The re-evaluation of single enantiomers  not without problems. The chiral switches of fluoxetine and fenfluramine are classical examples. The development of (R )-fluoxetine was terminated after patients developed abnormal heart rhythms. The chiral switch of fenfluramine, dexfenfluramine was withdrawn from world marker due to pulmonary hypertension. The table below enumerates couple of chiral switches aborted or withdrawn due stereochemically engineered toxicity.