median lethal dose

The median lethal dose, or LD50, is a measure used in pharmacology to determine the toxicity of a substance. It represents the amount of a toxic agent, such as a poison or radiation, that is sufficient to kill 50 percent of a population of animals, typically within a specified time frame.1

This concept is crucial in understanding the dose-response relationship of drugs and chemicals. While the LD50 indicates a lethal dose, the median effective dose (ED50) measures the dose required to produce a desired effect in 50 percent of a population. The ratio between these two, known as the therapeutic index, helps assess a drug's safety margin. Toxicity is relative, and the adage "the dose makes the poison" highlights that even seemingly harmless substances can be harmful in large quantities, while potent toxins can be used therapeutically in small doses.23