- Subjects cannot take mega-doses of vitamins during the trial. This is defined as more than 400 IU of vitamin E, 200 ug of selenium, or 1 gm of vitamin C per day, or more than the tolerable upper limits of any other supplement as defined in the Dietary Reference Intake panels developed by the Food and Nutrition Board under the Institute of Medicine. Such vitamin therapy must be discontinued at least 2 weeks prior to study entry.