While fermented milk may not appeal to everyone as a beverage, its popularity in the UK is soaring. Known as kefir, this health drink is attracting a growing number of enthusiasts.

Experts have noted a corresponding rise in reports of side effects from kefir consumers. Historically enjoyed for millennia, kefir has its roots in Russia and Mongolia.

Recently, its reputation has surged due to widely publicized health benefits. In 2024, Tesco revealed a dramatic 400% increase in kefir sales over the previous year and a half.

Packed with billions of beneficial bacteria, or probiotics, kefir is celebrated for promoting gut health. Research indicates that having a higher concentration of these microbes in the digestive system can lead to numerous health advantages.

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 earlier.

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. Just 250ml supplies around 40 per cent of your 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 symptoms, 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 and is flying 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 – a painful condition that affects around one in ten Britons. It often leads to abdominal pain and diarrhoea.

There are no licensed treatments, as it’s still unclear what triggers IBS. But growing evidence suggests that bacteria–packed drinks such as 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 aggravate the already irritated bowel lining.

Likewise, drinking kefir can also worsen symptoms for patients with a condition called small intestinal 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 build-up, says Mr Haworth. ‘Kefir can lead to bloating in IBS and SIBO patients,’ he adds. ‘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 such as 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 patients 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 problems 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 problems – such as HIV or recent cancer treatment – should speak to their doctor before starting to drink kefir.