Folic acid is most often taken at a dosage of 400 to 800 micrograms (μg), i.e., 0.4 to 0.8 mg, daily. The tolerable upper intake level (UL) of folic acid is 1,000 μg (1 mg) daily. This represents the maximum daily dose unlikely to cause adverse effects and was set based on the concern of folic acid masking vitamin B12 deficiency.^[1] In some circumstances, a higher dosage may be warranted.

For the prevention of NTDs during pregnancy, folic acid can be dosed at 400 μg (0.4 mg) daily, ideally starting a minimum of 4 weeks before conception and continuing until at least the end of the first trimester. In women with a previous pregnancy affected by NTDs, the recommended dose may be much higher (4 to 5 mg daily).^[2]

For reducing homocysteine levels, the maximum benefit can be achieved with folic acid 800 μg (0.8 mg) daily.^[3]

When used for depression in addition to antidepressant medication, the best evidence currently suggests a dose of L-methylfolate 15 mg daily.^[4][5]

For reducing the side effects of low-dose methotrexate (as used for some autoimmune conditions), a dosage of 500 to 1000 μg (0.5 to 1 mg) daily appears to be effective.^[6]

The recommended daily allowance (RDA) of folate for adults is 400 μg dietary folate equivalents (DFEs) daily, as per the Institute of Medicine. This represents the average amount of folate that is thought to meet the nutritional requirements of most healthy people. Natural dietary folate has a lower bioavailability than folic acid, so DFEs are used to convert between the two. For example, 1 μg of dietary folate (1 DFE) is approximately equal to 0.6 μg (taken with food) or 0.5 μg (taken without food) of folic acid. So, to get 400 μg DFEs entirely from supplementation, you would need to take approximately 200 to 240 μg of folic acid.^[7]