Rosiglitazone's antidiabetic action is related to the (S)-enantiomer. A carbonyl group is present in the proximity of the chiral centre; therefore, keto-enol tautomerism can convert the (S)-enantiomer to the (R)-enantiomer or vice versa. This rate of interconversion is very high after S-oxidation of rosiglitazone. One of the notorious examples is thalidomide, which shows therapeutic activity by the S-enantiomer, while the R-enantiomer shows teratogenicity. These enantiomers undergo racemization at physiological conditions very easily via keto-enol tautomerism.