Kefir is a probiotic fermented milk drink made with multiple cultures of lactic acid bacteria and yeasts. There is no single starter culture, and the bacteria and yeasts vary quite a bit between different brews.
The human clinical trials on kefir to date are all small. Between the small trial sizes and the massive variation in kefirs, there isn’t strong evidence that whichever kefir you take will have any given effect on you. On the other hand, if you’re a generally healthy adult, kefir is unlikely to hurt you, and may benefit your oral health.
Kefir may also be a useful adjunct to a common treatment for stomach ulcers, and for people with type 2 diabetes, regular kefir consumption may give slight improvements to blood sugar and glycated hemoglobin (but not enough to be considered a treatment).
Kefir has roughly the same caloric content as the milk it was brewed from, and is often additionally flavored and sweetened.
As it contains live, active cultures, kefir should be taken with caution by some.
Kefir is lower-lactose than milk, but may still contain enough lactose to trigger symptoms of lactose intolerance.
Kefir grains can, rarely, be contaminated with pathogenic fungi or bacteria.
Kefir can be drunk with meals, or alone. Kefir is also used as an ingredient in several cuisines, just as other fermented milk products (yogurt, sour cream) are. When used as a probiotic, kefir shouldn’t be cooked, because that will kill the beneficial bacteria. As with other fermented milk products, there is no recommended daily minimum or maximum for kefir consumption.