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 Pharmacodynamics: Drug-Receptor Interactions). In the elderly, the effects of similar drug concentrations at the site of action (sensitivity) may be greater or smaller than those in younger people (see Table 2: Drug Therapy in the Elderly: Effect of Aging on Drug Response). Differences may be due to changes in drug-receptor interaction, in postreceptor events, or in adaptive homeostatic responses and, among frail patients, are often due to pathologic changes in organs.

Elderly patients are particularly sensitive to anticholinergic drug effects. Many drugs (eg, tricyclic antidepressants, most nonselective antihistamines, some antipsychotic drugs, antiparkinsonian drugs with atropine

-like activity, many OTC hypnotics and cold
preparations) are anticholinergic. The elderly, most notably those with dementia, are particularly prone to CNS adverse effects of such drugs and may become more confused and drowsy. Anticholinergic drugs also commonly cause constipation, urinary retention (especially in elderly 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.

Table 2

Effect of Aging on Drug Response

Class
Drug
Action
Effect of Aging

Analgesics
Aspirin

Acute gastroduodenal mucosal damage
↔

Morphine

Acute analgesic effect
↑

Pentazocine

Analgesic effect
↑

Anticoagulants
Heparin

PTT
↔

Warfarin

PT
↑

Bronchodilators
Albuterol

Bronchodilation
↓

Ipratropium

Bronchodilation
↔

Cardiovascular drugs
Adenosine

Minute ventilation and heart rate response
↔

Venodilation
↔

Angiotensin II

receptor blockers
Decreased BP
↑

Diltiazem

Acute antihypertensive effect
↑

Dopamine

Increased creatinine clearance
↓

Enalapril

Acute antihypertensive effect
↑

Felodipine

Antihypertensive effect
↑

Histamine
Venodilation
↔

Isoproterenol

Increased heart rate
↓

Increased ejection fraction
↓

Venodilation
↓

Nitroglycerin

Venodilation
↔

Norepinephrine

Acute vasoconstriction
↔

Phenylephrine

Acute venoconstriction
↔

Acute hypertensive effect
↔

Prazosin

Acute antihypertensive effect
↔

Propranolol

Decreased heart rate
↔

Timolol

Decreased heart rate
↔

Verapamil

Acute antihypertensive effect, cardiac conduction effects
↑

Diuretics
Bumetanide

Increased urine flow and Na excretion
↓

Furosemide

Latency and size of peak diuretic response
↓

Oral hypoglycemics
Glyburide

Chronic hypoglycemic effect
↔

Tolbutamide

Acute hypoglycemic effect
↓

Psychoactive drugs
Diazepam

Sedation
↑

Diphenhydramine

Psychomotor function
↑

Haloperidol

Acute sedation
↑

Midazolam

EEG activity
↑

Sedation
↑

Temazepam

Postural sway
↑

Psychomotor effect
↑

Sedation
↑

Thiopental

Anesthesia
↔

Triazolam

Sedation
↔

Others
Atropine

Impaired gastric emptying
↔

Levodopa
Adverse effects
↑

Metoclopramide

Sedation
↔