The LD50 test determines the dosage of a substance that will kill 50% of the animals given that dosage. The LD50 value is that dosage which kills 50% of the animals. The LD50 value for a substance will vary according to the species involved. The substance may be administered any number of ways, including orally, topically, intravenously, or through inhalation. The most commonly used species for these tests are rats, mice, rabbits, and guinea pigs. Substances tested might include household products, drugs or pesticides.
The LD50 test is controversial because the results have limited, if any, significance when applied to humans. Determining the amount of a substance that will kill a mouse has little value to human beings. Also controversial is the number of animals frequently involved in an LD50 trial, which may be 100 or more animals.
Because of these controversies, the LD50 test is not used as much now as it was in the 1980s and 1990s. According to the National Institute of Heath, the Consumer Product Safety Commission "strongly discourages" the use of the LD50 test, while the Environmental Protection Agency discourages its use, and the Food and Drug Administration does not require it.