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Bone marrow suppression or myelotoxicity (adjective myelotoxic) is a serious side-effect of chemotherapy and certain drugs affecting the immune system such as azathioprine. The risk is especially high in chemotherapy for leukaemia.

Non-steroidal anti-inflammatory drugs, in some rare instances, may also cause bone marrow suppression.

Because the bone marrow is the manufacturing center of blood cells, the suppression of bone marrow activity causes a deficiency of blood cells. This condition can rapidly lead to life-threatening infection, as the body cannot produce leukocytes in response to invading bacteria and viruses, as well as leading to anaemia due to a lack of red blood cells and spontaneous severe bleeding due to deficiency of platelets.

Bone marrow suppression due to azathioprine can be treated by changing to another medication such as mycophenolate mofetil (for organ transplants) or other disease-modifying drugs in rheumatoid arthritis or Crohn's disease. Bone marrow suppression due to anti-cancer chemotherapy is much harder to treat and often involves hospital admission, strict infection control, and aggressive use of intravenous antibiotics at the first sign of infection.