Drug+action+and+Adverse+Reactions

= Welcome to the Drug Action and Adverse Reactions Development Page =


 * Zero and 1st Order Kinetics**

For almost all drugs, the metabolism rate in any given pathway has an upper limit (capacity limitation). However, at therapeutic concentrations of most drugs, usually only a small fraction of the metabolizing enzyme's sites are occupied, and the metabolism rate increases with drug concentration. In such cases, called **1st-order elimination (or kinetics),** the metabolism rate of the drug is a constant fraction of the drug remaining in the body (rather than a constant amount of drug per hour); ie, the drug has a specific half-life. For example, if 500 mg is present in the body at time zero, after metabolism, 250 mg may be present at 1 h and 125 mg at 2 h (illustrating a half-life of 1 h). However, when most of the enzyme sites are occupied, metabolism occurs at its maximal rate and does not change in proportion to drug concentration; ie, a fixed amount of drug is metabolized per unit time **(zero-order** **kinetics).** In this case, if 500 mg is present in the body at time zero, after metabolism, 450 mg may be present at 1 h and 400 mg at 2 h (illustrating a maximal clearance of 50 mg/h and no specific half-life). As drug concentration increases, metabolism shifts from **1st-order to zero-order kinetics**.