U01.21.006 Therapeutic Index (TI)

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Learning Objective:
Understand the concept and calculation of the therapeutic index (TI), which measures the safety margin of a drug by comparing toxic and effective doses.

Formula:

$TI = frac{text{Median toxic dose}}{text{Median effective dose}} = frac{TD_{50}}{ED_{50}}$

Explanation:

$TD_{50}$ = Dose that produces toxicity in 50% of the population
$ED_{50}$ = Dose that produces the desired therapeutic effect in 50% of the population

A high therapeutic index indicates a safer drug (a large margin between effective and toxic doses).
A low therapeutic index indicates a narrow safety margin, requiring careful monitoring (e.g., digoxin, lithium, warfarin).

Example:

Given:
$TD_{50} = 200 text{ mg}$
$ED_{50} = 20 text{ mg}$