The pharmacokinetics and pharmacodynamics pathway of gemcitabine, a cytidine analogue that is administered as a prodrug for the treatment of certain types of cancer. The diphosphate form is an inhibitor of ribonucleotide reductases, essential enzymes of deoxyribonucleotide synthesis. Genetic variations can result in changes in drug availability and can cause differences in the response of the organism to the drug.