With the exception of rare cases of allergic reactions,¹⁶ folic acid is not generally associated with side effects,¹⁷ although there have been rare case reports of allergic reactions to the vitamin.¹⁸ 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.¹⁹ Although vitamin B12 deficiency is uncommon, no one should supplement with 1,000 mcg or more of folic acid without consulting a doctor.

In a double-blind trial, people with diabetes who also had with kidney disease received a daily placebo or 2.5 mg of folic acid, 1 mg of vitamin B12, and 25 mg of vitamin B6 for three years. Compared with the placebo, vitamin supplementation accelerated the decline in kidney function and increased the incidence of cardiovascular events (such as heart attacks) and heart disease-related deaths.²⁰ Based on this study, diabetics with kidney disease should not take these vitamins without a doctor's supervision.

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.