Chemotherapy drugs are used to treat cancer cells. However, they can also affect bone marrow and reduce the ability to make certain types of blood cells. Low white blood cell counts can leave patients vulnerable to infection. Low red blood cell counts can lead to anaemia and feelings of fatigue and weakness. Low platelet counts can lead to bruising and bleeding. Blood counts are therefore monitored. If they fall too low, the dose of chemotherapy may be reduced or the time between doses extended. This may affect survival as well as quality of life.