Question

Which of the following words fulfils the definition of therapeutic index?
"The therapeutic index is a quantitative measure of a drug's ________".

• A. Efficacy
• B. Safety
• C. Toxicity
• D. Therapeutic value

Hint:

Therapeutic Index mnemonic: $\text{TI} = \text{TILE} \implies \frac{\text{Lethal Dose } (LD_{50})}{\text{Effective Dose } (ED_{50})}$. A high ratio means a safer drug!

Answer 1

The Correct Option is B

Concept: The Therapeutic Index (TI) is a quantitative comparative ratio used to evaluate the overall safety margins of a drug. It measures the gap between the dose required to produce therapeutic benefits and the dose that causes unacceptable toxicity. \[ TI = \frac{TD_{50}}{ED_{50}} \quad \text{or} \quad \frac{LD_{50}}{ED_{50}} \] Where:
• $LD_{50}$ or $TD_{50}$ is the median lethal or toxic dose in 50% of a test population.
• $ED_{50}$ is the median effective dose that produces the desired therapeutic outcome in 50% of the population.

Step 1: Determine the core property measured by TI.
A larger Therapeutic Index indicates a wider safety margin between effective and toxic doses, whereas a narrow Therapeutic Index (e.g., lithium, warfarin, digoxin) requires routine therapeutic drug monitoring due to safety risks. Therefore, the therapeutic index serves as a direct quantitative measure of a drug's safety.