In toxicology, the median lethal dose, LD50 (abbreviation for “Lethal Dose, 50%”), or LCt50 (Lethal Concentration & Time) of a toxic substance or radiation is the dose required to kill half the members of a tested population. LD50 figures are frequently used as a general indicator of a substance's acute toxicity. The test was created by J.W. Trevan in 1927. It is being phased out in some jurisdictions in favor of tests such as the Fixed Dose Procedure, however the concept, and calculation of the median lethal dose for comparison purposes, continues in wide use. As a measure of toxicity, LD50 is somewhat unreliable and results may vary greatly between testing facilities due to factors such as the genetic characteristics of the sample population, environmental factors and mode of administration.
The term semilethal dose is occasionally used with the same meaning, particularly in translations from non-English-language texts, but can also refer to a sublethal dose; because of this ambiguity, it is usually avoided.
