Pharmacology Basics, Drug Forms, Dosage and Routes of Administration
Domain overview
This domain tests basic pharmacokinetics, drug forms, routes and dose calculation. The aim is safe administration: the right patient, drug, dose, route and time.
Core rules and concepts
Pharmacokinetics describes absorption, distribution, metabolism and excretion; pharmacodynamics describes what the drug does at its target. An agonist activates a receptor, an antagonist blocks it, and half-life determines the dosing interval. First-pass metabolism in the liver reduces oral bioavailability, which is why some drugs are given sublingually or parenterally.
Drug forms carry instructions: shake suspensions and emulsions before measuring; do not crush enteric-coated tablets, which are designed to pass the stomach, or sustained-release tablets, which would release the dose at once. Sublingual nitroglycerin is absorbed under the tongue and avoids first-pass metabolism. Suppositories are given rectally; inhalers deliver drug directly to the airways.
Routes include oral, sublingual, buccal, topical, rectal, inhalation, subcutaneous, intramuscular, intravenous and intradermal. IV gives essentially 100% bioavailability and rapid onset; IM and SC are slower and depend on blood flow; intradermal is used for BCG and some skin tests. Calculate doses in mg, then convert to mL using the strength on the label.
Worked example
A 12 kg child needs paracetamol at 15 mg/kg per dose. The syrup is 125 mg in 5 mL. Dose = 12 × 15 = 180 mg. Volume = 180 mg × 5 mL ÷ 125 mg = 7.2 mL. Give with a proper oral syringe, not a household spoon, and check the maximum daily dose and interval before dispensing.
Traps that cost marks
Confusing milligrams with millilitres; moving a decimal point; using an adult dose for a child; failing to shake a suspension; crushing a sustained-release tablet; and mixing two drugs in one syringe without checking compatibility.