If you're sensitive to it, you'll know right away because of stomach cramps. :) Any other symptoms are most likely due to the microbes adjusting to their new environment, or calcium binding up bile salts. If you're prone to reacting a lot to microbes, just keep the intake to 1 cup a day or thereabouts, you should be fine on that. :) If you're prone to calcium binding you up, be sure to drink kefir alone, not with a meal, and it should not happen near as much because there won't be any bile for it to bind up! :)