Thalidomide is a tranquilizer as well as a notorious human teratogen, not a disease. This teratogen was associated with perhaps the most publicized birth defects tragedy of the last century, especially of the 1960s. A teratogen is an agent that can cause fetal malformations or monstrosities. Introduced in 1956 in West Germany, thalidomide soon became a drug prescribed to many pregnant women for the treatment of morning sickness. Yet it also soon became clear that when used by pregnant women during the first trimester, this drug would impair the proper growth of the fetus causing a very rare and characteristic limb deformity such as amelia (absence of the limbs) and phocomelia (absence/reduction of the proximal portion of the limbs). Congenital heart disease and other typical anomalies may also result. The thousands of children born with these deformities in the late 1950s and early 1960s became known as Thalidomide babies.

Historically, the suspicion that thalidomide was the teratogen was not launched by academic epidemiologists, but by several German and Australian clinicians who noticed several limb deformity cases that were rarely seen before the late 1950s. Thalidomide was withdrawn from the world market at the end of 1961, shortly after the causal association was established. The drug did not receive marketing approval in the USA during this epidemic. Today, thalidomide is approved for use in the USA for the treatment of erythema nodosum in leprosy. It is also being tested as an immunosuppressant drug including for the treatment of AIDS.