We don’t all respond to the same drugs in the same way. Sometimes a drug will work for one person and not for another, or may cause different side-effects in different people. Our individual responses can be due to the genes we have inherited. With respect to drugs, our unique genetic make-up and our individual characteristics may mean that a drug that is effective for one person is less effective for another, or that a drug that is safe for one person may be dangerous for another person—even at the same dose.
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 present just long enough to do its job. However, some people have enzymes that don’t work in quite the same way as other people, and they may metabolize the drug too quickly or too slowly or not at all — meaning that it may be gone before it has its intended effect, or it may hang around for too long and build up beyond safe levels, leading to side-effects.
A person’s response to a drug may also be related to variation in the way in which a drug reaches or interacts with the part of the body where it has to have an effect – for example, in a protein that the drug binds to in order to produce its particular effect. Also, individuals may experience severe side-effects (known as hypersensitivity reactions) from some drugs due to variations in the proteins involved in the body’s immune response.
Pharmacogenomics is the study of genetic variability that causes differences in individual responses to medications. By analysing the genes that are linked to the the enzymes that metabolize a drug that is to be prescribed or in it interacting the part of the body where it has its effect, a doctor may decide to raise or lower the dose, or even to use a different drug. The decision about which drug to prescribe may also be influenced by other drugs the patient is taking, to avoid interactions between drugs.
The terms “pharmacogenetics” and “pharmacogenomics” are sometimes used interchangeably. There are subtle differences between the two terms, and no consensus on their exact definitions. In general, pharmacogenomics refers to the overall study of the various genes that contribute to drug response, while pharmacogenetics is the study and evaluation of the inherited difference that affect an individual’s response to drugs. The term pharmacogenetics will be used in this article.