Fiction: Many people think if a little of something is good, a lot is even better. "But taking mega-doses of specific vitamins can be harmful," says Conrad L. Flick, M.D., F.A.A.F.P., a board member with the American Academy of Family Physicians. "For example, excessive amounts of vitamin A in pregnancy are associated with birth defects." Also, high doses of vitamin D can increase the risk of side effects, such as nausea, vomiting or heart rhythm problems. In fact, high doses of any vitamin may cause side effects not seen at the usual label-recommended doses. To avoid overdosing, always follow the label instructions.