Pharmacodynamics The study of the

mechanism of drug actions is called phar-macodynamics. Most (but not all) drugs exert their action by binding to specific RECEPTORS. This binding may initiate changes that lead to the characteristic effects of the drug on body functions.

A central question in drug therapy (medication) is the proper dose of the drug that produces a desired action without many harmful side effects. To clarify this problem, pharmacologists analyze the relationship between dose and response. Most dose-response curves are sigmoidal (shaped like anS).

The peyote cactus, from which is derived the hallucinogenic mescaline. (Drug Enforcement Administration)

The log-dose-response can be viewed as having four parameters: potency, slope, maximal efficacy, and variability. Potency describes the strength of drug effects. It is usually employed to calculate relative strengths among drugs of the same class. Slope is the central part of the curve that is approximately straight. It is used to analyze drug concentration (dose) from the observed corresponding responses. Maximal efficacy, or simply "efficacy," is the greatest effect produced by the drug. This is one of the major characteristics of a drug. Efficacy and potency of a drug are not necessarily correlated, and the two characteristics should not be confused.

Many drugs, including drugs of abuse, produce Tolerance—when it becomes necessary to take progressively larger doses to achieve the same drug effect. In some cases, the brain and other tissues on which a drug acts undergo adaptive changes (neuroadaptations) that tend to offset the drug effect. When a drug that produces neuroadaptation is withdrawn, the brain and other tissues have to readapt, because they are no longer balanced by drug effect. The adaptation produces a variety of signs and symptoms called withdrawal syndrome. The severity of this syndrome depends on the degree of adaptive changes in the nervous system— which, in turn, depends on the dose and the duration of exposure to the drug. The particular characteristics of the withdrawal syndrome depend on the pharmacological effects of the drug(s) and typically are opposite to the drug effects. For example, MORPHINE constricts the pupil; the morphine withdrawal symptom includes pupil dilation.

Most drugs of abuse produce pleasant effects in humans. For example, some people use AMPHETAMINES or other stimulants (e.g., COCAINE) to achieve a sense of well-being and euphoria. Some people use DEPRESSANTS—ALCOHOL, OPIOIDS, or Tranquilizers —to relax. Still others use either stimulants or depressants to relieve boredom or reduce anxiety or pain. The common feature is that people use drugs because somehow the drug is rewarding to the user, either by producing a feeling of well-being (e.g., euphoria, elation) or by taking away a negative feeling (e.g., anxiety).

(SEE ALSO: Addiction: Concepts and Definitions; Drug Interaction and the Brain; Drug Metabolism; ED50; LD50)

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