LD50 is the median lethal dose, the dose at which 50% of treated experimental animals die.

• A. The maximum tolerated dose.
• B. The median lethal dose.
• C. The dose causing toxicity in 50% of population.
• D. The dose producing therapeutic effect in 50% of patients.

Answer: (B)

The main idea is that LD50 is a measure of acute lethality. It represents the dose at which half of the treated animals would be expected to die, so the word “median” directly corresponds to 50% mortality and the term is about death, not just any toxicity or therapeutic effect. This makes it the standard way to compare how dangerous a substance is in terms of lethal dose.

This differs from the other concepts: the maximum tolerated dose is about the highest dose that can be given with acceptable effects, not necessarily causing death; the dose causing toxicity in 50% of the population (TD50) refers to nonlethal toxic effects in half the population; and the dose producing a therapeutic effect in 50% of patients (ED50) refers to efficacy rather than lethality.