An inhibitor of ribonucleotide reductase, gemcitabine is used in the treatment of various carcinomas, particularly non-small cell lung cancer, pancreatic cancer, bladder cancer and breast cancer. It has a role as a photosensitizing agent, a DNA synthesis inhibitor, a prodrug, an EC 1.17.4.1 (ribonucleoside-diphosphate reductase) inhibitor, an environmental contaminant, a xenobiotic, a radiosensitizing agent, an antineoplastic agent, an antimetabolite, an antiviral drug and an immunosuppressive agent.