A doctor has issued a caution regarding Mounjaro, a new drug set to become a popular weight loss aid in the UK. The GP explained the potential risky side effects of this diabetes medication, which is also set to be prescribed for obesity.
Mounjaro, commercially known as tirzepatide, is a medication that reduces blood sugar levels. It functions by activating two receptors, GLP-1 and GIP, thereby increasing the level of incretins – hormones found in the gut.
This process enables the body to produce more insulin when necessary, decreases the amount of sugar produced by the liver, and slows down the rate at which food is digested. While these effects have been demonstrated to facilitate “rapid” weight loss, Doctor Mark Porter advises caution.
In his column for The Times, Dr Porter – recognised for his appearances on The One Show and Radio 4 – outlined some potential drawbacks of taking Mounjaro.
The medical expert confessed he hasn’t yet prescribed any of these new slimming injections for weight loss, but he’s aware of “plenty” of people, “both patients and friends”, who are obtaining them through private clinics, reports the Express.
He elaborated that Mounjaro is now becoming the preferred option over semaglutide, commonly known as Ozempic or Wegovy, with sales projected to hit $34 billion annually 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,” Dr. Porter stated. “They may even have a role in helping people with addictions to alcohol, tobacco and opioid drugs.”
By June 2025, certain NHS GPs will be authorised to prescribe Mounjaro for weight management, although its availability will be “limited”. At present, it’s only possible to obtain a prescription through NHS GPs for treating type 2 diabetes.
Dr. Porter said: “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.”
Consequently, some individuals might opt for private healthcare to get the medication, which is expected to cost around £130 monthly. While Dr. Porter expressed his endorsement for “any advance” in the battle against obesity and its associated health issues, he admitted having “reservations” about the use of such medications specifically for the purpose of losing weight.
Dr Porter has voiced concerns over the increasing use of weight loss injections, fearing they may be considered a “substitute” for vital diet and exercise routines, potentially diminishing the motivation for individuals to adopt healthier lifestyles. He further highlighted anxieties about the implications on a person’s weight if they were to stop taking the medication.
He remarked: “The drugs may save money in the long term, but at what cost to other services in the short term?” Alongside that, he pointed out the risk of unwanted side effects.
Decrease in muscle mass
One of those side effects includes the potential decrease in muscle mass and strength due to rapid drug-induced weight reduction. Dr Porter stated: “A study following 140 volunteers on semaglutide showed that about 40 per cent 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.”
He continued: “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.”
Dr Porter added: “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.”
Digestive problems
People using certain weight loss medications might experience digestive issues, according to Dr Porter. He 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.”
Side effects of tirzepatide are reportedly similar to those of semaglutide (Ozempic/Wegovy), primarily affecting the gastrointestinal system with common symptoms (for more than one in ten people) including nausea, vomiting, diarrhoea, and constipation.
While serious side effects like gallstones and pancreatitis are less common, they do occur. The Medicines and Healthcare products Regulatory Agency highlighted the key adverse reactions upon approving Mounjaro for weight management: “The most common side effects of the medicine are nausea, diarrhoea, vomiting (which usually goes away over time), and constipation.”
Low blood sugar
Dr Porter said: “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.”
Administered weekly via injection, Mounjaro’s patient information for UK residents can be found in section four of an online pamphlet accessible here.
Manufacturer Eli Lilly and Company said: “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.”
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