When an anticancer drug is captured and transported to the interior of a cell, many biochemical processes intervene in the metabolism of the drug. First, the Phase I and Phase II enzymes produce metabolites, which are then excreted via the ABC-transporter enzymes. Finally, the remaining drug binds to its molecular target. Genetic alterations known as polymorphisms in any of the proteins that intervene in these processes can affect the efficacy of treatment. Interestingly, these polymorphisms can be detected in both normal and tumour cells. In this article, we will review the most effective method of detecting single-nucleotide polymorphisms (SNPs) and describe the principal SNPs in enzymes involved in drug metabolism and in DNA repair. Finally, we will briefly describe promising lines of future research in this field.