More than half of American adults take vitamin pills. I've watched in wonder as some of my more health-conscious friends kick off their morning with an impressive array of multicolored supplements: A, C, D, calcium, magnesium, you name it. And it's not just my friends: data from the National Health and Nutrition Examination Survey (NHANES) indicate a trend away from all-in-one multivitamins and toward specific supplements—especially fish oil and vitamin D. Most of this is self-prescribed. According to a 2016 analysis of the NHANES data, less than a quarter of supplements are taken at the recommendation of a health professional.