Kat lost more than 6st on Mounjaro and thought all her prayers had been answered… until she ended up in hospital in agonising pain. Now, doctors reveal devastating jab side effect surging among users – and who’s most at risk

For nearly two years while taking weight-loss medication Mounjaro, Kat Smith experienced few, if any, problems. Indeed, her experience was entirely positive. 

The mother of three, 41, who works as a nurse in London, saw her weight fall from 14st 3lb to 7st 12lb – shedding more than 6st of excess weight. 

She had more energy, felt less out of breath when active and saw her cholesterol and blood pressure fall to healthy levels. 

So when Kat found herself, at 2am one morning, curled in a ball on the bathroom floor with chest pain so severe she felt she was having a heart attack, it never even crossed her mind that it could be caused by the jab. 

The pain had started as a tingle, she explains, and a heat that radiated through her upper body. Waking up in the middle of the night, Kat at first thought she might be having a hot flush, or was experiencing the early signs of a stomach bug. 

But when she reached the bathroom to splash some water on her face, the sensation suddenly shifted to an intense pain that spread from underneath her chest to her neck and arms. 

Within 20 minutes, Kat could barely raise her left arm. 

‘I was sure I was having a heart attack,’ she explains. ‘It’s a pain that I’ve not experienced ever. Even childbirth pales in comparison.’

At the time, she was staying at her parents’ house. Finding her on all fours in the bathroom, Kat’s mother immediately called 999. After she was rushed to hospital, scans revealed Kat was suffering from a severe gallstone attack. 

Gallstones are hard lumps that can form inside the gallbladder, a small pouch that sits beneath the liver that stores bile, a digestive fluid that is released into the gut after eating to help break down fats.

In the gallbladder, these pebble-like deposits can range in size from tiny grains of sand to large pebbles.

Millions of Britons are thought to have gallstones, although most will never know it because they cause no symptoms.

But when a stone moves and blocks one of the narrow tubes that carry bile from the liver and gallbladder into the gut, preventing the fluid from draining normally, the consequences can be excruciating.

Symptoms include sudden, severe pain – known as a gallstone attack or biliary colic – and, if the blockage persists, potentially life-threatening infections and inflammation can occur that ultimately mean the gallbladder has to be surgically removed.

Strangely, says Kat, one of the first questions she was asked by doctors was whether she was taking weight-loss medication. The gallstones, they told her, were almost certainly caused by the jabs. 

She is currently waiting to undergo surgery. 

‘I knew there were some side effects of the drug, but no one ever clarified to me that this could happen – or that I could end up in hospital in the worst pain of my life,’ she says. 

‘I wish someone had at least warned me, so that I could prepare – or know what to look out for. I thought I was dying.’

Kat’s experience is not unique – and the risk is not confined to Mounjaro.

Gallstones are a recognised potential side effect of weight-loss jabs, also known as GLP-1s, with patient safety information warning of the problem.

The patient leaflet for Wegovy, which contains the GLP-1 drug semaglutide, lists gallstones as a ‘common’ side effect, meaning they may affect up to one in ten patients.

And research suggests the increased risk may extend across the wider family of GLP-1 drugs, which also includes Ozempic.

A major 2022 analysis of 76 clinical trials involving more than 103,000 patients found those given GLP-1 drugs were 37 per cent more likely to develop gallbladder or bile duct problems than those given a placebo or other treatment.

They were 27 per cent more likely to develop gallstones and 36 per cent more likely to suffer inflammation of the gallbladder, while the likelihood of needing their gallbladder removed was 70 per cent higher.

Crucially, the risk appeared substantially greater when the drugs were being used specifically for weight loss, when they are given at higher doses.

Experts speaking to the Daily Mail say the issue may be more serious – and commonplace – than patients realise.

‘It’s clear that GLP-1 medications affect gallbladder function and substantially increase the risk of gallstone disease,’ says Mr Saqib Rahman, consultant in general GI and emergency surgery at University Hospitals Southampton NHS Trust.

‘As an emergency gastrointestinal surgeon, it’s something I now see quite a lot. We have multiple people a week coming in for emergency surgery who have been taking GLP-1s.

‘These drugs can be life-changing but it’s important that people know about these risks.

‘Gallbladder issues are often trivialised. But if people have symptoms, they need to know to seek help.’

Amid an extraordinary rise in the use of weight-loss jabs, there has also been a striking increase in the number of Britons undergoing gallbladder surgery.

