A substance that relieves pain and is being studied in the treatment of some types of cancer. Opioid growth factors bind to cells in the body, including tumor cells, which have opioid growth factor receptors on the surface. This may help stop the growth of the tumor cells. It may also prevent the growth of blood vessels that tumors need to grow. An opioid growth factor is a type of biological response modifier and a type of antiangiogenesis agent. Also called OGF.