A doctor has issued a warning over a new drug set to become a popular weight loss aid in the UK. The GP highlighted the potential risky side effects of Mounjaro, a diabetes medication that will also be prescribed to people living with obesity.

Mounjaro, which is the brand name for tirzepatide, is a drug that lowers blood sugar levels. It works by activating two receptors known as GLP-1 and GIP to increase the level of incretins – hormones found in the gut.

This allows the body to produce more insulin when needed as well as reducing the amount of sugar produced by the liver, and slowing down how quickly food is digested. While this has been shown to help people lose weight “rapidly”, it should be approached with caution, according to Doctor Mark Porter.

Writing for The Times, Dr Porter – who is known for his appearances on The One Show and Radio 4 – shared some of the potential downsides of taking Mounjaro.

The medical professional admitted he had not yet prescribed any of the new slimming injections for weight loss, but he knew “plenty” of people, “both patients and friends”, who are taking them through private clinics.

He explained that Mounjaro is now becoming the preferred choice over semaglutide, which is better known as Ozempic or Wegovy, with sales set to reach reach $34 billion a year by 2029. Dr Porter acknowledged that it’s “not all about weight loss”.

“There is growing evidence that they may protect against stroke, heart attack, some types of cancer and dementia,” he said. “They may even have a role in helping people with addictions to alcohol, tobacco and opioid drugs.”

As of June 2025, Mounjaro is set to be available for prescription for weight loss from some NHS GPs. However, access will be “limited”. Currently, NHS GPs can only prescribe it for type 2 diabetes.

Dr Porter explained: “Initially it looks like GPs will only be able to prescribe Mounjaro to very obese people with a BMI of 40 or more (37.5 for some higher risk ethnic groups) and at least four of the following: type 2 diabetes, high blood pressure, high cholesterol levels, obstructive sleep apnoea (severe snoring) or heart disease. Lower thresholds can apply for those patients referred to specialist weight-management clinics, but access is very limited.”

Some people may, therefore, go private to access the medication. This will cost around £130 a month. Dr Porter admitted that he welcomed “any advance” to help tackle obesity and its complications, but he said he had “reservations” about using these drugs to treat weight loss.

Concerns about weight loss jabs

Firstly, Dr Porter is concerned that any of these drugs will be seen as a “substitute” for diet and lifestyle measures, meaning people will feel less inclined to eat healthily and exercise regularly if they are in receipt of a weight loss jab. He was also worried about the impact on people’s weight when they stop taking them.

He said: “The drugs may save money in the long term, but at what cost to other services in the short term?” On top of this are the potential side effects.

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One such side effect is the effect of rapid drug-induced weight loss on muscle bulk and strength. Dr Porter continued: “A study following 140 volunteers on semaglutide showed that about 40% of the weight loss seen was muscle rather than fat, with more recent studies suggesting that this muscle loss may be even more marked with tirzepatide. 

“Muscle loss (sarcopenia) happens to some degree with all weight-loss programmes but appears more pronounced with the new slimming jabs. Ensuring you are eating enough protein and practising resistance/weight training should help to mitigate this loss, but most of the people I know taking the drugs are not doing either. 

“And it shows, at least to my eyes. The older you are the greater the significance, as we all lose muscle bulk and strength from early middle age on – as much as 10% during our sixties alone. Be careful.”

Certain people on weight loss drugs will also experience gastrointestinal problems. Dr Porter said: “Research suggests that for most people taking these drugs to lose weight, tirzepatide (Mounjaro) is the most effective.

“They all work by decreasing appetite/increasing satiety: you feel full sooner and for longer.

Tirzepatide side-effects appear much the same as with semaglutide (Ozempic/Wegovy) and are mainly gastrointestinal, the most common complaints (one in 10 or more) being nausea, vomiting, diarrhoea and constipation.”

Serious side effects are rarer but can include gallstones and inflammation of the pancreas (pancreatitis). Announcing the approval of Mounjaro for weight loss, the Medicines and Healthcare products Regulatory Agency previously said: “The most common side effects of the medicine are nausea, diarrhoea, vomiting (which usually goes away over time), and constipation. 

“Low blood sugar (hypoglycaemia) is also very common in patients with diabetes. Symptoms of this can include headache, drowsiness, weakness, dizziness, feeling hungry, confusion, irritability, fast heartbeat and sweating.”

Mounjaro is typically given as an injection once a week. UK patient information about Mounjaro is available in section four of this online leaflet here.

Manufacturer Eli Lilly and Company commented: “Patient safety is Lilly’s top priority, and we take any reports regarding patient safety extremely seriously. Mounjaro (tirzepatide) should only be used when prescribed by a licensed healthcare professional and prescriptions should be fulfilled and supplied by registered pharmacies and providers. We encourage patients to consult their doctor or other healthcare professional to discuss any possible side effects they may be experiencing and to ensure that they are getting genuine Lilly medicine.”