Fluorouracil is an analogue of uracil, a component of ribonucleic acid. The drug is believed to act as an antimetabolite. After intracellular conversion to the active deoxynucleotide, it interferes with DNA synthesis by blocking the conversion of deoxyuridylic acid to thymidylic acid by the cellular enzyme thymidylate synthetase. Fluorouracil may also interfere with RNA synthesis.