Kilograms in large animals, grams in small animals, and milligrams in tiny animals.
heres what wiki says about it -
In
toxicology, the
median lethal dose,
LD50 (abbreviation for Lethal Dose, 50%),
LC50 (Lethal Concentration, 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, is still widely used.
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, animal species tested, environmental factors and mode of administration.
[3] Another weakness is that it measures acute toxicity only (as opposed to
chronic toxicity at lower doses), and does not take into account toxic effects that do not result in death but are nonetheless serious (e.g.
brain damage). There can be wide variability between species as well; what is relatively safe for rats may very well be extremely toxic for humans, and vice versa. In other words, a relatively high LD50 does not necessarily mean a substance is harmless, but a very low one is always a cause for concern.
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.