LD50 is defined as?

• A. The dose that will kill 50% of the subjects in a dose/response experiment.
• B. The dose that guarantees 50% survival.
• C. The dose that causes cancer in 50% of subjects.
• D. The maximum dose tested in an experiment.

Answer: (A)

LD50 is the lethal dose that kills about half of the individuals exposed under defined test conditions. It’s determined in a dose–response study by giving groups increasing amounts of a substance and recording mortality, then identifying the dose at which roughly 50% die. This metric helps compare acute toxicity between substances and guide safety decisions, but it depends on species, route of exposure, and other experimental factors, and it doesn’t account for chronic effects or nonlethal endpoints. The idea that it guarantees 50% survival isn’t correct, since LD50 focuses on death, not survival. A dose that causes cancer in 50% of subjects relates to carcinogenic risk, not acute lethality. And the highest dose tested isn’t the LD50 by definition; it’s simply the maximum dose used in the study.