The hidden danger of kefir: How the trendy ‘off-milk’ drink harms gut health
The Rise of Kefir and Its Potential Side Effects
Fermented milk might not sound like everyone’s drink of choice. But in the UK, the number of people consuming kefir, a health beverage with a long history, is on the rise. According to experts, this surge in popularity has also led to an increase in reports of kefir side effects.
Kefir has been consumed for thousands of years and originated in Russia and Mongolia. In recent years, it has gained significant traction due to its purported health benefits. In 2024, Tesco reported a 400% increase in kefir sales compared to just 18 months earlier.
The drink contains billions of beneficial bacteria—often referred to as friendly microbes that support gut health. Studies suggest that having an above-average level of these bacteria in the gut can have wide-ranging positive impacts, including reducing the risk of heart disease, obesity, infections, and even bowel cancer.
“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 yogurt, as well as protein, calcium, and other vitamins.”
However, experts caution that kefir is not without risks. Some individuals may experience uncomfortable side effects from consuming the milk drink. For others, kefir can exacerbate existing medical conditions. In rare cases, it can even trigger a serious infection.
So, what are the dangers of kefir? And how can you avoid them?
Beware of Drinking Too Much Kefir Too Soon
The sudden increase in bacteria in the gut caused by kefir can lead to painful symptoms, according to experts. Kefir, made by combining aged milk with grains packed with bacteria and yeast, typically contains around two billion microbes per portion size.
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, diarrhea, or loose stools. This is because the new bacteria begin fermenting carbohydrates—like bread or pasta—in the gut, producing excess gas.
Experts recommend starting with small portions of kefir and gradually 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
Despite being touted as a cure-all for gut problems, kefir can make some digestive issues worse, say experts. One such condition is irritable bowel syndrome (IBS), a painful condition affecting around one in ten Britons. It often leads to abdominal pain and diarrhea.
There are no licensed treatments for IBS, as it’s still unclear what triggers the condition. However, growing evidence suggests that probiotics—drinks like kefir—can help combat 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 intestine bacterial overgrowth (SIBO). This occurs when excess bacteria in the small intestine leads to a build-up of gas, triggering bloating, pain, and diarrhea.
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 patients 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 the hospital, he was advised to start taking a probiotic to tackle ongoing gut issues he was experiencing. However, two months later, he was admitted to the hospital with severe, continuous abdominal pain due to internal swelling caused 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 attributed his weakened immune system 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 advise that patients with severe underlying health issues—such as HIV, an organ transplant, or recent cancer treatment—should consult their doctor before starting to drink kefir.
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