If a person is deficient in vitamin B12 and takes 1,000 mcg or more of folic acid per day, the folic acid supplementation can improve the anaemia caused by vitamin B12 deficiency. The effect of folic acid on vitamin B12 deficiency-induced anaemia is not a folic acid toxicity. Rather, the folic acid supplementation is acting to correct one of the problems caused by B12 deficiency. The other problems caused by a lack of vitamin B12 (mostly neurological) do not improve with folic acid supplements, and can become irreversible if vitamin B12 is not provided to someone who is vitamin B12 deficient.

Some doctors are unaware that vitamin B12 deficiencies often occur without anaemia—even in people who do not take folic acid supplements. This lack of knowledge can delay diagnosis and treatment of people with vitamin B12 deficiencies. This can lead to permanent injury. When such a delayed diagnosis occurs in someone who inadvertently erased the anaemia of vitamin B12 deficiency by taking folic acid supplements, the folic acid supplementation is often blamed for the missed diagnosis. This problem is rare and should not occur in people whose doctors understand that a lack of anaemia does not rule out a vitamin B12 deficiency. Anyone supplementing 1,000 mcg or more per day of folic acid should be initially evaluated by a doctor before the folic acid can obscure a proper diagnosis of a possible B12 deficiency.