Updated: 12/24/2017

Elimination of Drugs

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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 (Cp) 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 (Cp) decreases exponentially with time
    • first-order drugs have a characteristic, constant half-life of elimination

 

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