Fermented milk might not sound like everyone’s drink of choice.
But, in the UK, the number of people consuming the health beverage, also known as kefir, is sky–rocketing.
And, according to experts, so too are the amount of patients reporting kefir side effects.
Kefir has been consumed for thousands of years and first originated in Russia and Mongolia.
But, in recent years, it has grown increasingly popular due to its much–hyped health benefits. In 2024, Tesco reported a 400 per cent increase in kefir sales compared to just 18 months previously.
The drink contains billions of good bacteria – so called friendly microbes that boost gut health. Studies show that having an above average level of these bacteria in the gut has wide–ranging positive impacts.
These include reducing the risk of heart disease, obesity, infections and even bowel cancer.
Kefir is a fermented milk, similar to a thin yoghurt, that is rising in popularity thanks to its gut health benefits and drinking just 250ml supplies around 40 per cent of daily calcium needs
‘I drink kefir every day,’ says Jordan Haworth, a digestive health expert at The Microbiome Clinic, in London, which offers gut microbiome testing.
‘It’s arguably a superfood. It contains about ten to 20 times more good bacteria than yoghurt, as well as protein, calcium and other vitamins.’
However, experts say kefir is not without risks. Some people can experience uncomfortable side effects from consuming the milk drink. For others, kefir can aggravate existing medical problems.
And, in rare occasions, kefir can even trigger a serious infection.
So, what are the dangers of kefir? And how can you avoid them?
Read on to find out more…
Beware of drinking too much kefir too soon
The sudden increase in bacteria in the gut caused by kefir can lead to painful symptom, experts say.
Kefir, which is made by combining aged milk with grains that are packed with bacteria and yeast, typically contains around two billion microbes per portion size.
The sharp tang of kefir may not be to everyone’s taste, but the fermented milk drink has a growing legion of fans, as it flies off the shelves
While, in the long–term, this boosts gut health, it can have short–term negative effects, says Mr Haworth.
Research shows that people who start drinking kefir for the first time often develop gas, bloating, cramps, diarrhoea, or loose stools.
This is because the new bacteria begin fermenting carbohydrates – like bread or pasta – in the gut. This produces excess gas.
Experts say the best way to avoid this problem is to start by consuming small portions of kefir and slowly increasing the dose over time.
‘It’s best to start low and go slow with kefir,’ says Mr Haworth. ‘Most people should be able to handle a small glass a day. A large glass could cause problems.’
Kefir could make stomach issues worse – not better
It’s often touted as a cure–all for gut problems – but kefir can make some digestive issues worse, say experts.
One of these is irritable bowel syndrome, also known as IBS, the painful condition that affects around one in ten Britons. It often leads to abdominal pain and diarrhea.
There are no licensed treatments, as it’s still unclear what triggers IBS. But growing evidence suggests that bacteria–packed drinks like kefir – known collectively as probiotics – can help combat the symptoms.
This is because research suggests that IBS patients tend to have low levels of good gut bacteria.
However, for others, experts say kefir can have the opposite effect. This is because the increase in gas levels – triggered by greater levels of carbohydrate fermentation – can further aggrevate the already irritated bowel lining.
Likewise, drinking kefir can also worsen symptoms for patients with a condition called small intestine bacterial overgrowth, or SIBO.
This occurs when excess bacteria in the small intestine – part of the gut – leads to a build–up of gas, triggering bloating, pain and diarrhoea.
Consuming kefir can increase this painful gas buildup, says Jordan Haworth.
‘Kefir can lead to bloating in IBS and SIBO patients,’ says Mr Haworth. ‘You don’t necessarily need to avoid it, just limit your consumption.’
For some, the ‘good’ bacteria in kefir can be dangerous
In extreme cases, probiotics like kefir can trigger dangerous infections.
This is because, for patients with severely weakened immune systems, the new bacteria gained from the drink can damage the body.
Last year, Japanese doctors revealed how one of their patient had died after taking a probiotic, similar to kefir.
In a medical review, published in the British Medical Journal, the medics detailed how the 70–year–old man had initially been admitted to hospital with Covid and an unrelated bacterial infection.
While recovering in hospital, he was advised to start taking a probiotic in order to tackle ongoing gut issues he was experiencing.
However, two months later, he was admitted to hospital with ‘severe, continuous’ pain in his abdomen, which was due to internal swelling brought on by an infection.
When the doctors tested his blood, they found that the infection was caused by a bacteria called Clostridium butyricum – the same contained in the probiotic supplement he took.
The man developed sepsis – where the immune system overreacts to an infection – leading to organ failure and death.
The doctors put his weakened immune system down to a long–standing alcohol problem combined with the Covid infection. They concluded that this allowed the bacteria found in the probiotic to infect the body and spread.
For this reason, experts say that patients with severe underlying health issues – such as HIV, an organ transplant or recent cancer treatment – should speak to their doctor before starting to drink kefir.
