Drug treatment requires getting a drug to its specific target site or sites in tissues where the drug has its action. Typically, the drug is introduced into the body (the process of administration), sometimes far from this target site. The drug must move into the bloodstream (the process of absorption) and be transported to the target sites where the drug is needed (the process of distribution). Some drugs are chemically altered (the process of metabolism) by the body before they take effect, others are metabolized afterward, and still others are not metabolized at all. The final step is the removal of the drug and its metabolites (byproducts) from the body (the process of elimination).
Many factors, including a person’s weight, genetic makeup, and kidney or liver function, can influence these kinetic processes (see Overview of Response to Drugs and Genetic Makeup and Response to Drugs). Changes due to aging also affect how the body processes drugs (see Aging and Drugs).