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Drug Interactions Overview

Jul 3, 2025

Overview

This lecture covers the concept of drug interactions, their types, causes, common examples, and important considerations for pharmacists and patients.

What is a Drug Interaction?

  • A drug interaction is a reaction between a drug and another substance that changes the effect or side effects of the drug.
  • Interactions can decrease drug effectiveness or increase side effects and toxicity.

Types and Causes of Drug Interactions

  • Drug interactions can be pharmacodynamic (interaction at the drug effect level) or pharmacokinetic (interaction changes drug concentration in the body).
  • Pharmacodynamic interactions include additive (sum of effects), synergistic (greater effect), or antagonistic (reduced effect) outcomes.
  • Pharmacokinetic interactions occur through changes in absorption, distribution, metabolism, or excretion.

Types of Drug Interactions & Examples

Drug-Drug Interactions

  • Most common; occur between two drugs and may increase toxicity or cause ineffectiveness.
  • Examples:
    • Warfarin + aspirin: increased bleeding risk
    • ACE inhibitors (e.g., lisinopril) + spironolactone: hyperkalemia
    • Digoxin + amiodarone/verapamil: increased digoxin toxicity
    • Fluoroquinolones + theophylline: increased theophylline toxicity
    • Warfarin + fluoroquinolone/macrolide: increased bleeding risk
    • Azole antifungals + statins: increased statin side effects

Drug-Dietary Supplement Interactions

  • Occur between drugs and supplements (vitamins, botanicals, enzymes).
  • Examples:
    • Antidepressants + St. John's Wort: serotonin syndrome
    • Warfarin + ginkgo: increased bleeding risk
    • Benzodiazepines + kava: increased drowsiness

Drug-Nutrient (Food) Interactions

  • Occur between drugs and nutrients in food or supplements.
  • Examples:
    • Warfarin + vitamin K-rich foods: decreased warfarin effectiveness
    • Tetracyclines + calcium: decreased antibiotic levels
    • Fluoroquinolones + calcium/magnesium/iron/aluminum: decreased antibiotic levels
    • ACE inhibitors + potassium: risk of hyperkalemia

Drug-Food Interactions

  • Occur between drugs and foods.
  • Examples:
    • MAO inhibitors + tyramine-rich foods: dangerously high blood pressure
    • Statins + grapefruit: increased statin side effects due to CYP3A4 inhibition
    • Several other drugs (e.g., alprazolam, amiodarone) also interact with grapefruit

Drug-Disease Interactions

  • Occur when a drug worsens a disease.
  • Examples:
    • Beta blockers: can worsen asthma
    • Nasal decongestants: increase blood pressure in hypertension
    • NSAIDs/aspirin: worsen peptic ulcers or heart failure
    • Steroids: worsen blood sugar control in diabetes

Drug-Laboratory Interactions

  • Drugs can alter lab test results, sometimes causing false readings.
  • Example: Cephalosporins can affect urine glucose and ketone tests.

Key Terms & Definitions

  • Pharmacodynamic Interaction — Interaction affecting the drug’s effect at its site of action.
  • Pharmacokinetic Interaction — Interaction changing the concentration of a drug in the body.
  • Additive Effect — Combined effect equals the sum of individual effects.
  • Synergistic Effect — Combined effect is greater than the sum of effects.
  • Antagonistic Effect — One drug reduces the effect of another.
  • CYP3A4 — A liver enzyme involved in drug metabolism, inhibited by grapefruit.

Action Items / Next Steps

  • Review common drug interactions and memorize high-risk combinations.
  • Be alert for patients taking supplements or specialty diets.
  • Prepare for questions on drug interactions for exams or clinical practice.

Full transcript