Nearly 40 percent of American adults take a daily multivitamin, and more than 50 percent of American adults take some sort of dietary supplement (the most popular is a daily calcium supplement) [source: Fetters]. While evidence suggests that calcium may actually have a positive impact on your health, there doesn't seem to be any evidence any other ingredient in your multivitamin does anything positive for you. At all. And in addition to that bombshell, some research suggests those of us who take vitamins may die sooner than our friends who skip the supplements. Yes, you read that correctly; in 2011 The Iowa Women's Health Study found multivitamins didn't offer any health benefits to the women taking them -- and also found an association between taking dietary supplements and a shortened lifespan [source: Mursu]. It's entirely possible that this is due to people having a false sense of security about their health; taking a multivitamin may make them less likely to take care of themselves in other ways.
The best thing to do is ditch the daily vitamins and instead make sure you eat a healthy, balanced diet.