LD50

LD50 is the bluntest measure of acute danger: the dose that kills half of a test population. It is the lethal counterpart of ED50, read from a quantal curve whose all-or-none endpoint is death.

Historically derived from animal toxicity testing, LD50 is reported per body weight (for example, milligrams per kilogram) and depends heavily on species, route of administration, and conditions, so a single number must always be quoted with its context. A small LD50 signals high acute lethality; a large one signals that a great deal of drug is needed to kill.

LD50 has become controversial on both scientific and ethical grounds. It tells you nothing about long-term or non-lethal harm, varies widely between species, and requires killing many animals, so modern regulatory toxicology has largely replaced the classic LD50 test with refined methods that use far fewer animals while still estimating acute lethality. In humans, LD50 cannot be measured and is inferred from poisoning case reports and animal data.