The therapeutic index (TI) of a drug is defined as TD50/ED50. A drug with a TI of 2 compared to one with a TI of 100 means:

• A The drug with TI=2 requires twice the dose to achieve effect compared to the drug with TI=100
• B The drug with TI=100 is 100 times more potent
• C The drug with TI=2 has a narrow safety margin — the dose producing toxicity in 50% of subjects is only twice the dose producing efficacy in 50%, leaving little room between therapeutic and toxic doses ✓
• D The drug with TI=2 requires therapeutic drug monitoring only if plasma protein binding exceeds 90%

Explanation

The therapeutic index (TI = TD50/ED50) measures the ratio between the dose causing toxicity in 50% of subjects and the dose producing therapeutic effect in 50% of subjects. A TI of 2 means the toxic dose is only twice the effective dose — a very narrow margin where small variations in dose, absorption, or clearance can shift a patient from therapeutic to toxic range. This is the pharmacological basis for requiring therapeutic drug monitoring (TDM) for narrow TI drugs like digoxin (TI ~2), lithium, phenytoin, aminoglycosides, and warfarin. A TI of 100 indicates a wide safety margin.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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