Toxicity

Folic acid works closely with vitamin B12 and the two vitamins share similar symptoms of deficiency.

Folic acid can mask vitamin B12 deficiency.
Indeed, high levels of folic acid can interfere with diagnosis of a serious blood disorder related to a vitamin B12 deficiency (pernicious anemia).
Delays in the diagnosis can lead to serious and permanent nerve damage, including paralysis.
However, these cases are rare and usually result from very high supplementation doses of folic acid.

Large doses of folic acid should not be taken unless the possibility of vitamin B12 deficiency has been ruled out.

Strict vegetarians who consume large amounts of folic acid-rich foods and little or no vitamin B12 are at particular risk.