GOSERELIN treats prostate cancer and breast cancer. It works by decreasing levels of the hormones testosterone and estrogen in the body. This prevents prostate and breast cancer cells from spreading or growing. It may also be used to treat endometriosis. This is a condition where the tissue that lines the uterus grows outside the uterus. It works by decreasing the amount of estrogen your body makes, which reduces heavy bleeding and pain. It can also be used to help thin the lining of the uterus before a surgery used to prevent or reduce heavy periods. With the Patient Assistance Program from AstraZeneca, contact the program to see how they can financially help you out.