Glass of kefir beside a jar of kefir grains

Dangers of Kefir: What You Need to Know

ByCraig Brockie Last updated

Is kefir dangerous? The short answer

For most healthy adults, kefir made from pasteurized milk is a low-risk food. The dangers of kefir are mostly practical: gas and bloating when you start, lactose for people who are very sensitive, added sugar in flavored bottles, and a few groups who should check with a doctor first. Kefir made from raw milk, and careless home brewing, carry the most serious risks.

This is general information, not medical advice. If you have a health condition or take prescription medicine, ask your doctor or pharmacist before making kefir a daily habit.

Gas and bloating when you start

Kefir is a fermented milk drink made with kefir grains, which hold lactic acid bacteria, acetic acid bacteria and yeast (UF/IFAS Extension). A sudden jump in fermented food can upset your stomach for a while, and this is the most common of the kefir side effects people notice.

A Cleveland Clinic dietitian says an adjustment period with gas and bloating is common and advises adding kefir slowly (Cleveland Clinic). Stanford Medicine’s nutrition program gives the same advice for fermented foods in general: start with small amounts and one serving a day, then increase over time, because too much too quickly can cause bloating, gas and discomfort (Stanford Medicine).

The NIH Office of Dietary Supplements notes that in healthy people, probiotics may cause gas but rarely cause infections or other health problems (NIH ODS). Our guide on how to drink kefir lays out a gradual way to build up.

Lactose: less than milk, about the same as yogurt

Fermentation uses up part of the milk sugar, but not all of it. USDA FoodData Central lists 3.7 grams of lactose per 100 grams of Lifeway lowfat plain kefir, compared with 5.01 grams in 2% milk and 3.35 grams in plain whole-milk yogurt.

In a small Ohio State study of 15 adults with lactose maldigestion, plain kefir and plain yogurt produced far less breath hydrogen than milk with the same 20 grams of lactose. All the kefirs and yogurts cut how bad people rated their flatulence by 54% to 71% compared with milk, while raspberry-flavored kefir did less well than plain on breath hydrogen (Hertzler and Clancy, 2003).

If you are very sensitive, start with a small glass or look for a lactose-free kefir, which UF/IFAS notes is available. A cow’s milk allergy is a different problem: UF/IFAS says dairy kefir is unsuitable for people with that allergy (UF/IFAS Extension).

Raw-milk kefir is the biggest risk

Fermentation does not make raw milk safe. UF/IFAS notes that kefir with a pH below 4.5 suppresses many pathogens, but E. coli, Listeria and Salmonella can survive (UF/IFAS Extension).

The FDA reports that since 1987 there have been 143 outbreaks of illness linked to raw milk and raw milk products contaminated with bacteria such as Listeria, Campylobacter, Salmonella and E. coli, some involving miscarriages, stillbirths, kidney failure and deaths (FDA). The CDC says children under 5, adults over 65, pregnant women and people with weakened immune systems face a higher risk of serious illness from raw milk and products made from it (CDC).

Choose kefir made from pasteurized milk, and use pasteurized milk if you make your own. Our article on raw milk yogurt covers the same question for yogurt.

Home fermentation: hygiene and alcohol

Jar of milk kefir fermenting under a cloth cover

UF/IFAS advises making kefir with pasteurized milk and quality grains, taking care to prevent contamination, and refrigerating it at 41°F or below after fermentation. It adds that people with compromised immune systems may want to avoid homemade kefir because of the risk of foodborne illness (UF/IFAS Extension).

  • Start clean: wash jars, lids, strainers and spoons in hot, soapy water before each batch.
  • Use pasteurized milk: it removes the raw-milk risk described above.
  • Chill when done: move finished kefir to the refrigerator at 41°F or below (UF/IFAS).
  • Throw out moldy batches: USDA says to discard moldy yogurt and sour cream because high-moisture foods can be contaminated below the surface (USDA FSIS). Treat kefir the same way.

Kefir yeasts also make a little alcohol. UF/IFAS says kefir may contain alcohol, which would not be appropriate for children and pregnant women (UF/IFAS Extension). Lifeway says it formulates its kefir not to contain alcohol (Lifeway FAQ).

Home ferments are less predictable. In a small forensic study of one homemade water kefir recipe, Swiss researchers measured up to 16 grams of ethanol per liter after two days and up to 38 grams per liter after 7 to 10 days, roughly 2% and 5% alcohol by volume (Rabl et al., 1994). Water kefir is fermented sugar water, not milk, so the numbers don’t carry over directly, but the lesson does: longer, warmer ferments make more alcohol.

Sugar in flavored kefir

Plain kefir contains only the sugar left from the milk. Flavored kefir adds more. Here is how two Lifeway labels compare (labels checked October 2026).

Product (1 cup)CaloriesTotal sugarsAdded sugarsSource
Lifeway Lowfat Plain Kefir1109 g0 gLifeway product page
Lifeway Lowfat Strawberry Kefir14018 g8 g (cane sugar)Lifeway product page

The FDA Daily Value for added sugars is 50 grams a day on a 2,000-calorie diet (FDA), so one cup of the strawberry version uses about 16% of it. Buying plain and adding fruit keeps you in control. Our best kefir brands guide compares more labels.

Who should ask a doctor first

  • People with weakened immune systems or serious illness: the NIH ODS says probiotics may cause bacterial infections in people who are already seriously ill or have weak immune systems (NIH ODS), and NCCIH says the risk of harm is greater in people with severe illnesses or compromised immune systems (NCCIH).
  • Hospital patients: in an open-label Mayo Clinic safety study, 54 critically ill adults received kefir in escalating doses with close monitoring. None showed signs of kefir-related bacteremia, and diarrhea occurred in two patients who were also taking laxatives (Gupta et al., 2024). It was a small, early-phase study under medical supervision, not a reason to give kefir to a sick person at home.
  • Pregnant women: avoid raw-milk kefir (CDC) and long home ferments that may contain alcohol (UF/IFAS).
  • People taking MAOI antidepressants: Mayo Clinic notes that fermented drinks such as kombucha and kefir may have higher levels of tyramine, which people on MAOIs are told to limit (Mayo Clinic).
  • People who react to histamine: Cleveland Clinic says aged and fermented foods can contain large amounts of histamine, though it doesn’t name kefir. It also calls histamine intolerance a proposed condition with no proven way to diagnose it (Cleveland Clinic).

Don’t use kefir as a reason to put off seeing a doctor about a health problem, a point NCCIH makes about probiotics in general (NCCIH). For the wider picture on this food group, see fermented foods for gut health.

When to see a doctor

Mild gas in the first week or two is common. NIDDK says to talk with a doctor if gas symptoms bother you, change suddenly, or come with abdominal pain, constipation, diarrhea or weight loss (NIDDK).

After raw-milk or homemade kefir, watch for the CDC’s signs of severe food poisoning (CDC):

  • Bloody diarrhea
  • Diarrhea that lasts more than 3 days
  • Fever over 102°F
  • Vomiting so often that you cannot keep liquids down
  • Signs of dehydration, such as peeing very little, a dry mouth and throat, or feeling dizzy when standing up

Related topics

Sources

Craig Brockie is the founder of Ultimate Health Solutions, the company behind Ultimate Gut, and the author of Ultimate Health, The Ultimate Gut Health Guide and The Unlimited Probiotics Guide. After injuries and chronic health problems in his twenties that conventional care didn’t resolve, he began researching health on his own and has studied natural health for 30 years. He is not a medical professional, and his articles are general information, not medical advice.

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