Drug Interactions

Drug interactions occur when one drug affects the action of another, altering effectiveness or causing side effects.

Drug Interactions

  • Drug interactions occur when the effect of one drug is altered by the presence of another drug, food, or substance.
  • These can be classified as pharmacokinetic or pharmacodynamic.
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  1. Pharmacokinetic Interactions

    • These involve changes in the Absorption, Distribution, Metabolism, or Excretion (ADME) of a drug.
    • Absorption:

      • Altered by pH, food, or other drugs
      • Example: Antacids reduce tetracycline absorption
    • Distribution:

      • Displacement from plasma proteins can increase free drug levels
      • Example: NSAIDs displacing warfarin
    • Metabolism:

      • Enzyme induction increases metabolism, reducing drug levels (e.g., rifampin induces CYP450)
      • Enzyme inhibition decreases metabolism, increasing drug levels (e.g., erythromycin inhibits CYP3A4)
    • Excretion:

      • Competition at renal transporters
      • Example: Probenecid reduces excretion of penicillin
  2. Pharmacodynamic Interactions

    • These involve direct or indirect effects on drug receptors or physiological systems.
    • Additive effect:

      • The combined effect is equal to the sum of individual effects
      • Example: Aspirin and paracetamol for pain relief
    • Synergism:

      • The combined effect is greater than the sum of individual effects
      • Example: Sulfonamide and trimethoprim
    • Antagonism:

      • One drug reduces the effect of another
      • Can be competitive, non-competitive, or physiological
      • Example: Naloxone antagonizing morphine

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