Elimination of Drugs

Overview

• Elimination of drugs 
  • rate of elimination is rate of disappearance of active molecules from bloodstream or body
  • rate of elimination and dosage determine duration of action of a drug
  • drug elimination ≠ drug excretion
• e.g., a drug may be eliminated by metabolism before excretion from body

Zero-order elimination

• Zero-order elimination
  • implies rate of elimination is constant, regardless of drug concentration
    • constant amount of drug is eliminated per unit time
  • drug concentration in plasma (C_p) decreases linearly with time
  • e.g., phenytoin, ethanol, aspirin
• aspirin is zero-order drug at high therapeutic or toxic concentrations

First-order elimination

• First-order elimination
  • implies rate of elimination is proportional to drug concentration
    • constant fraction of drug is eliminated per unit time 
  • drug concentration in plasma (C_p) decreases exponentially with time
  • first-order drugs have a characteristic, constant half-life of elimination