With the exception of rare cases of allergic reactions,²⁴⁶ folic acid is not generally associated with side effects,²⁴⁷ although there have been rare case reports of allergic reactions to the vitamin.²⁴⁸ Folic acid supplementation can interfere with the laboratory diagnosis of vitamin B12 deficiency, possibly allowing the deficiency to progress undetected to the point of irreversible nerve damage.²⁴⁹ Although vitamin B12 deficiency is uncommon, no one should supplement with 1,000 mcg or more of folic acid without consulting a doctor.

Vitamin B12 deficiencies often occur without anaemia (even in people who do not take folic acid supplements). Some doctors do not know that the absence of anaemia does not rule out a B12 deficiency. If this confusion delays diagnosis of a vitamin B12 deficiency, the patient could be injured, sometimes permanently. This problem is rare and should not happen with doctors knowledgeable in this area using correct testing procedures.