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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[1]. It is being phased out in some jurisdictions in favor of tests such as the Fixed Dose Procedure.[2], 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 very strongly between testing facilities, due to factors such as the genetics of the testing population, environmental factors and mode of administration.[3] 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. Medical Subject Headings, Medical Subject Headings (MeSH) (ID D007928) defines LD50 as The LD50 is usually expressed as the mass of substance administered per unit mass of test subject, such as grams of substance per kilogram of body mass. Stating it this way allows the relative toxicity of different substances to be compared, and normalizes for the variation in the size of the animals exposed (although toxicity does not always scale simply with body mass). Typically, the LD50 of a substance is given in milligrams per kilogram of body weight. In the case of some neurotoxins such as batrachotoxin, one of the most deadly toxins known, the LD50 may be more conveniently expressed as micrograms per kilogram (µg/kg)of body mass.
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