Hormones are naturally occurring substances in the body that can stimulate or modulate the growth of hormone sensitive tissues, such as the breast or prostate gland. When cancer arises in certain tissues such as breast or prostate tissue, its growth and spread may be facilitated by the body's own hormones. Therefore, drugs that block hormone production or change the way hormones work are often very effective and tolerable ways of treating cancer. Hormone therapy, similar to chemotherapy, is a systemic treatment in that it is designed to affect cancer cells throughout the body. Some hormones may be used to enhance appetite, decrease inflammation and swelling, improve breathing. Some may stimulate the production or re-growth of good cells damaged by certain treatments.