Bone marrow suppression 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

The bone marrow is where blood cells are formed, and this process is slowed or stopped when bone marrow suppression is caused. This can rapidly lead to life-threatening infection as the body cannot produce leukocytes in response to invading bacteria and viruses, as well as 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.