Drug interactions are a perennial exam favourite. Understanding them mechanistically is far more useful than rote memorisation.
Pharmacokinetic interactions
The most clinically relevant interactions occur at the CYP450 enzyme level:
- Inhibitors raise drug levels: ketoconazole, erythromycin, cimetidine
- Inducers lower drug levels: rifampicin, carbamazepine, phenytoin, St John’s Wort
Classic question: a patient on warfarin starts rifampicin → CYP2C9 induction → ↑ warfarin metabolism → ↓ INR → warfarin dose must increase.
Pharmacodynamic interactions
- Synergism: co-trimoxazole (sequential folate block)
- Antagonism: naloxone reverses opioid toxicity
- Additive toxicity: aminoglycosides + loop diuretics → ↑ oto/nephrotoxicity
Remember: not all interactions are harmful — some are exploited (carbidopa + levodopa, clavulanic acid + amoxicillin).