Which statement accurately defines the therapeutic index?

Which statement accurately defines the therapeutic index?

• A. TI = LD50/ED50
• B. TI = ED50/LD50
• C. TI is the same as the safety margin
• D. TI is unrelated to safety

Answer: (A)

Therapeutic index measures how wide the safety window of a drug is by comparing the dose that causes toxicity with the dose that produces the desired therapeutic effect. It is defined as the ratio of the toxic dose for 50% of the population (LD50 or TD50) to the effective dose for 50% of the population (ED50). Therefore, TI = LD50/ED50. A larger TI means a bigger gap between the effective dose and the toxic dose, indicating a safer drug with less risk of harm from small dosing errors. Using the reciprocal, ED50/LD50, would not describe safety in the same meaningful way. The concept is inherently about safety margins—the TI quantifies how close therapeutic use is to potential harm.

Therapeutic index measures how wide the safety window of a drug is by comparing the dose that causes toxicity with the dose that produces the desired therapeutic effect. It is defined as the ratio of the toxic dose for 50% of the population (LD50 or TD50) to the effective dose for 50% of the population (ED50). Therefore, TI = LD50/ED50. A larger TI means a bigger gap between the effective dose and the toxic dose, indicating a safer drug with less risk of harm from small dosing errors. Using the reciprocal, ED50/LD50, would not describe safety in the same meaningful way. The concept is inherently about safety margins—the TI quantifies how close therapeutic use is to potential harm.