Two forms of thalidomide. Molecular models of the S- (left) and R- (right) forms of the drug thalidomide. Thalidomide was used as a sedative for pregnant women during the 1950s, but it was found to cause deformities in foetuses. Further research discovered that only S-thalidomide caused the deformities. The S- and R- forms are enantiomers, mirror images of each other. This property arises due to the molecule possessing a chiral centre (yellow atom). The arrangement of the other parts of the molecule around this centre can occur in two ways, which affects the molecule's properties, despite it having the same chemical formula (C13.H10.N2.O4).

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