Pharmacology and Therapeutics
Pharmacology and Therapeutics tests prescribing decisions: pharmacokinetics, interactions, adverse reactions, dosing in renal or hepatic impairment, pregnancy, and antimicrobial stewardship. The exam rewards practical prescribing and monitoring.
Core rules
- Warfarin is monitored with INR; many drugs alter it. Fluconazole and metronidazole inhibit CYP2C9 and raise INR.
- ACE inhibitor plus potassium-sparing diuretic plus NSAID increases hyperkalemia and acute kidney injury risk.
- Renal dosing: review eGFR for metformin, gabapentin, DOACs, and many antibiotics.
- Pregnancy: avoid ACE inhibitors, ARBs, warfarin, methotrexate, and isotretinoin; use paracetamol for pain and penicillin for many infections.
- Elderly: Beers criteria flag benzodiazepines, antihistamines, and long-acting sulfonylureas.
Worked example
A patient stable on warfarin with INR 2.5 develops candidiasis and is prescribed fluconazole. Reasoning: fluconazole inhibits CYP2C9 and can raise INR and cause bleeding. Reduce the warfarin dose and check INR within a few days, or choose a non-interacting antifungal. Do not simply continue unchanged.
Traps
- Missing the triple whammy of ACE inhibitor, diuretic, and NSAID.
- Prescribing a macrolide with a QT-prolonging drug.
- Forgetting grapefruit inhibits CYP3A4.
- Using a standard adult dose in renal impairment.