When Drugs Meet Drugs
Most drugs are metabolized by cytochrome P450 enzymes in the liver. When two drugs share the same metabolic pathway, one can alter the clearance of the other — sometimes with life-threatening consequences. Understanding these interactions is essential for safe polypharmacy, which affects the majority of hospitalized patients and elderly populations taking multiple medications.
CYP Inhibition: Raising Drug Levels
Enzyme inhibition reduces clearance, causing the affected drug to accumulate. The effect is often immediate and proportional to the inhibitor concentration. A strong CYP3A4 inhibitor like ketoconazole can increase midazolam AUC by 15-fold, turning a safe sedative dose into a dangerously deep sedation. This simulation shows how inhibition factor directly translates to AUC changes.
CYP Induction: Lowering Drug Levels
Enzyme induction upregulates CYP enzyme expression, accelerating drug clearance. Rifampin, the most potent known inducer, can reduce oral contraceptive levels below effective thresholds — a common cause of unintended pregnancy. Unlike inhibition, induction takes 1-2 weeks to develop fully as new enzyme protein accumulates, and the same time to reverse after the inducer is stopped.
Protein Binding Displacement
Highly protein-bound drugs (>95% bound) are vulnerable to displacement interactions. When a second drug competes for albumin binding sites, the free fraction of the first drug increases. While this transiently boosts pharmacological effect, the body typically compensates: higher free drug means higher clearance, so steady-state free concentration often returns to baseline. The clinical significance of binding displacement alone is frequently overstated.