vitamin B9 Any B-vitamin that exhibits biological activity against vitamin B<small>₉</small> deficiency. Vitamin B<small>₉</small> 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 B<small>₉</small> leads to anemia, a condition in which the body cannot produce sufficient number of red blood cells. Symptoms of vitamin B<small>₉</small> deficiency include fatigue, muscle weakness, and pale skin. CHEBI:176842 B vitamin