‘I started Wegovy just over a month ago – and within 24 hours of my first dose I got terrible diarrhoea,’ says Bethany, 34, who works as a mental health nurse.
‘It happened again and again for days and I was late to work because of it.
‘The medication has worked amazingly for weight loss – I have lost 10lb in just a month and my weight has gone from 15st 8lb to 14st 7lb, but the diarrhoea is taking its toll, and sometimes makes me anxious about going out.’
Bethany tried seeking help from her GP but, because she sourced the medication herself through an online pharmacy, says the GP ‘doesn’t want to know’.
‘I’ve decided to stay on the starter dose of 0.25mg longer and not go up – as I don’t want the diarrhoea to get worse,’ she says.
‘I’m also eating no refined sugar, fatty or greasy or fatty foods as I’d read those can make it worse.’
Visit any online weight-loss jab support group and you will encounter members sharing experiences of lavatory near-misses, of having to stop and ‘go’ in a field – or even having accidents in public and in their cars – resulting in shame and discomfort.
‘Diarrhoea is a very common side effect of GLP-1 agonist drugs [such as semaglutide, used in Wegovy], with up to 30 per cent of patients reporting it in clinical trials,’ says Dr Hal Brindley, a consultant gastroenterologist and hepatologist at Barts Health NHS Trust and London Bridge Hospital, both in London.
‘That’s because the drugs work by significantly slowing down the stomach’s emptying rate, which is good for blood sugar control and beneficial for satiety – it makes you feel fuller for longer and reduces appetite.
‘However, this slowdown can create a mismatch with the bowel, as the medication can also speed up movement in the intestines. The gut stimulation starts as soon as the stomach senses food – it is called the gastrocolic reflex – and instructs the gut to release more fluid.’
This mismatch of slow activity at the top of the stomach and fast movement at the bottom of the bowel create a kind of digestive ‘traffic jam’.
When food moves too quickly through the small intestine, the body doesn’t have enough time to absorb water from it and it gets passed out in our stool, which becomes more watery.
On top of that, adds Dr Brindley, because the stomach is emptying more slowly, some food – especially fatty or sugary items – may reach the bowel without being fully digested.
‘These undigested bits act like a sponge, drawing even more water into the bowel and leading to loose, watery stools.’
He adds: ‘The severity is often dose-dependent, meaning higher doses of the drug may lead to higher rates of diarrhoea.’
The degree to which this effects people ‘varies considerably’, he says. ‘And in clinical trials, 6 to 8 per cent of people stop the drug due to gastrointestinal side effects.’
Some, such as those with irritable bowel syndrome, are more prone to this side effect that others says Dr Brindley – although they can still use the drugs as for most people it is a temporary problem that ‘peaks during the initial few weeks of treatment or after a dose increase – and then subsides, as the body develops tolerance over several months.
‘However for a minority it may be a longer-term problem.’
Zaher Toumi, a bariatric surgeon based at the Spire Washington Hospital in Newcastle, says that diarrhoea becomes worse when people start to increase their dose, as ‘the sudden rise in medication levels can overstimulate the gut before the body has had time to adjust’.
‘That is why doctors advise on a gradual dosing schedule, increasing the medication step by step over several months,’ he says.
‘This gives the digestive system time to adapt and significantly reduces the risk of side effects.’
Sally Jones, 69, a retired shop assistant from Kent, started taking Mounjaro in June 2025, when she weighed 19st (she’s 5ft 8in). When she went on to the higher dose she was hit by such severe diarrhoea she ‘could not leave the house’.
Starting on 2.5mg, she said she initially got ‘a very upset tummy’, but when she went up to a 7.5mg dose (as the 5mg was not working), she says: ‘I was literally running to the loo day and night for seven days.
‘I tried over-the-counter anti-diarrhoea tablets and was taking two every two hours – but they didn’t help. I felt very dehydrated and was just drinking water and eating crackers.
‘One day I was in the car with my husband and suddenly got the urge to go. He had to speed back home so I could get to the loo in time.’
Yet Sally says the drug has worked wonders for her.
‘It’s changed my life,’ she says. ‘I am already down to 15st.’
She’s still taking the medication and says the side effects are a price worth paying.
Dr Brindley says many people are prepared to put up with the downside of these drugs because of the weight loss they bring.
‘On the whole, side effects of medications are downplayed and people want to focus on the potential benefit before acknowledging the downsides,’ he says.
‘This isn’t just specific to GLP-1s, but many other medications as well.’
Indeed, some people choose to continue with the jabs despite being hit by severe diarrhoea.
Samantha Caris, 51, a clerical assistant from Swindon, started taking Mounjaro last year, when she weighed 10st 12lb (she is 5ft 1in) and was delighted to shed 3st in 18 months.
Having hit her goal weight, she stopped the jabs – but then piled the weight back on over the summer.
So she started using the injections again in September, starting on 2.5mg and going up to 5mg, and this time she was hit with diarrhoea.
According to the experts, the severity of the diarrhoea ‘is often dose-dependent’ and ‘varies considerably’
‘It became crippling,’ she says.
‘I struggled to even go out, but I had to go to work so I had to take Imodium constantly.
‘One day at a work meeting I had to suddenly rush out and go to the loo. Another time I had to jump off my bus early and waddle into a pub to use their loo and only just got there in time.
‘I’ve even considered a nearby park on occasion in a panic. I love the results of Mounjaro – but not this side effect.’
Despite all this, she says: ‘I am still taking it.’
Mr Toumi says there are several ways to manage ‘troublesome diarrhoea’.
‘Temporarily lowering the dose or pausing the medication until the diarrhoea resolves can give the body time to recover,’ he says.
He also advises people to tweak their diet.
‘Food that is greasy, spicy or high in sugar can irritate the gut – and make diarrhoea worse.
‘On the other hand, simple, bland foods containing soluble fibre – such as oats, bananas and apples – are easier on the stomach and can help settle things down by absorbing excess fluid.’
What’s more, he says ‘staying well hydrated is essential to replace lost fluids’.
Mr Toumi adds: ‘Over-the-counter remedies such as loperamide may help by slowing down the bowel – but they should only be used under medical advice, to ensure they don’t mask other conditions that diarrhoea can be linked to, such as inflammatory bowel disease or bowel cancer.
‘If the problem doesn’t improve, it is important to speak with a GP. In many cases, switching to a different GLP-1 medication or adjusting the treatment plan can make a big difference.’
He says that for some, the side effects mean ‘they have to give up the drugs altogether’.
But he adds: ‘With the right support and a personalised approach, most people can continue their weight-loss treatment safely and comfortably, allowing them to still benefit from the long-term health improvements these medications offer.’