LD50

Lethal Dose, 50%. Refers to the dosage of a given drug required to kill 50% of a test population. The LD50 is usually measured in units of mass of drug per units of body mass; for example, mg/kg. This type of measurement is used to help ensure that the size of the specimen has no effect on the dosage measurement, as a large specimen can generally take a larger dose of a toxin without fatal effects. The measure is often known colloquially as the "semilethal dose;" while it may not always be lethal to a given subject, it will be lethal around half the time.