Pharmacology and Therapeutics
Pharmacology tests mechanism, dosing, interactions, and adverse effects in the patient in front of you.
Core rules
- Pharmacokinetics: absorption, distribution, metabolism, excretion. Most drugs follow first-order kinetics; phenytoin, ethanol, and high-dose aspirin follow zero-order and saturate.
- CYP interactions: CYP3A4 inhibitors (macrolides, azoles, grapefruit) raise levels of statins, calcium channel blockers, and tacrolimus; inducers (rifampicin, carbamazepine) lower them.
- Warfarin: narrow therapeutic index; interacts with amiodarone, azoles, metronidazole, and trimethoprim-sulfamethoxazole; monitor INR.
- Renal dosing: adjust for eGFR; avoid NSAIDs, nitrofurantoin at low eGFR, and metformin below 30.
- Adverse-effect patterns: ACE inhibitors cause cough and angioedema; aminoglycosides cause nephro- and ototoxicity; fluoroquinolones cause tendon rupture and QT prolongation.
Worked example
A patient stable on warfarin with INR 2.5 is prescribed trimethoprim-sulfamethoxazole for a urinary infection. TMP-SMX inhibits CYP2C9 and displaces protein binding, raising INR. Choose another antibiotic (nitrofurantoin if eGFR allows) or reduce the warfarin dose and recheck INR in 3–5 days.
Traps
- Prescribing without checking renal function and the interaction list.
- Missing QT prolongation and serotonin toxicity when combining psychotropics.
- Assuming all antibiotics reduce contraceptive efficacy - rifampicin does substantially, most others do not.
- Recording an adverse effect as an allergy, restricting future options.