Pharmacology
What you must understand
Pharmacology questions test mechanism, kinetics and adverse effects. Start with pharmacokinetics: absorption, distribution, metabolism (phase I and II reactions), and excretion, plus the concepts of bioavailability, volume of distribution, half-life and clearance. Then pharmacodynamics: receptor types (agonist, antagonist, partial agonist, inverse agonist), dose–response curves, potency vs efficacy, and therapeutic index. Organise drugs by class and know the first-line agent, its mechanism, and its signature toxicity (e.g., aminoglycosides → ototoxicity and nephrotoxicity; ACE inhibitors → cough and hyperkalaemia).
Traps that appear in questions
- Confusing potency (dose needed) with efficacy (maximal effect).
- Forgetting that zero-order kinetics (e.g., phenytoin, ethanol, aspirin at high dose) means a constant amount is eliminated per unit time, so small dose increases can cause toxicity.
- Mixing up competitive vs non-competitive antagonism on the dose–response curve (competitive shifts right, non-competitive lowers the maximum).
- Overlooking enzyme induction (e.g., rifampicin, carbamazepine) and inhibition (e.g., erythromycin, ketoconazole) in drug-interaction questions.
Worked example (rule of thumb)
A patient on warfarin develops bleeding after starting an antibiotic. Warfarin is highly protein-bound and metabolised by CYP enzymes; many antibiotics inhibit CYP, raising warfarin levels. Rule of thumb: inhibitors raise the level of the substrate drug; inducers lower it. Check INR and adjust the dose.
Exam strategy
For every drug, recall the class, mechanism, key adverse effect and one interaction. That framework answers most clinical pharmacology stems.