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Pharmacodynamics in Older Adults

By

J. Mark Ruscin

, PharmD, FCCP, BCPS, Southern Illinois University Edwardsville School of Pharmacy;


Sunny A. Linnebur

, PharmD, BCPS, BCGP, University of Colorado Anschutz Medical Campus

Last full review/revision Jul 2021| Content last modified Jul 2021
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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.

Table
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Effect of Aging on Drug Response

Class

Drug

Action

Effect of Aging

Analgesics

Morphine

Acute analgesic effect

Pentazocine

Analgesic effect

Anticoagulants

Heparin

PTT (partial thromboplastin time)

Warfarin

PT/INR (prothrombin time/international normalized ratio)

Bronchodilators

Albuterol

Bronchodilation

Ipratropium

Bronchodilation

Cardiovascular drugs

Angiotensin II

receptor blockers

Decreased blood pressure

Diltiazem

Acute antihypertensive effect

Enalapril

Acute antihypertensive effect

Felodipine

Antihypertensive effect

Isoproterenol

Increased heart rate

Increased ejection fraction

Venodilation

Nitroglycerin

Venodilation

Norepinephrine

Acute vasoconstriction

Phenylephrine

Acute venoconstriction

Acute hypertensive effect

Prazosin

Acute antihypertensive effect

Propranolol (and other beta-blockers)

Decreased heart rate

Verapamil

Acute antihypertensive effect, cardiac conduction effects

Diuretics

Bumetanide

Increased urine flow and sodium excretion

Furosemide

Latency and size of peak diuretic response

Oral hypoglycemics

Glyburide*

Chronic hypoglycemic effect

Psychoactive drugs

Diazepam

Sedation

Diphenhydramine

Psychomotor dysfunction

Haloperidol

Acute sedation

Midazolam

Electroencephalogram (EEG) activity

Sedation

Temazepam

Postural sway

Psychomotor effect

Sedation

Thiopental

Anesthesia

Triazolam

Sedation

Others

Atropine

Impaired gastric emptying

Levodopa

Adverse effects

Metoclopramide

Sedation

=unchanged; = increased; = decreased.

* Older adults are particularly susceptible to the hypoglycemic action of glucose-lowering drugs. Hypoglycemia may be difficult to recognize in older adults.

Adapted and updated from Cusack BJ, Vestal RE: Clinical pharmacology: Special considerations in the elderly. In Practice of Geriatric Medicine, edited by E Calkins, PJ Davis, and AB Ford. Philadelphia, WB Saunders Company, 1986, pp. 115–136; used with permission.

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