The B vitamin, essential for women who may become or are pregnant to prevent birth defects, is a double-edged sword when it comes to cancer risk. Consuming too much of the synthetic form (not folate, found in leafy green veggies, orange juice, and other foods) has been linked to increased colon cancer risk, as well as higher lung cancer and prostate cancer risks. Rethink your multivitamin, especially if you eat a lot of cereal and fortified foods. A CDC study discovered that half of supplement users who took supplements with more than 400 mcg of folic acid exceeded 1,000 mcg per day of folic acid. Most supplements pack 400 mcg. Individual supplements (of vitamin D and calcium, for instance) may be a smarter choice for most women who aren't thinking of having kids.