Chemotherapy drugs are a group of medicines known as antineoplastics ("against cancer"). Most antineoplastics work by stopping cell division. This causes cell death in any dividing cell, and since most cells are not dividing all the time, the resulting effect of these medications is to kill cancer cells. The side effect is that they do kill other cell types which divide frequently: cells in the gastrointestional tract, the bone marrow, and hair follicles. Fortunately, these effects will reverse when the drug is discontinued, but the cancer cells stay dead.