||Any B-vitamin that exhibits biological activity against vitamin B9 deficiency. Vitamin B9 refers to the many forms of folic acid and its derivatives, including tetrahydrofolic acid (the active form), methyltetrahydrofolate (the primary form found in blood), methenyltetrahydrofolate, folinic acid amongst others. They are present in abundance in green leafy vegetables, citrus fruits, and animal products. Lack of vitamin B9 leads to anemia, a condition in which the body cannot produce sufficient number of red blood cells. Symptoms of vitamin B9 deficiency include fatigue, muscle weakness, and pale skin.