Uracil reductase inactivators, notably a 5-substituted uracil or 5,6-dihydro-5-substituted uracil, potentiate 5-flourouracil and find use particularly in the treatment of cancer. The 5-substituent is selected from bromo, ido, cyano, halo-substituted C.sub.1-4 alkyl C.sub.2-6 alkenyl, 1-halo-C.sub.2-6 alkenyl and halo-substituted C.sub.2-6 alkynyl.