Enantiopure Medications in the Body
For some drugs, only one enantiomer is effective. A pure enantiomer of the effective form would, in theory, only require half the effective dose of a 50/50 racemic mixture. The clinical picture is more complicated.
For example, the results of studies of enantiopure escitalopram versus racemic citalopram have mixed results. Some single studies show that escitalopram is no better or worse than citalopram and other SSRI antidepressants (e.g. Svensson & Mansfield 2004); others, including meta-analyses (studies of studies), show it to be more effective or to have fewer side effects (e.g. Kennedy et al. 2006).
Armodafinil (Nuvigil) is a newly-approved drug that is the R-enantiomer of modafinil (Provigil). This enantiomer stays in the body longer than the S-enantiomer, so theoretically, it may work better or longer than racemic modafinil. However, while studies have proved that armodafinil is effective, there are no data yet on whether it actually works better than racemic modafinil.
The most dramatic case of enantiomers that have different effects in the body is that of thalidomide. "Right-handed" thalidomide stops nausea and vomiting, while "left-handed" thalidomide causes severe birth defects. Thousands of babies were born in the late 1950s and early 1960s, mostly in Europe, with birth defects caused by the thalidomide their mothers were given for morning sickness. (Even if pregnant women had access to the "safe" enantiomer, the body changes it back and forth into both enantiomers (Teo et al. 2004), so it would still cause birth defects.)
Not all medications benefit from being enantiopure. For example, ibuprofen is a racemic drug, but the body has an enzyme that converts one enantiomer to the other, so there is no benefit to purifying one enantiomer.