Therapeutic Index — FRCA Primary MCQ
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Correct answer: B — LD50 / ED50
The therapeutic index = LD50 / ED50 where LD50 is the dose lethal to 50% of the population and ED50 is the dose effective in 50% of the population. A larger therapeutic index indicates a wider margin of safety. Drugs with narrow therapeutic indices such as digoxin lithium phenytoin theophylline and warfarin require careful dose titration and monitoring.
Reference: Peck Hill Williams 2014 Pharmacology for Anaesthesia and Intensive Care