Pharmacokinetics Applied

Pharmacokinetics describes what the body does to a drug through four processes: Absorption (drug entry into systemic circulation — affected by route, formulation, GI pH, food, and first-pass hepatic metabolism), Distribution (drug movement from blood into tissues — affected by protein binding, lipid solubility, volume of distribution, and blood-brain barrier penetration), Metabolism (biotransformation primarily by hepatic cytochrome P450 enzymes — CYP3A4 metabolizes ~50% of drugs; Phase I reactions add functional groups, Phase II conjugation reactions increase water solubility), and Excretion (elimination primarily via kidneys — glomerular filtration, tubular secretion, and reabsorption; also via bile, lungs, and sweat). Half-life (t½) is the time for plasma concentration to decrease by 50%; steady state is reached after approximately 4-5 half-lives of continuous dosing. These principles determine dosing intervals, loading doses, and dose adjustments for organ dysfunction.