Pharmacology
Pharmacology is the domain that asks how drugs act, move and cause harm; RRB pharmacist questions commonly combine general pharmacology with autonomic, CNS, cardiovascular, antimicrobial and endocrine examples.
Core rules
- Pharmacokinetics is ADME: absorption, distribution, metabolism and excretion. First-order elimination removes a constant fraction per unit time; zero-order elimination removes a constant amount and is seen with ethanol, high-dose phenytoin and high-dose aspirin.
- Bioavailability is the fraction of an administered dose reaching systemic circulation unchanged; IV bioavailability is 100% by definition. Volume of distribution relates the amount of drug in the body to plasma concentration.
- Agonists activate receptors. Competitive antagonists shift the dose-response curve right without lowering the maximum; noncompetitive antagonists reduce the maximum. Therapeutic index is toxic dose divided by effective dose.
Worked example
A drug has a half-life of 8 h. After 4 half-lives, about 94% of steady state is reached (32 h); after 5 half-lives, about 97% is reached (40 h). If a loading dose is required, use LD = Vd × target concentration / bioavailability. A large Vd increases the load but does not by itself change the maintenance dose rate.
Traps
Potency is not efficacy: a more potent drug needs a lower dose, not necessarily a greater maximum effect. Beta-blockers can cause bronchospasm in asthma; ACE inhibitors cause dry cough and angioedema; aminoglycosides are nephrotoxic and ototoxic; rifampicin induces CYP enzymes and can lower warfarin and oral contraceptive effectiveness.