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 after a specified test duration. 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, 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. 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, since its relative harmfulness depends on its usual dose, but a very low one is always a cause for concern.

Animal-rights and animal-welfare groups, such as Animal Rights International, have campaigned against LD50 testing on animals in particular as, in the case of some substances, causing the animals to die slow, painful deaths. Several countries, including the UK, have taken steps to ban the oral LD50, and the Organization for Economic Co-operation and Development (OECD) abolished the requirement for the oral test in 2001 (see Test Guideline 401, Trends in Pharmacological Sciences Vol 22, February 22, 2001).

Source: Wikipedia (All text is available under the terms of the Creative Commons Attribution-ShareAlike License)