Pharmacogenetics is the study of how genetic factors affect the response to drugs (efficacy, adverse effects). Variation in genes can affect either a drug’s pharmacokinetics (how the drug is handled in the body) or its pharmacodynamics (how it interacts with proteins in the body to produce its effects). Such variation needs to be evaluated in combination with clinical and environmental factors in order to personalize either drug choice or drug dose in individual patients. There are some well-characterized examples of pharmacogenetic variation in clinical practice.