Developments in synthetic and analytical chemistry have provided the tools to differentiate between two enantiomers (mirror images) of drugs or between the parent compound and metabolite(s) with respect to desired and undesired pharmacological effects. Several drugs are now marketed or being developed as single enantiomers in place of a previous racemic mixture, a process known as "chiral switching". It is easier to understand "pure" as opposed to "mixture" pharmacology but whether the promise of chiral (and metabolite) switches will translate into real clinical advances remains to be seen.