For many drugs, pharmacogenetic polymorphisms are known affecting biotransformation and clinical outcome. The clinical importance of these variants depends on allele-frequency and the effect size of the clinical outcome parameters. Further, it depends on the therapeutic range of the drug which is affected, on predictability of drug response as well as on duration until onset of therapeutic efficacy. Consequences which arise from genotyping might be: adjustment of dose according to genotype, choice of therapeutic strategy or even choice of drug. In antidepressant drug treatment, most drugs are metabolized via the polymorphic cytochrome P450 enzyme CYP2D6. Huge differences in pharmacokinetic parameters have been consistently shown for many tricyclics, some SSRIs, and other antidepressant drugs whereas the effects on therapeutic efficacy and adverse events have been described controversially. In cardiovascular disease, oral anticoagulants, nonsteroidal anti-inflammatory drugs, oral hypoglycemic drugs and other drugs are affected by genetic polymorphisms of the cytochrome P450 drug metabolizing enzyme CYP2C9. Studies in patients or healthy volunteers revealed up to 10-fold differences in pharmacokinetic parameters due to genetic polymorphisms of CYP2C9. Pharmacogenetics based dose adjustments are one tool to individualize drug treatment according to genetic factors. They can be derived from pharmacokinetic data with the aim to obtain equal drug concentrations in each individual. Prospective validation of dose adjustments based on pharmacogenetics should be performed before routine application of such strategies. A controlled prospective clinical trial with one arm receiving genotype-based dose adjustments and the other arm receiving therapy as usual will elucidate the benefit of pharmacogenomics-based individualization of certain drug therapies.