OUTLINE: This is a randomized, double-blind, placebo-controlled, multicenter study. Patients are stratified according to prior anterior resection of the rectum (yes vs no), total planned cumulative dose of radiotherapy, including boost fields (4,500-5,350 cGy vs 5,351-6,000 cGy vs more than 6,000 cGy), use of concurrent fluorouracil (none vs bolus vs continuous infusion), use of concurrent leucovorin calcium (yes vs no), use of concurrent cisplatin (yes vs no), superior border of initial field (at or inferior to the L4-5 interspace vs superior to the L4-5 interspace), planned intracavitary brachytherapy (yes vs no), and primary site of disease (rectal cancer vs prostate cancer vs gynecological cancer vs other). Beginning no later than the fourth day of radiotherapy, patients are randomized to one of two treatment arms.