Mounjaro vs Wegovy: What’s the Difference in the UK? Side Effects, Weight Loss Results & Prescribing Guidance Explained

Thinking about starting weight loss treatment and unsure whether Mounjaro® (tirzepatide) or Wegovy® (semaglutide) is right for you? You’re not alone. These two GLP-1 based medications are among the most talked-about weight loss injections in the UK.

On this page, we break down everything you need to know:

✅ How Mounjaro and Wegovy work (GLP-1 vs dual GIP/GLP-1 action)

✅ Differences in weight loss results from clinical trials

✅ Side effects and safety considerations

✅ Who can get a prescription in the UK (NHS and private criteria)

Whether you’re searching “Mounjaro vs Wegovy,” “best GLP-1 weight loss injection,” or “UK prescribing rules for Wegovy and Mounjaro,” this guide is designed to give you clear, reliable answers in one place.

Q: What’s the difference between Wegovy® and Mounjaro®?

Both are injectable weight-loss medicines that work on the body’s appetite and blood sugar signals, but they’re not the same drug:

  • Wegovy® (semaglutide) is a GLP-1 receptor agonist. In trials, people lost on average around 15% of body weight after 68 weeks. It has proven benefits in reducing cardiovascular risk for people with obesity and existing heart disease.

  • Mounjaro® (tirzepatide) is a dual GIP and GLP-1 receptor agonist. In head-to-head studies, people on Mounjaro lost more weight on average (about 20% after 72 weeks). It’s also shown benefits in conditions linked to obesity, such as sleep apnoea.

    In simple terms: Wegovy works on one hormone (GLP-1), while Mounjaro works on two (GIP + GLP-1). That’s why many people see stronger weight-loss results with Mounjaro in clinical trials.

Q: What are GIP and GLP-1, and why do they matter?

Both GLP-1 and GIP are natural hormones your body makes after eating. They send signals that help control appetite, blood sugar, and how your body uses food for energy.

  • GLP-1 (glucagon-like peptide-1): Helps you feel fuller for longer, slows stomach emptying, reduces hunger, and keeps blood sugar steady. Wegovy® works on this pathway.

  • GIP (glucose-dependent insulinotropic polypeptide): Also helps with blood sugar and may enhance how the body burns fat and stores energy.

  • Mounjaro® works on both GLP-1 and GIP receptors, which is why it can lead to greater weight loss for some people.

    In simple terms:

    • Wegovy® = GLP-1 only → reduces appetite and supports weight loss.

    • Mounjaro® = GLP-1 + GIP → adds an extra pathway, which may boost weight loss and energy balance.

GLP-1 vs GIP infographic: Wegovy targets GLP-1, Mounjaro targets GLP-1 and GIP for dual weight-loss effect.

Q: What’s the difference in dosing between Mounjaro and Wegovy?

Wegovy® (semaglutide) is taken once a week, starting at 0.25 mg and gradually increasing to a maximum of 2.4 mg weekly.

Mounjaro® (tirzepatide) is also a once-weekly injection, but it has a different dosing range: it begins at 2.5 mg and can increase step-by-step up to 15 mg weekly.

In simple terms: Wegovy maxes out at 2.4 mg, while Mounjaro can go up to 15 mg, giving doctors more flexibility to adjust the dose.

Q: What are the common side effects of Mounjaro vs Wegovy?

Both Mounjaro® (tirzepatide) and Wegovy® (semaglutide) are GLP-1 based weight loss injections, and so they share many of the same side effects. The most common are:

  • Nausea

  • Vomiting

  • Diarrhoea or constipation

  • Stomach pain or bloating

  • Headaches and tiredness

For most people, side effects are mild to moderate and improve as the body adjusts to treatment. Eating smaller meals, avoiding rich or greasy foods, and staying hydrated can help reduce symptoms.

In simple terms: both drugs can upset the stomach at first, but side effects usually ease with time and gradual dose increases.

Q: Can my GP prescribe Mounjaro or Wegovy in the UK?

From June 2025, NHS England introduced very limited GP prescribing for Mounjaro® (tirzepatide). At this stage, only patients with a BMI of 40 or higher and at least four obesity-related health conditions are eligible. This is part of a phased rollout that may expand over the next 2–3 years.

What this means right now:

  • Mounjaro® (England): GPs can prescribe, but only to a very restricted group. Wider rollout expected in future.

  • Wegovy® (semaglutide): Still specialist-only across the UK. GPs cannot prescribe it.

  • Scotland & Wales: Access rules differ by local health authorities, but remain specialist-led.

  • Northern Ireland: Wegovy and Mounjaro remain specialist-only.

In simple terms: For most people, this means access to Mounjaro or Wegovy directly from your GP may be limited. Instead, you’ll need referral to an NHS weight-management service or go through a licensed private provider.

For general, nationwide information on how these medications are approved and should be used, see: GOV.UK

Q: Can I switch from Mounjaro® to Wegovy® ?

NICE guidance for Wegovy® (semaglutide) requires patients to start at the lowest 0.25 mg dose and increase step-by-step:
0.25 → 0.5 → 1.0 → 1.7 → 2.4 mg. (NICE TA875)

  • This gradual escalation is to reduce side effects such as nausea, diarrhoea, or vomiting. Even if you tolerated a high dose of Mounjaro® (tirzepatide), your body still needs to adapt to semaglutide separately.

  • There is no specific NICE guidance on switching directly from Mounjaro® to Wegovy®. Some specialists may adjust titration for patients already experienced with GLP-1 medicines, but this should only ever be done under medical supervision.

In simple terms: There is no shortcut to matching your Mounjaro® dose in Wegovy® milligrams. Always follow the dose-escalation plan set by your prescriber.

Q: How can I buy GLP-1 Medicines Safely in the UK?

GLP-1 weight-loss injections (like Mounjaro® and Wegovy®) can be prescribed privately, but only after a consultation with a registered healthcare professional. This ensures safety checks and that you understand the risks and benefits.

Never buy GLP-1 medicines from unregulated sources such as beauty salons, social media, or sellers offering powders in vials. These are unauthorised, unsafe, and often counterfeit. Even fake pre-filled pens that look genuine have been linked to severe side effects.

✅ To guarantee your treatment is genuine, only use a licensed UK pharmacy (including online). You can check registration here:

In simple terms: The only safe way to get GLP-1 medicines in the UK is with a prescription from a healthcare professional and dispensed by a registered pharmacy.

Information correct -17/08/2025