What is Mounjaro and how does it work?

Mounjaro is the brand name for tirzepatide, a once-weekly injection you give yourself under the skin with a pre-filled pen. Unlike earlier weight-loss injections, it mimics two gut hormones — GLP-1 and GIP — which reduce appetite, help you feel full sooner, and slow stomach emptying. That dual action is why it produced more weight loss than semaglutide in head-to-head trials.

Tirzepatide is also licensed for type 2 diabetes. This page covers its use for weight loss; if you have diabetes, your GP or diabetes team will advise on how it fits your treatment.

What are the side effects of Mounjaro?

Stomach and bowel side effects are the most common, particularly when you start or move up a dose. They include:

They often settle over a few weeks. Talk to a pharmacist or the prescriber if a side effect bothers you or does not go away, and keep using Mounjaro unless you are told to stop.

Serious side effects are rare. Call 111 or get urgent medical help for:

  • severe pain in your tummy or back that does not go away, which can be an inflamed pancreas (pancreatitis)
  • severe tummy pain from gallstones or an inflamed gallbladder
  • headache, blurred vision, dizziness, weakness, confusion, a fast heartbeat or sweating from low blood sugar, which is more likely if you also take other diabetes medicines

Call 999 for signs of a severe allergic reaction, such as a swollen tongue or throat, or difficulty breathing. If you feel dizzy or your vision changes, do not drive or use machinery until it has passed.

Who can get Mounjaro on the NHS?

You can currently be considered for NHS tirzepatide if both of these apply:

CriterionDetail
BMI40 or more — or 37.5 or more if you are of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background
Weight-related conditionsAt least four of: type 2 diabetes · high blood pressure · high cholesterol (dyslipidaemia) · heart disease · obstructive sleep apnoea

GPs in England have been able to prescribe it since June 2025, and NHS England plans to reach around 220,000 patients over three years, widening the criteria in stages. From 1 April 2026, GP practices can prescribe tirzepatide under the 2026/27 Quality and Outcomes Framework, so access through your own surgery is improving.

If you do not meet the criteria yet, you can still be referred to NHS weight-management services, which are open to far more people.

How do I ask my GP about Mounjaro?

Book a routine appointment and say you would like to discuss weight-management options, including whether you meet the NHS criteria for tirzepatide. Your GP will check your BMI, your medical history and which weight-related conditions you have. If you are not eligible, ask what NHS weight-management support is available locally — programmes combining diet, activity and behavioural support remain available to everyone.

How much weight do people lose?

In the first large head-to-head trial (published in the New England Journal of Medicine, 2025), people on tirzepatide lost 20.2% of their body weight on average at 72 weeks, compared with 13.7% on semaglutide. Weight loss is maintained only while treatment continues, which is why NHS care always includes support to change eating and activity habits alongside the injection.

Mounjaro vs Wegovy vs Saxenda

Mounjaro targets two gut hormones and produced the largest average weight loss in trials; Wegovy (semaglutide) targets one and is prescribed through specialist services with different criteria; Saxenda (liraglutide) is an older, daily injection. See our full weight-loss injections comparison for eligibility, routes and availability across the UK.

Is it safe to buy Mounjaro online?

Only from a UK-registered pharmacy with a valid prescription — check the pharmacy on the General Pharmaceutical Council register. Never buy from social media sellers or unregistered websites: the NHS and MHRA warn that unregulated “skinny jabs” have caused serious harm and deaths in the UK, and the BMJ reported black-market warnings while a fifth of areas still lacked NHS access. If in doubt, speak to your GP or pharmacist first.

Common questions about mounjaro

What are the most common side effects of Mounjaro?
The most common are nausea, vomiting, diarrhoea, constipation and reduced appetite, especially when starting or increasing the dose. These usually settle over a few weeks. Eating smaller meals and avoiding fatty food helps. Contact your GP or 111 if side effects are severe or persistent, and seek urgent help for severe tummy pain, which can rarely signal pancreatitis.
How much weight do people lose on Mounjaro?
In a major head-to-head trial published in 2025, people taking tirzepatide lost 20.2% of their body weight on average at 72 weeks, compared with 13.7% on semaglutide (Wegovy). Individual results vary, and weight loss is only maintained while treatment and lifestyle changes continue. NHS treatment always pairs the injection with diet and activity support.
Can my GP prescribe Mounjaro?
Yes — GPs in England have been able to prescribe tirzepatide for weight loss since June 2025 for people meeting the NHS criteria, and from 1 April 2026 GP practices can prescribe it under the 2026/27 Quality and Outcomes Framework. Rollout is phased, so availability still varies by area — around a fifth of areas were still setting up access in early 2026.
How much does Mounjaro cost?
On the NHS, Mounjaro costs the standard prescription charge in England (free if you are exempt, and free in Scotland and Wales). Privately, it typically costs between £120 and £250 a month depending on dose and provider. Only buy from a UK-registered pharmacy with a valid prescription — check the pharmacy on the General Pharmaceutical Council register.

Sources

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