A drug used to treat cancers of the breast, colon, rectum, stomach, and pancreas. Under the brand names Carac, Tolak, Efudex, and Fluoroplex, it is used as a cream to treat actinic keratosis (a skin condition that may become cancer). It is also used under the brand name Efudex as a cream to treat basal cell skin cancer that is superficial (not invasive) and cannot be removed by surgery. Tolak is being studied in the treatment of other conditions and types of cancer. It stops cells from making DNA and may kill cancer cells. Tolak is a type of antimetabolite. Also called 5-fluorouracil, 5-FU, Carac, Efudex, Fluoroplex, and fluorouracil.