Folic acid is nontoxic but concern has been raised about high intakes. The adverse effects are established only at intakes greater than 5 milligrams. Potential adverse effects from excessive folic acid intakes are as follows:
Masking vitamin B12 symptoms: Administration of high potency of folic acid to patients with anemia can mask the anemic symptoms while allowing the neurologic disease (posterolateral spinal cord degeneration) of vitamin B12 deficiency to progress unchecked. Although there are a few reports of the masking effect related to amounts of folic acid less than 1 mg (results based almost entirely on studies with injected folic acid), the effect is unusual at such low doses and predictable only at doses greater than 5mg.
Interaction with low B12 levels: One recent study has demonstrated that high folic or folate levels when combined with low B12 levels are associated with significant cognitive impairment among the elderly.
Folic acid and drug interactions: At high intakes, folic acid has been reported to interfere with the effectiveness of anticonvulsant drugs such as Dilantin, used to control epilepsy. Folic acid doses of 5-30 milligram orally have produced some evidence of increased frequency of seizures in epileptics, but there is no evidence of such effects at lower intakes of folic acid. It might be expected that increased folic acid intakes could interfere with actions of folate-antagonizing drugs. Research suggests high levels of folic acid can also interfere with some antimalarial drugs.
Disruption of zinc function: Some reports suggest that as little as 350 micrograms of supplemental folic acid can adversely affect zinc function, but more recent reports indicate no adverse effects of folic acid at even higher intakes on zinc uptake or function. These apparently incompatible results are attributable to the widely different experimental approaches used.
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