Adverse drug reactions are important public health problem. Adverse drug reactions are an important public health problem. Inherited genetic variations in both host genome and the tumor genome affect drug response in cancer patients. Pro-drugs are metabolized through serial biochemical reactions (phase I, phase II etc.) by using detoxification enzymes. Functional polymorphisms in the genes encoding detoxification enzymes cause inter-individual differences, which contribute to adverse drug reactions as a result of standard chemotherapy protocols. Clinicians generally start the treatment with the average dose. The application of pharmacogenetic testing to cancer therapy is attractive because it helps to reduce the fatal adverse effect of therapeutics and creates effective individual medical strategies. Many cheap and fast molecular techniques are developed for pharmacogenetic screening. The most prominent two examples of pharmacogenetic tests used in oncology are the tests determining the toxicity of chemotherapeutics (5-fluorouracil, irinotecan, thiopurine etc.) and the tests determining the response to the treatment with specific molecular therapeutics (imatinib, rituximab and gefitinib etc.). Baseline pharmacogenetic testing before starting therapy is expected to be useful for the individualization and optimization of cancer chemotherapy. Recently high-throughput screening has been developed to identify the genes influencing polygenic drug response by using gene expression profiles.