question and answer
Best dose of folic acid before pregnancy
October 2008
“How much folic acid — 400 μg, 1 mg, 5 mg — is recommended before conception and during pregnancy? Will these amounts be affected by interactions with medications for diabetes or epilepsy?” BETTINA WINDISCH, MD, Halifax, NS
Though the efficacy of folic acid in primary prevention of neural tube defect has been shown clearly in a number of trials, these studies didn’t demonstrate the ideal dose. After thoughtful consideration, it’s been recommended that women planning pregnancy consume at least 400 μg/day of the vitamin. Given the usual Canadian diet, this means that women should take a supplement with at least 400 μg folic acid on a daily basis. Formulations containing 400 μg and 1 mg are available without prescription.

In cases where a previous pregnancy resulted in a child with neural tube defect, most experts would recommend a daily dose of folic acid in the range of 4-5 mg. In Canada, there’s a 5-mg tablet available by prescription. Diabetes and the medications used for its management shouldn’t alter the dose of folic acid needed. In the case of epilepsy, however, two commonly used drugs — valproic acid and carbamazepine — are associated with a 10-fold increase in the risk for neural tube defect. While the exact dose for prevention isn’t known, many experts would recommend that women being treated with these meds also receive a daily dose of 5 mg of folic acid.

Finally, recent work in Toronto and Hungary has suggested that women contemplating pregnancy should take a daily multivitamin, as it yields a slight reduction in overall risk of birth defects. So for those without epilepsy or a history of having a baby with spina bifida, taking a multivitamin containing 400 μg folic acid will meet both goals.
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