | Reasonable, if the saving is real after delivery and any code. Check both live prices; the gap narrowed in 2025 but moves month to month. |
| You have established heart disease | Reasonable to discuss. Wegovy has a completed cardiovascular outcome trial (SELECT) and a UK licence for reducing cardiovascular risk; Mounjaro’s equivalent trial is still running. |
| Wegovy 7.2 mg is available and you have plateaued on 2.4 mg elsewhere | Only relevant if you are already on Wegovy. It is not a reason to leave Mounjaro. |
| You have plateaued on Mounjaro and hope Wegovy will restart weight loss | Weak. The trial evidence points the other way. Talk to your prescriber about dose, adherence and diet before changing medicine. See when weight loss slows. |
| A pharmacy suggested it during a consultation | Ask why. A suggestion driven by stock or price is not a clinical reason, though it may still suit you. |
How the switch is done
Both medicines are given once a week, so the mechanics are simple: your last Mounjaro injection is followed a week later by your first Wegovy injection. No gap or washout is required. The harder question is the starting dose, because there is no licensed conversion between the two. Common prescriber practice is to start Wegovy at 0.5 mg or 1 mg rather than the 0.25 mg used for new starters, depending on how long you have been on Mounjaro and at what dose, then step up every four weeks. Some prescribers prefer to go back to 0.25 mg to minimise side effects. Either way, expect a few weeks at a lower level of appetite control while the dose builds.
Practically, you will need a fresh consultation with the new prescriber, your Mounjaro treatment history, and, if you are changing pharmacy at the same time, the steps in our switching pharmacy guide. Do not run both medicines at once.
What to expect in the first weeks
- Side effects may return. Nausea and bowel changes that had settled on Mounjaro can reappear as Wegovy is titrated, then settle again for most people.
- Appetite may be less controlled until you reach a maintenance dose. Planning meals and protein intake in that window helps; our nutrition guide applies to both medicines.
- Some weight regain is possible during the transition. Judge the switch over three months, not three weeks.
- The pens are different. Wegovy’s UK pen is single-dose per pen with four pens per pack; you do not dial a dose.
Switching back
If Wegovy does not suit you, moving back to Mounjaro is done the same way, and after a break of more than a few weeks prescribers usually restart at 2.5 mg or 5 mg rather than your previous dose. Keep your old treatment summary so the new assessment is quick. Our restarting guide explains what to expect.
Working out the real cost
The September 2025 price rise applied to Lilly’s list price, but what you pay is set by each pharmacy, and since then the spread between providers has widened for both medicines. The fair comparison is total monthly cost at your current dose, including delivery, against Wegovy at the dose your prescriber would start you on and the dose you would likely maintain. Monj’s Mounjaro comparison and its sister site iGovy for Wegovy both rank pharmacies by lowest total cost and are updated daily. A medicine that costs a little less per month but delivers less weight loss may not be the better value, and vice versa; only you can weigh that.
Common questions
Do I need a break between Mounjaro and Wegovy?
No. Take the first Wegovy dose a week after your last Mounjaro dose. Never take both in the same week.
What Wegovy dose is equal to my Mounjaro dose?
There is no official equivalence. Prescribers commonly start at 0.5 mg or 1 mg for someone established on Mounjaro, then step up every four weeks. Your prescriber decides based on your history.
Is Wegovy cheaper than Mounjaro now?
Sometimes, depending on dose and pharmacy. Compare live total costs at your dose on both medicines rather than relying on a headline.
Will I lose more weight on Wegovy?
On average, no. The SURMOUNT-5 trial found greater weight loss with tirzepatide. Individual responses vary, and tolerability may matter more than averages.
Sources
- Aronne LJ et al. Tirzepatide as compared with semaglutide for the treatment of obesity (SURMOUNT-5). New England Journal of Medicine, 2025.
- Lincoff AM et al. Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT). New England Journal of Medicine, 2023.
- Medicines and Healthcare products Regulatory Agency, via the electronic medicines compendium. Wegovy: Summary of Product Characteristics, sections 4.2 and 4.8.
- Medicines and Healthcare products Regulatory Agency, via the electronic medicines compendium. Mounjaro: Summary of Product Characteristics, sections 4.2 and 4.8.
- NHS Specialist Pharmacy Service. Switching between GLP-1 receptor agonists.
Monj is an independent price information service operated by Medstack Ltd. It does not sell, prescribe or supply medicines and does not give medical advice. Whether to switch, and at what dose, is a decision for your prescriber. Report suspected side effects through the MHRA Yellow Card scheme.