Aging and drug disposition--pharmacodynamics.

The pharmacodynamics of a drug is defined as the effect of the drug on physiological functions and pathological processes and can be modified in the elderly by (a) pharmacokinetics, (b) receptor sensitivity (or density), (c) impaired homeostasis or (d) pathology. The alterations in pharmacokinetics in the elderly often result in a reduction of the dose required to achieve the optimum pharmacodynamic effect. Changes in the sensitivity of receptors may be decreased in the elderly (perhaps by a diminution in the number of receptors) as in the case of the beta-adrenergic blockers, but increase in sensitivity may also be found as in the case of warfarin. Impairment of homeostatic mechanisms, while related to aging, are frequently produced by pathological processes and have an important influence on the pharmacodynamics of many drugs. The effect of the multiple pathology often found in the elderly is likely to be the dominant factor in changes in pharmacodynamics in this population. The interaction of the above factors results in a heterogeneity of pharmacodynamic response in the elderly which requires the careful monitoring of both the efficacy and toxicity of their drug treatment.