Similar to yogurt, kefir is a fermented milk beverage that contains probiotics, or friendly bacteria. It also contains calcium, B vitamins and other nutrients and is generally considered a safe and healthy beverage. In some cases, however, kefir may cause unpleasant side effects. In addition, flavored kefir is sometimes high in sugar, so read the label before buying.
Any side effects of kefir are typically due to the probiotic content. Among healthy people, the only known risk is excess gas, according to NYU Langone Medical Center. People with suppressed immune systems, however, may run a risk of infection when taking probiotics -- Langone notes that at least one person taking immunosuppressant medication died this way. Probiotics may also be unsafe for people with severe acute pancreatitis, notes Langone.