Pharmacy and Pharmacology: calculations, classes, and controlled drugs
Pharmacy and Pharmacology
This domain tests dose calculation, unit conversion, drug classes and their key adverse effects, dispensing accuracy, and legal handling of controlled substances.
Core rules and concepts
- Conversions: body weight in pounds ÷ 2.2 = kg. Percent solutions: 1% = 10 mg/mL (2% lidocaine = 20 mg/mL). Ratios: 1:1000 epinephrine = 1 mg/mL.
- Dose formula: volume (mL) = (dose mg/kg × weight kg) ÷ concentration mg/mL.
- Key adverse effects: NSAIDs - GI ulceration and renal injury; aminoglycosides - nephrotoxicity and ototoxicity; doxycycline in cats - esophageal stricture, so follow with water or food; alpha-2 agonists - bradycardia and hypertension, reversible with atipamezole; metoclopramide - extrapyramidal signs.
- Controlled substances: locked storage, DEA-required records, biennial inventory, running use logs with patient and amount. Wastage must be witnessed and documented.
- Withdrawal times apply to food-producing species and compete animals; check the label and regulatory source, not memory.
Worked example
A 15 kg dog is prescribed 2 mg/kg of a 50 mg/mL solution. Total dose = 15 × 2 = 30 mg. Volume = 30 ÷ 50 = 0.6 mL. Have a colleague check the calculation, draw up with a syringe no larger than needed for accuracy, label the syringe with drug, dose, and time, and record it in the treatment sheet.
Traps
Pounds versus kilograms is the classic error. Confusing mg with mL, or percent with mg/mL. Trailing/leading decimal points (write 0.5, never .5). Assuming two drugs in one syringe are compatible. Failing to log a partial vial or wastage.