A drug that blocks the action of opiates (drugs used to treat pain). It may be used in the treatment of intravenous opiate addiction or alcohol dependence. Naltrexone is also being studied in the treatment of breast cancer. It may block the effects of the hormone estrogen, which causes some breast cancer cells to grow, or block the blood flow to tumors. It is a type of opiate antagonist. Also called naltrexone hydrochloride, ReVia, and Vivitrol.