The drug Thalidomide as released in the 1950's is a racemic compound, meaning that it is an equal mixture of 2 different isomers. In this case the two different isomers are called enantiomers, meaning that they have the same chemical bond connectivity but they only differ in spacial arrangement. Enantiomers are mirror images of each other but are not superimposable. In the case of Thalidomide, one of the enantiomeric isomers was safe and effective against nausea and morning sickness but the other isomer is a teratogen, or a chemical that causes birth defects in pregnant women. Many drugs released today are now not mixtures of isomers but instead are composed of a single active isomer.