Clinical meaning
Polypharmacy — the concurrent use of five or more medications — creates a complex web of pharmacokinetic and pharmacodynamic interactions that exponentially increases risk of adverse drug events (ADEs). The pathophysiology of polypharmacy harm operates through several mechanisms. First, drug-drug interactions at the cytochrome P450 level alter metabolism: CYP3A4 inhibitors (clarithromycin, ketoconazole, grapefruit juice) increase levels of substrates like statins and calcium channel blockers, while CYP3A4 inducers (rifampin, phenytoin, carbamazepine) decrease levels of substrates including warfarin and oral contraceptives. Second, additive pharmacodynamic effects compound toxicity — multiple anticholinergic medications (diphenhydramine, oxybutynin, amitriptyline, promethazine) produce cumulative anticholinergic burden causing delirium, urinary retention, constipation, tachycardia, and falls. Third, prescribing cascades develop when adverse effects of one medication are misdiagnosed as new conditions and treated with additional medications — for example, an NSAID causes hypertension, an antihypertensive is added causing ankle edema, a diuretic is added causing hypokalemia, potassium supplementation causes GI upset, and an antiemetic is added. Age-related pharmacokinetic changes amplify polypharmacy risk: decreased hepatic blood flow reduces first-pass metabolism by 20-40%, reduced CYP450 activity (particularly Phase I reactions) prolongs drug half-lives,...