Thalidomide is a classical example highlighting the alleged role of chirality in drug toxicity. Thalidomide was a racemic therapeutic and prescribed to pregnant women to control nausea and vomiting. The drug was withdrawn from world market when it became evident that the use in pregnancy causes phocomelia (clinical conditions where babies are born with deformed hand and limbs). Later in late 1970s studies indicated that the (R)- enantiomer is an effective sedative, the (S)-enantiomer harbors teratogenic effect and causes fetal abnormalities. Later studies established that under biological conditions the (R)-thalidomide, good partner, undergoes an in vivo metabolic inversion to the (S)-thalidomide, evil partner and vice versa. It is a bidirectional chiral inversion. Hence the argument that the thalidomide tragedy could have been avoided by using a single enantiomer is ambiguous and pointless.
The salient features are presented in the table below.