Which type of anemia is associated with isoniazid therapy?

• A. Sideroblastic anemia
• B. Iron-deficiency anemia
• C. Megaloblastic anemia
• D. Aplastic anemia

Answer: (A)

Isoniazid can cause sideroblastic anemia by creating a deficiency of pyridoxine (vitamin B6). B6 is a required cofactor for δ-aminolevulinate synthase, the first step in heme synthesis inside the mitochondria. When B6 is deficient, heme production falters and iron begins to accumulate in the mitochondria, leading to the formation of ring sideroblasts and a microcytic, hypochromic anemia typical of sideroblastic anemia. This is also why patients on isoniazid are given pyridoxine to prevent both neuropathy and sideroblastic anemia. Other anemias don’t fit this mechanism: iron-deficiency anemia arises from insufficient iron rather than impaired heme synthesis; megaloblastic anemia stems from defective DNA synthesis (folate or B12 deficiency); aplastic anemia is due to bone marrow failure with pancytopenia, not a drug-induced defect in heme synthesis.