Fluorouracil is an antimetabolite that is used for the topical treatment of multiple actinic or solar keratosis and superficial basal cell carcinomas (skin cancers). The nucleotide derivative is also available for intravenous injection in the treatment of other types of cancers, including colorectal, pancreatic, and breast cancer. Fluorouracil interferes with the synthesis of deoxyribonucleic acid (DNA) and to the lesser degree ribonucleic acid (RNA). Side effects include diarrhea, heartburn, and sores in the mouth and on the lips.