Although all treatment using drugs can be termed chemotherapy, the word is most often used to describe treatment by strong cytotoxic drugs for serious diseases such as cancer. 'Cytotoxic' means that the drug destroys rapidly growing cancer cells. Cytotoxic drugs are useful in the treatment of many forms of cancer.

Drugs based on sex hormones are also used to treat cancer (eg tamoxifen for breast cancer). Although they are sometimes referred to as chemotherapy, they are more often known as hormone-therapy drugs.

Chemotherapy is often used as an additional safeguard after surgery or in conjunction with radiotherapy. Cytotoxic drugs are designed to cause more damage to cancer cells than to normal body cells, because in most cases the drugs can distinguish cancer cells from other cells by the speed with which cancer cells reproduce.

Normally, cells divide and reproduce in an orderly and controlled manner. In cancer, however, cells multiply without proper control. Chemotherapy works by interfering with the ability of the cancer cells to reproduce.

The ability to distinguish between the two cell types is not complete, and cytotoxic drugs cannot avoid causing some damage to normal cells. Consequently they always have some side effects.

The more rapidly normal cells reproduce, the more likely they are to be damaged by cytotoxic drugs. For this reason, these drugs cause most damage to cells in the lining of the bowel, hair-producing cells, the sex glands and the blood-forming tissue in the bone marrow.