Prenatal vitamins: OK for women who aren't pregnant?
You may be tempted to take prenatal vitamins because of unproven claims that they promote thicker hair and stronger nails. While prenatal vitamins are generally safe for healthy adults, they may not be suitable if you're not pregnant or planning to become pregnant. Prenatal vitamins are formulated specifically for a mother-to-be, her developing fetus and women who are breast-feeding, with particular emphasis on:

Folic acid. The Institute for Clinical Systems Improvement recommends that women get 800 micrograms (mcg) of folate or folic acid a day before conception through diet or supplements and then increase that to 1,000 mcg a day during pregnancy. Other healthy adults — both men and women — need only 400 mcg a day. While uncommon, getting too much folic acid by taking supplements can mask the symptoms of vitamin B-12 deficiency and delay diagnosis and treatment.
Iron. During pregnancy, the recommended intake of iron is 27 milligrams (mg) a day. Women between the ages of 19 and 50 who aren't pregnant need only 18 mg a day, and women age 51 or older and all adult men need only 8 mg a day. Getting too much iron can be toxic because it can build up in your body, causing constipation, nausea, vomiting, diarrhea and in severe cases, possibly death.
Calcium. Pregnant adult women and healthy men and women ages 19 to 50 all need 1,000 mg a day. Men and women age 51 and older need 1,200 mg a day. Because prenatal vitamins are intended to supplement calcium you get in your diet, they generally contain only 200 to 300 mg of calcium. If you rely on prenatal vitamins to meet your calcium needs, you likely won't get enough, raising your risk of osteoporosis and other health problems.
It's best to take a multivitamin tailored to your sex, age and specific medical needs.