Pharmacology and Toxicology: Mechanisms, Kinetics and Poisoning
Core concepts
Expect integrated questions on pharmacodynamics (receptors, agonists/antagonists, dose–response, potency vs efficacy, therapeutic index), pharmacokinetics (absorption, distribution, metabolism, excretion; first-order vs zero-order; loading and maintenance doses), and systems pharmacology for autonomic, cardiovascular, CNS, endocrine, antimicrobial and chemotherapeutic drugs. Toxicology covers dose–response for toxicity, mechanisms of organ damage, antidote selection, and management principles (decontamination, enhanced elimination, supportive care).
Traps examiners use
- Confusing potency (dose needed) with efficacy (maximal effect); a drug can be highly potent yet a partial agonist.
- Mixing competitive (surmountable, rightward shift) with non-competitive (non-surmountable) antagonism.
- Applying first-order half-life logic to zero-order drugs such as phenytoin or ethanol at high dose.
- Naming an antidote without the poison-specific caveat: e.g. N-acetylcysteine for paracetamol, naloxone for opioids, atropine plus pralidoxime for organophosphate poisoning, chelators for heavy metals.
Worked example
Loading dose = (Vd × target concentration) / bioavailability. If Vd = 40 L and target = 10 mg/L with F = 1, loading dose = 400 mg. Maintenance dose rate = clearance × target concentration. If clearance = 4 L/h, maintenance = 40 mg/h. This pairing - load to fill the compartment, maintain to match clearance - answers most kinetic calculations. For toxicology, always state the mechanism before the antidote: organophosphates inhibit acetylcholinesterase, so atropine blocks muscarinic effects while pralidoxime reactivates the enzyme.