Deoxycytidine analog that inhibits DNA synthesis. Metabolized to form gemcitabine triphosphate (dFdCTP) and gemcitabine diphosphate (dFdCDP). dFdCTD inhibits ribonucleotide reductase causing a reduction in cellular nucleotides. dFdCTP is incorporated in DNA resulting in DNA strand termination. Displays antitumor activity in vitro and in vivo.