More than 80,196 gallbladder operations were carried out by the NHS in 2024-25 – the highest number in the past decade and a 15 per cent increase on the previous year, official figures show.

In comparison, around 39,000 gallbladder removals were performed in England in 2000 – meaning the number has more than doubled in 25 years.

The organ is not essential – the liver continues to produce the bile needed to digest fats after it has been removed. Instead of being stored in the gallbladder and released after meals, however, the bile flows continuously from the liver into the gut.

Most people can eat normally afterwards, although some experience temporary digestive problems such as diarrhoea, indigestion or abdominal pain while their body adjusts.

Experts say part of the link between GLP-1 drugs and gallbladder problems is partly explained by the rapid weight loss they can produce.

‘The overall guidance is that any type of weight loss – whether by surgery, medication or lifestyle – will increase the risk of gallstones,’ says Dr Naveed Sattar, professor of metabolic medicine at the University of Glasgow.

‘This is because when you lose weight, you have a lot of cholesterol from fat material that has to go somewhere. And often it goes into and supersaturates the body’s bile, which then creates gallstones.

‘Evidence we have so far suggests that gallstones are probably not a specific effect of the GLP-1s, but are simply linked to the process of losing weight and having to get rid of that extra cholesterol.’

However, scientists have also investigated whether GLP-1 drugs themselves could affect the way the gallbladder works, potentially providing another explanation for the increased risk.

Normally, after a meal, the gallbladder contracts to squeeze stored bile into the intestine, where it helps digest fat.

But a 2018 trial by researchers in Copenhagen found that in patients taking liraglutide – the GLP-1 drug sold for weight loss as Saxenda – the gallbladder took significantly longer to reach its maximum contraction after a meal.

This slower response could mean bile remains sitting in the gallbladder for longer, creating more opportunity for cholesterol within it to crystallise and eventually form stones.

The researchers suggested this delayed emptying could therefore be one reason gallstones occur more frequently among patients taking GLP-1 drugs.

Mr Rahman says: ‘GLP-1s help reduce hunger by slowing gastric emptying, and this also affects the gallbladder. 

‘The absolute risk per patient remains modest, but the very large and growing number of people on these medications means the absolute number of patients presenting with biliary complications is substantial and rising.’

Certain patients are more likely than others to develop gallbladder issues, say experts.

Some patients may already be more vulnerable to gallstones before they begin taking a GLP-1 drug.

Women, people over 40 and those with a personal or family history of gallstones are all at greater risk of developing them generally. Rapid or substantial weight loss – including while taking a weight-loss jab – can increase the risk further.

There is no guaranteed way to prevent gallstones from forming. But it is important that people taking GLP-1 drugs – particularly those who already have risk factors or are losing weight rapidly – know the warning signs to watch out for, says Mr Rahman. 

‘Any sudden pain in the upper right or upper central abdomen, as well as pain that radiates to the right shoulder or centre of the back, should prompt assessment,’ he says.

Persistent nausea or vomiting beyond the expected GLP-1 side-effect window, as well as any fever, chills or yellowing of the eyes or skin, should also be cause for concern.

The risk of gallstones shouldn’t put eligible patients off of the jabs altogether, clarifies Mr Rahman.

‘In my perspective, these are very good drugs,’ he says. ‘But patients need to know their side effects.’

Adds Professor Sattar: ‘Even if the risks are explained properly to patients, most people who are eligible will still choose to take them. But they need to be made clear in the first place.’ 

A spokesman for Mounjaro maker Eli Lilly said: ‘Patient safety is Lilly’s top priority and we actively monitor, evaluate, and report safety information for all our medicines to the MHRA. 

‘Anyone experiencing side effects when taking any Lilly medicine should consult their doctor or other healthcare professional, and should ensure that they are getting genuine Lilly medicine.’

A spokesman for Wegovy and Ozempic maker Novo Nordisk said GLP-1 drugs ‘are a well-established class of medicines, which have been studied rigorously in clinical trials’. According to Novo Nordisk, acute gallstone disease was reported in 1.6 per cent of patients, which led to cholecystitis, or inflammation of the gallbladder, in 0.6 per cent of patients.

‘For this reason, acute gallstone disease is listed as a “common” potential adverse reaction for Wegovy in the product’s UK SMPC (product summary) and should be considered when patients are being evaluated for this medicine,’ it said.