Pharmacokinetics for NCLEX: Absorption, Distribution, Metabolism & Excretion
Understand pharmacokinetics fundamentals for the NCLEX. Learn ADME principles, half-life, bioavailability, and drug interaction mechanisms.
Pharmacokinetics Fundamentals for NCLEX
Pharmacokinetics describes what the body does to a drug through four processes: Absorption, Distribution, Metabolism, and Excretion (ADME). Understanding these processes helps you predict drug behavior, anticipate interactions, and answer NCLEX pharmacology questions with confidence.
Absorption
Absorption is the movement of a drug from its site of administration into the bloodstream. Factors affecting absorption include route of administration (IV provides 100% bioavailability), dosage form (liquid absorbs faster than tablet), GI motility and pH, blood flow to the absorption site, and food interactions. Enteric-coated tablets resist stomach acid and dissolve in the intestine, so they should never be crushed. Sublingual and buccal medications are absorbed through oral mucosa, bypassing first-pass metabolism. Teach patients not to swallow sublingual nitroglycerin.
Distribution
Distribution is the transport of a drug from the bloodstream to target tissues. Factors include blood flow to tissues, protein binding (highly protein-bound drugs have less free drug available for action), lipid solubility (lipid-soluble drugs cross the blood-brain barrier more easily), and body composition. Elderly patients have increased body fat and decreased lean muscle, affecting distribution of fat-soluble medications. Albumin levels affect distribution of protein-bound drugs. Low albumin increases free drug levels, potentially causing toxicity at standard doses.
Metabolism (Biotransformation)
Most drugs are metabolized in the liver by cytochrome P450 enzymes. First-pass metabolism significantly reduces bioavailability of oral medications. Factors affecting metabolism include liver function (hepatic impairment decreases metabolism, increasing drug levels), age (neonates and elderly have reduced hepatic function), genetic variations, and drug interactions. CYP450 inducers (rifampin, phenytoin, carbamazepine) increase metabolism and decrease drug effectiveness. CYP450 inhibitors (grapefruit juice, ketoconazole, erythromycin) decrease metabolism and increase drug levels.
Excretion
Most drugs are excreted by the kidneys. Monitor renal function (creatinine clearance, GFR) for drugs with renal excretion. Dose adjustments are required for renal impairment. Other excretion routes include bile, lungs, breast milk, and sweat. Half-life is the time for plasma drug concentration to decrease by 50%. Steady state is reached after approximately 4-5 half-lives. Study more with our pharmacology flashcards and pharmacology mini-exams.