Pharmacogenetics is a field that deals with the relationship between genetic variations and the effects and side-effects of drugs. Genetic variations in drug-metabolizing enzymes, transporters, receptors, and other drug targets have been linked to interindividual differences in the efficacy and safety of many drugs. Genotyping is performed increasingly in primary and secondary health care in patients with abnormal plasma drug levels, in insufficient efficacy or in the presence of side-effects. Trastuzumab and abacavir are examples of drugs where patients are tested before treatment is started, in order to ascertain whether they carry a particular genetic variant. Trastuzumab is effective only when a particular protein is expressed in breast cancer tissue and the AIDS inhibitor may cause serious hypersensitivity reactions in patients carrying a particular variant. It is expected that screening for genetic variants prior to drug treatment will occur more often. This depends on the intensity of the association between genotype and outcome, the severity or relevance of the clinical outcome and the availability of alternative treatment.