About half of American women ages 40 and over take some kind of dietary supplement. But recent studies have questioned the effectiveness of one type of pill, the multivitamin, in heading off major problems like heart disease. Now there’s a suggestion that multis may actually cause harm: They’ve been associated with an increased risk of breast cancer. In a 10-year Swedish study of 35,329 women ages 49 and up, participants who reported using multivitamins every day were 19 percent more likely to develop the disease, a finding that held up after researchers adjusted for various risk factors. One theory is that a component of multivitamins—possibly folic acid, which another study recently linked to breast cancer in a small minority of women—might play a role, says author Susanna Larsson, PhD, of the Karolinska Institute in Stockholm. To be clear: This observational study did not definitively establish that multivitamins contribute to the development of breast cancer. But the research does underscore the idea that it’s generally safer (and more effective) to “get as many nutrients as we can from food instead of pills,” says Andrea Giancoli, RD, MPH, spokeswoman for the American Dietetic Association.