The process whereby a drug is moved from the site of appli­cation into the body and into the extracellular fluid compartment.

The tendency of a drug to combine with a specific drug receptor.

A drug or other substance that binds with a specific drug re­ceptor and causes a physiological response.

Protective mechanism that selectively allows the en­try of specific compounds into the brain. It is an effective boundary be­tween the central nervous system and the peripheral nervous system.

A phenomenon that occurs when a drug is adminis­tered in several doses, causing an increased effect. It is usually due to a buildup of a drug in the blood.

The power of a drug to produce a therapeutic effect.

The process whereby a drug is removed from the body by ex­cretion into the urine, feces, bile, saliva, sweat, breast milk, or expired air.

The elimination of waste products from the body. Excretion is often used interchangeably with the term elimination.

The time required for a level of a drug in the blood to be reduced by 50 percent of its beginning level.

The dose of a drug necessary to maintain a constant therapeutic plasma level.

The sum total of all physical and chemical changes that oc­cur within the body. In pharmacology it is often used interchangeably with the term biotransformation.

The minimum amount of drug needed in the bloodstream to cause the desired therapeutic effect.

The time interval between the administration of a drug and the first sign of its onset; onset of action is influenced by the physical and chemical properties of a drug as well as by its route of administration.

A scientific method of expressing the acidity or alkalinity of a solu­tion, which is the logarithm of the hydrogen ion concentration divided by 1. The higher the pH, the more alkaline the solution; the lower the pH, the more acidic the solution.

The study of how drugs enter the body, reach their site of action, and eventually are eliminated.                                  

The amount of the drug present in the plasma. The peak plasma level refers to the highest concentration produced by a spe­cific dose.

The tendency of a drug to dissolve.

An index of the drug's safety profile, which is deter­mined by calculating the difference between the drug's therapeutic threshold and toxic level. It is typically determined in the laboratory.

The difference between the minimal therapeutic and toxic concentrations of a drug. Drugs with a low therapeutic range pre­sent a higher risk of toxicity than do drugs with a high therapeutic range. The therapeutic range is also referred to as the margin of safety.

The minimum amount of drug needed in the bloodstream to cause a desired therapeutic effect.

The degree to which a substance is poisonous. At high doses drugs can produce toxic effects that are not seen at low doses.