Pharmacology
What is tested
Pharmacology in FMGE is prescriber's pharmacology: which drug, at what dose, by which mechanism, with which predictable toxicity and which antidote. Kinetics calculations and adverse-effect matching appear regularly.
Core rules
- Kinetics: first-order elimination removes a constant fraction per unit time; zero-order eliminates a constant amount. Phenytoin, ethanol, high-dose salicylate and high-dose warfarin saturate and become zero-order.
- Half-life arithmetic: after n half-lives the fraction remaining is (1/2)^n; four half-lives leave 6.25%.
- Volume of distribution: Vd = dose / plasma concentration; a large Vd means tissue sequestration (chloroquine, digoxin).
- Mechanisms: beta-lactams and vancomycin act on the cell wall; aminoglycosides and tetracyclines on the 30S subunit; macrolides, clindamycin and chloramphenicol on the 50S; fluoroquinolones on DNA gyrase; rifampicin on RNA polymerase.
- Antidotes: organophosphate - atropine plus pralidoxime; paracetamol - N-acetylcysteine; warfarin - vitamin K; heparin - protamine; benzodiazepine - flumazenil; opioid - naloxone; methanol - fomepizole.
Worked example
A patient on phenytoin 300 mg/day develops nystagmus and ataxia. The level is 32 µg/mL (therapeutic 10–20). Reasoning: phenytoin follows Michaelis–Menten kinetics, so a small dose increase produces a disproportionate rise in concentration. The correct step is to hold the drug and recheck, not merely to halve the dose, because elimination is already saturated.
Traps
Treating phenytoin as a first-order drug; forgetting pyridoxine cover with isoniazid and ethambutol's optic neuritis; and confusing prodrugs (enalapril, codeine, clopidogrel) with active drugs.