Contraceptive activity as well as anti-viral protection can be provided by contraceptive compositions containing a pyrimidine-based nucleoside or nucleotide exhibiting spermicidal or sperm-immobilizing activity. In one example, the active ingredient contains a thymine ring (e.g. AZT) that has 5-halo, 6-alkoxy substitution. Additional improvements in anti-HIV activity for certain AZT derivatives are seen by providing the pentose ring of AZT with aryl phosphate substitution.