Therapeutic index of a drug signifies: September 2005

Ans. B: Safety Margin The therapeutic index/therapeutic ratio is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount that causes death. It signifies safety margin. Quantitatively, it is the ratio given by the lethal dose divided by the therapeutic dose. A therapeutic index is the lethal dose of a drug for 50% of the population (LD50) divided by the minimum effective dose for 50% of the population (ED50). A high therapeutic index is preferable to a low one: this corresponds to a situation in which one would have to take a much higher dose of a drug to reach the lethal threshold than the dose taken to elicit the therapeutic effect Generally, a drug or other therapeutic agent with a narrow therapeutic range (i.e. with little difference between lethal and therapeutic doses) may have its dosage adjusted according to measurements of the actual blood levels achieved in the person taking it.