Relevance of pharmacodynamic principles in therapeutics.

Therapeutics is the art of recommending an appropriate dose of an appropriate medicine to achieve a target therapeutic effect. The rational basis for therapeutics is founded on the complementary disciplines of pharmacokinetics and pharmacodynamics. The relevance of pharmacodynamics for therapeutics is discussed in terms of simple models for concentration-effect relationships and the time course of drug action. Explanations are offered for the apparent lack of correlation between plasma concentration and effect which incorporates existing knowledge of pathophysiology and may be applied to individual patients. The delay in onset of drug effect in relation to the time course of plasma concentration can be described in terms of the kinetics of distribution of the drug to its site of action and the kinetics of physiological substances which mediate the observed effect.