Chemicals (molecules) can exist as two mirror images that are not superimposed on each other--such as your right and left hands. The two chemical images are referred to as isomers. However, often it is only one isomer that provides the desired effects and the other is either inactive (not needed) or sometimes even causes side effects.

Xopenex™ is an example of a drug in which the desired isomer has been purified. It is a single isomer version of albuterol commonly used by asthma sufferers for quick-relief.  

Xopenex™ is preservative-free and can be given in smaller doses than albuterol which may lessen shakiness or nervousness that some patients experience with albuterol. Xopenex™ can last longer than albuterol.