Chemotherapy is the administration of drugs that are toxic to cancer cells proliferate rapidly. Cancer cells are some of the cells that proliferate most rapidly in the body. Other cells, such as contributing to the formation of hair shafts and nails, also reproduce quickly. So while chemotherapy drugs preferentially destroy cancer cells, drugs also can destroy the cells responsible for normal growth of hair and nails. That's why cancer patients sometimes shed the hair and nails during treatment. No hair growth stimulants, shampoos, conditioners or other cosmetic treatments can prevent or inhibit hair loss. The good news is that once chemotherapy is completed, the hair usually grows back within six months to a year.