as everything in the world there are good yeast that help your body and bad yeast the one that comes in kefir is the good yeast, don't worry about it, I have been drinking for 2 months 1.5 litres of kefir per day and feeling better day by day. However the commercial kefir is not the right one you must try to get the real kefir grains, so that you can prepair kefir at home with not additional price other than the milk.
Conclusion: Kefir is the best probiotic and food you can put in your guts.