What does therapeutic index indicate?

• A. The time to onset of effect.
• B. A higher index indicates a wider safety margin.
• C. The ratio of the toxic dose to the effective dose.
• D. The maximum safe dose.

Answer: (B)

Therapeutic index reflects the safety margin between the dose that provides benefit and the dose that causes toxicity. A higher index means there’s a larger gap between the effective dose and the toxic dose, so the drug is safer to use at typical therapeutic levels. In practice, it’s calculated as the toxic dose (often TD50 or LD50) divided by the effective dose (ED50). While that ratio defines the index, the reason it matters is how it communicates safety: a larger number indicates a wider safety margin. The other ideas—how fast it acts or the maximum safe dose—don't capture this safety gap between benefit and harm.