Clinical Pharmacology · MU Sofia

18. Drug interactions. Types. Importance for clinical practice.

Covers what a drug interaction is and the main types: pharmaceutical incompatibility outside the body, and pharmacokinetic and pharmacodynamic interactions. A table sets out the pharmacokinetic processes (absorption, distribution, metabolism, excretion, transporters) with mechanisms and examples. It then explains why most interactions are harmless, which drugs with narrow therapeutic windows matter most, and how to prevent problems through history-taking, alternatives, dose adjustment and monitoring.

Inside: 3 sections · 1 table · 15 flashcards · 2 min

Topic 18 of 30 in the MU Sofia syllabus for Clinical Pharmacology (Medicine, Year 5). The free AceMed note covers it in 3 sections, about 223 words (2 min of reading), with 15 flashcards.

In this note

  1. Introduction
  2. Types
  3. Clinical importance

Introduction

A drug interaction occurs when the effect of one drug is changed by another drug, food, drink (alcohol, grapefruit juice) or herbal product. The risk rises steeply with the number of drugs taken (polypharmacy), especially in the elderly.

Types

Pharmaceutical (in vitro)

Physical or chemical incompatibility before the drug enters the body, e.g. mixing drugs in the same infusion (precipitation), gentamicin inactivated by penicillins in one syringe.

Pharmacokinetic

Pharmacodynamic

  • Synergistic or additive: alcohol + benzodiazepines (sedation); warfarin + aspirin or NSAIDs (bleeding); several QT-prolonging drugs (torsades de pointes); ACE inhibitor + spironolactone (hyperkalaemia); SSRI + tramadol or MAO inhibitor (serotonin syndrome).
  • Antagonistic: β-blockers + salbutamol; NSAIDs reduce the effect of antihypertensives and diuretics; vitamin K and warfarin.

Clinical importance

Most interactions are harmless or even used deliberately (e.g. amoxicillin + clavulanic acid; levodopa + carbidopa; combination antihypertensives). They matter clinically when the affected drug has a narrow therapeutic index or a steep dose–response curve:

  • Warfarin, digoxin, lithium, theophylline, phenytoin, ciclosporin and tacrolimus, antiarrhythmics, oral contraceptives, insulin and sulfonylureas.

Prevention

  • Take a full drug history (including OTC and herbal products); use few drugs; check interactions (SmPC, BNF, databases).
  • Choose a non-interacting alternative, adjust the dose, or monitor (INR, levels, potassium, ECG) when starting or stopping an interacting drug.

Enzyme inhibitors raise levels within days; enzyme inducers lower them over weeks, and stopping an inducer can cause toxicity.

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