Folic acid is generally considered safe and well-tolerated when taken at appropriate dosages.
When taken during pregnancy, observational studies suggest higher doses of folic acid may be associated with an increased risk of gestational diabetes,[1][2] childhood asthma,[3] and impaired psychomotor and neurological development.[4][5]
Folic acid can mask the signs of vitamin B12 deficiency. Left untreated, a vitamin B12 deficiency can lead to irreversible cognitive impairment and neuropathy (a type of nerve damage). This could be avoided by supplementing with vitamin B12 at the same time or having your blood levels of vitamin B12 checked to ensure they are normal. Importantly, natural folates found in food and supplemental L-methylfolate don’t carry this risk.[6]
Folic acid might lower the effectiveness of some medications, including anti-epileptic medications (e.g., phenytoin), which could increase the risk of seizures, and high-dose methotrexate used for cancer treatment.[7][8]