Oral folic acid (pteroylglutamic acid) is generally regarded as not toxic for normal humans but it may cause neurological injury when given to patients with undiagnosed pernicious anemia. The vitamin should be given with caution to drug-treated epileptic patients because seizure control may be affected. Some studies suggest that folic acid supplements interfere with intestinal zinc absorption in humans and animals but others do not confirm such an effect. The weight of current evidence favors the view that daily supplements of 5-15 mg folic acid do not have significant adverse effects on Zn nutriture in healthy nonpregnant subjects. Because antifolate medications are now being used to treat a wide range of malignant and nonmalignant disorders, further investigation is needed concerning folate metabolism and the safety of supplements in patients with these disorders.