Drug Interactions: A Clinical Classification

Drug interactions look endless until you split them in two: those that change how much drug is present (pharmacokinetic) and those that change what the drug does (pharmacodynamic). Sort every interaction onto the tree below and the topic becomes systematic.

Pharmacokinetic interactions — changing the level

• Absorption: chelation (tetracyclines/quinolones with calcium, iron, antacids), altered gut pH, or motility. Usually separate dosing solves it.
• Distribution: displacement from plasma protein (warfarin + sulfonamides) transiently raises free drug — clinically minor for most, but relevant for narrow-index drugs.
• Metabolism — the big one:
  • Enzyme inducers (rifampicin, carbamazepine, phenytoin, barbiturates, chronic alcohol, St John’s Wort) lower levels of co-administered drugs over days–weeks — e.g. oral contraceptive failure.
  • Enzyme inhibitors (macrolides, azole antifungals, grapefruit juice, cimetidine, valproate, ciprofloxacin) raise levels quickly → toxicity.
• Excretion: competition for renal tubular transport (probenecid + penicillin) or reduced clearance (NSAIDs and thiazides raise lithium; verapamil raises digoxin).

Pharmacodynamic interactions — changing the effect

• Additive / synergistic: alcohol + benzodiazepines (CNS depression), ACE inhibitor + potassium-sparing diuretic (hyperkalaemia), warfarin + aspirin (bleeding), QT-prolonging combinations.
• Antagonistic: β-blocker opposing salbutamol; naloxone reversing opioids; vitamin K opposing warfarin.

Who is at highest risk?

• Patients on narrow-index drugs — warfarin, digoxin, lithium, phenytoin, theophylline.
• The elderly and those on polypharmacy.
• Patients with renal or hepatic impairment.

Exam tip: name your inducers and inhibitors. Inducers — “CRAP-GPS“: Carbamazepine, Rifampicin, Alcohol (chronic), Phenytoin, Griseofulvin, Phenobarbitone, Sulfonylureas/St John’s Wort. Inhibitors — azoles, macrolides, grapefruit, cimetidine, ciprofloxacin, valproate. Inducers lower levels (slow); inhibitors raise them (fast).

Two branches, a short list of culprit enzymes, and the high-risk drugs — that framework lets you predict, prevent, and explain almost any interaction you’ll meet in the exam or on the ward.