Some of us respond differently than others to the same medications that we take, or we may experience different side effects from drugs. The way we respond can be due to the genes we have inherited. With respect to drugs, our unique genetic make-up and our individual response may mean that a drug that is effective for one person may be less effective for another or that a drug that is safe for one person may be less safe for another person—even at the same dosage.
Most drugs are broken down (metabolized) in the body by various enzymes. In some cases, an active drug is made inactive (or less active) through metabolism. In other cases, an inactive (or less active) drug is made more active through metabolism. The challenge in drug therapy is to make sure that the active form of a drug stays around long enough to do its job. However, some people have variable enzyme action so that they may metabolize the drug too quickly or too slowly or not at all — meaning that the drug may not produce its intended effect or it may remain in a person's system too long and may lead to side effects.
Individual response to a drug may also be related to variability in the drug target, for example a protein that the drug binds to in order to produce its specific effect. Furthermore, individuals may experience side effects (known as hypersensitivity reactions) from certain medications due to variability in proteins involved in the immune response.
Pharmacogenetics is the study of genetic variability that causes individual responses to medications. By analyzing the genes that produce the specific drug targets or enzymes that metabolize a medication or are associated with immune response, a healthcare practitioner may decide to raise or lower the dose or even change to a different drug. The decision about which drug to prescribe may also be influenced by other drugs the person is taking in order to avoid drug-drug interactions.
The terms "pharmacogenetics" and "pharmacogenomics" are sometimes used interchangeably. There are subtle differences in the meaning of the two terms and there is no consensus on the exact definitions. In general, pharmacogenomics refers to the overall study of the many various genes that contribute to drug response. Pharmacogenetics is the study and evaluation of the inherited differences (genetic variations) that affect drug metabolism and an individual's response to medications. For the purposes of this article, the term pharmacogenetics will be used.