Medication Safety, Pharmacovigilance and Drug Interactions
Core concepts
Know the classification of adverse drug reactions (Type A augmented, dose-dependent; Type B bizarre, idiosyncratic), the difference between adverse events and adverse reactions, and the reporting system in Brazil (Notivisa, operated by Anvisa). Understand pharmacovigilance signals, under-reporting, and the importance of spontaneous reporting. For drug interactions, distinguish pharmacokinetic (absorption, distribution, metabolism, excretion) from pharmacodynamic interactions, and know key CYP inhibitors (ketoconazole, clarithromycin, ritonavir) and inducers (rifampicin, carbamazepine, phenytoin, St John's wort).
Traps in questions
A common trap is assuming all interactions are clinically relevant - many are theoretical. Another is confusing a contraindication with a precaution. Watch for questions that forget that grapefruit inhibits CYP3A4 while St John's wort induces it. Also, Type B reactions are not dose-related and are often immune-mediated.
Worked rule of thumb
Use the "mechanism then magnitude" rule: first identify the mechanism (CYP inhibition, QT prolongation, additive bleeding), then assess whether the magnitude matters clinically (dose, therapeutic index, patient risk). For a patient on warfarin started on clarithromycin, the interaction is CYP3A4/mechanism-based and raises INR - monitor closely.