In contrast to pharmacokinetic effects, pharmacodynamics is defined as what the drug does to the body or the response of the body to the drug; it is affected by receptor binding, postreceptor effects, and chemical interactions (see Drug–Receptor Interactions). In older adults, the effects of similar drug concentrations at the site of action (sensitivity) may be greater or smaller than those in younger people (see table Effect of Aging on Drug Response). Differences may be due to changes in drug-receptor interaction, postreceptor events, or adaptive homeostatic responses and, among frail patients, are often due to pathologic changes in organs. However, clinical differentiation between pharmacodynamic and pharmacokinetic effects can sometimes be difficult.
Older adults are particularly sensitive to anticholinergic drug effects. Many drugs (eg, tricyclic antidepressants, sedating antihistamines, urinary antimuscarinic agents, some antipsychotic drugs, antiparkinsonian drugs with atropine-like activity, many over-the-counter hypnotics and cold preparations) have anticholinergic effects. Older adults, most notably those with cognitive impairment, are particularly prone to central nervous system (CNS) adverse effects of such drugs and may become more confused and drowsy. Anticholinergic drugs also commonly cause constipation, urinary retention (especially in older men with benign prostatic hyperplasia), blurred vision, orthostatic hypotension, and dry mouth. Even in low doses, these drugs can increase risk of heatstroke by inhibiting diaphoresis. In general, older adults should avoid drugs with anticholinergic effects when possible.