Pharmacology: pharmacokinetics, dosing and receptor logic
What is tested
Pharmacology is examined through pharmacokinetics (absorption, distribution, metabolism, elimination), pharmacodynamics (receptors, dose–response, agonism and antagonism), autonomic pharmacology, antimicrobials and major adverse effects.
Core rules
- Volume of distribution is Vd = dose/concentration; it describes distribution, not a real anatomical space.
- Clearance is the volume of plasma cleared per unit time; half-life t½ = 0.693 × Vd/CL.
- Loading dose = (Vd × target concentration)/F. Maintenance dose = (CL × target concentration × interval)/F.
- First-order kinetics means a constant fraction is eliminated per unit time; zero-order means a constant amount, seen with phenytoin, ethanol and high-dose salicylate.
Worked example
A 70 kg patient needs a drug with Vd 40 L, target plasma concentration 15 mg/L and bioavailability 0.8. Loading dose = (40 × 15)/0.8 = 750 mg. If clearance is 4 L/h and the dosing interval is 8 hours, maintenance dose = (4 × 15 × 8)/0.8 = 600 mg every 8 hours. Doubling the dose without measuring levels risks toxicity when metabolism is saturable.
Traps that cost marks
- Using the maintenance formula as a loading dose.
- Expecting half-life to predict duration of action; it predicts the time to steady state, about four to five half-lives.
- Forgetting that enzyme inducers such as rifampicin lower co-prescribed drug levels while inhibitors such as erythromycin raise them.
- Describing a partial agonist as simply a weak agonist; it produces a reduced response and blocks the full agonist.