Section 2. Drugs

Chapter 11. Drug Administration and Kinetics

Topics: Introduction | Drug Administration | Drug Absorption | Drug Distribution | Drug Metabolism | Drug Elimination

Drug Metabolism

Drug metabolism is the chemical alteration of a drug by the body.

Some drugs are chemically altered by the body (metabolized). The substances that result from metabolism (metabolites) may be inactive, or they may be similar to or different from the original drug in therapeutic activity or toxicity. Some drugs, called prodrugs, are administered in an inactive form, which is metabolized into an active form; the resulting metabolites produce the desired therapeutic effects. Metabolites may be metabolized further instead of being excreted from the body. The subsequent metabolites are then excreted.

Most drugs must pass through the liver, which is the site of most drug metabolism. Liver enzymes convert prodrugs to active metabolites or inactivate drugs. The group of P-450 enzymes is the liver's primary mechanism for chemically altering drugs. The levels of P-450 enzymes control the rate at which many drugs are metabolized. The capacity of the enzymes to metabolize is limited, so they can become overloaded when blood levels of a drug are high (see Section 2, Chapter 13).

Because metabolic enzyme systems are only partially developed at birth, newborns have difficulty metabolizing certain drugs. As people age, enzymatic activity decreases, so that older people, like newborns, cannot metabolize drugs as well as younger adults and children do. Consequently, newborns and older people often need smaller doses per pound of body weight than young or middle-aged adults do.