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 Drug Administration Drugs are introduced into the body by several routes. They may be Taken by mouth (orally) Given by injection into a vein (intravenously, IV), into a muscle (intramuscularly, IM), into the space... read more ), sometimes far from this target site. The drug must move into the bloodstream (the process of absorption Drug Absorption Drug absorption is the movement of a drug into the bloodstream after administration. (See also Introduction to Administration and Kinetics of Drugs.) Absorption affects bioavailability—how quickly... read more ) and be transported to the target sites where the drug is needed (the process of distribution Drug Distribution Drug distribution refers to the movement of a drug to and from the blood and various tissues of the body (for example, fat, muscle, and brain tissue) and the relative proportions of drug in... read more ). Some drugs are chemically altered (the process of metabolism Drug Metabolism Drug metabolism is the chemical alteration of a drug by the body. (See also Introduction to Administration and Kinetics of Drugs.) Some drugs are chemically altered by the body (metabolized)... read more ) 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 Drug Elimination Drug elimination is the removal of drugs from the body. (See also Introduction to Administration and Kinetics of Drugs.) All drugs are eventually eliminated from the body. They may be eliminated... read more ).
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 Overview of Response to Drugs Everyone responds to drugs differently. The way a person responds to a drug is affected by many factors, including Genetic makeup Age Body size Use of other drugs and dietary supplements (such... read more and Genetic Makeup and Response to Drugs Genetic Makeup and Response to Drugs Differences in genetic (inherited) makeup among individuals affect what the body does to a drug and what the drug does to the body. The study of genetic differences in the response to drugs... read more ). Changes due to aging also affect how the body processes drugs (see Aging and Drugs Aging and Drugs read more ).