Pharmacology and Therapeutics
Pharmacology items test whether you can predict what a drug will do in a real patient: how it is cleared, what interacts with it, and which adverse effect matters most.
Core rules
- First-order kinetics: a constant fraction is eliminated per unit time, so half-life is constant. Steady state is reached after about 4–5 half-lives.
- Zero-order kinetics: a constant amount is eliminated per unit time - phenytoin, ethanol and high-dose aspirin. Small dose increases can cause toxicity.
- Loading dose = volume of distribution × target concentration ÷ bioavailability; maintenance dose depends on clearance.
- CYP inducers (rifampicin, phenytoin, carbamazepine) lower levels of warfarin and oral contraceptives; inhibitors (ketoconazole, erythromycin, ciprofloxacin) raise them.
- Therapeutic drug monitoring is useful for digoxin, vancomycin, gentamicin, phenytoin and lithium.
Worked example
A drug has a half-life of 6 hours and is given at a fixed dose. Steady state is reached after roughly 4–5 half-lives, that is 24–30 hours. If a rapid effect is needed, give a loading dose calculated from volume of distribution rather than repeating maintenance doses.
Traps
- Confusing loading and maintenance doses.
- Missing enzyme inhibition when a patient on warfarin develops bleeding after a new antibiotic.
- Using aminoglycosides without checking renal function and levels.
- Prescribing tetracycline or fluoroquinolones to children or pregnant women.