Pharmaceutical companies are selling billions of dollars' worth of single-enantiomer drugs. Why might it be important to use only one form of an enantiomer?

Pharmaceutical companies use only one form of an enantiomer because while one enatiomer may be effective, the other may be ineffective, toxic and even cause adverse effects

Explanation:

Enantiomer members often have different chemical reactions with another enantiomer substances. Many biological molecules made up of enantiomers, so sometimes there is a marked difference in the effects of two enantiomers on biological organisms. For example, often only one of the enantiomers of a drug is responsible for desired physiological effects, while the other enantiomer is less active, inactive, or sometimes even productive of adverse effects.

Take an example of eszopiclone (Lunesta), which is consist a single enantiomer of an older racemic drug called zopiclone. One enantiomer produce the desired effects, with the other enantiomer is being inactive, and the dose of eszopiclone is half that of zopiclone

This is because  the  potency of enantiomers may fluctuates, as one enantiomers may produce  the desired therapeutic effects, while the other may be ineffective.

Explanation:

During the   drugs productions, two chiral images of drugs( as mirror images) of one another are usually produced .Theses mirror images are refereed to as enantiomers or( rancemic mixtures); and the concerned drug is  a  mixture of good therapeutic effects and toxicity. The high demand technicalities to separate this rancemic mixtures made pharmaceutical companies to sell these drug as enantiomers.

However,  recent advancements in separation techniques leads to switching from 'rancemic mixtures' to 'single-enantiomer drugs'.(rancemic switch). This assured drug effectiveness and potency, so that the desired therapeutic drug effect is obtained by the user.

It also enables the pharmaceutical company to gain patent on the drug for an extended period of time.

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