Pharmacology and Therapeutics
What to know
This domain rewards mechanism-based reasoning over memorised lists. For each major drug class, know the molecular target, the downstream physiological effect, the main therapeutic uses, the dose-limiting adverse effects, and the key contraindications. Understand receptor concepts: agonist versus antagonist, partial agonism, inverse agonism, competitive versus non-competitive antagonism, potency versus efficacy, and the meaning of a therapeutic index.
Pharmacokinetics is examined heavily. Know absorption, bioavailability, first-pass metabolism, volume of distribution, protein binding, hepatic metabolism (including phase I and phase II reactions and enzyme induction or inhibition), and renal versus biliary elimination. Understand half-life and steady state: steady state is reached in roughly four to five half-lives, and loading doses exist to bypass that delay. Apply these ideas to special populations - renal impairment, hepatic impairment, pregnancy, lactation, and older adults - and to clinically important interactions.
Traps
- Confusing potency (dose needed) with efficacy (maximum effect).
- Assuming a longer half-life always means a better drug.
- Forgetting that enzyme inhibition raises substrate levels quickly, while induction develops slowly.
- Overlooking active metabolites when assessing renal impairment.
- Applying adult doses to children or older adults without adjustment.
Worked example / rule of thumb
Half-life rule: after starting a maintenance dose, expect about 50% of steady state at one half-life, 75% at two, 87.5% at three, and roughly 94% at four. If a drug has a half-life of 12 hours and the patient needs an effect today, a loading dose is the logical step. If the half-life is 12 hours and you stop the drug, roughly 94% clears in 48 hours - useful when judging washout before starting an interacting agent.