U01.05.009 Therapeutic index

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The therapeutic index is a key measure of drug safety, quantifying the margin between a drug’s therapeutic and toxic doses.

Definition

$\text{Therapeutic Index (TI)} = \frac{\text{TD}_{50}}{\text{ED}_{50}}$

Where:

TD50 (Median Toxic Dose): Dose at which 50% of patients experience toxicity.
ED50 (Median Effective Dose): Dose at which 50% of patients experience the desired therapeutic effect.

Key points:

Higher TI → Safer drug
Lower TI → Drug requires careful monitoring

Examples of drugs with low TI (high risk):

Warfarin
Theophylline
Digoxin
Antiepileptic drugs
Lithium

Therapeutic Window

The therapeutic window is the range of drug concentrations that is both effective and safe.

Parameter	Definition
ED50	Dose producing effect in 50% of patients
TD50	Dose producing toxicity in 50% of patients
Therapeutic window	Safe and effective range of drug concentration
Therapeutic index (TI)	TD50 ÷ ED50; higher = safer drug

Efficacy rises at ED50
Toxicity rises at TD50
Therapeutic Index = distance between ED50 and TD50

Learning Objective (Step 1)

Understand the concepts of therapeutic index and therapeutic window, identify drugs with low TI that require careful monitoring, and interpret ED50 and TD50 values to assess drug safety and dosing.