Just as our hands are mirror images of each other, molecules can also have mirror images that can not be superimposed on them. These molecules are called enantiomers.
An example of this enantiomer is sedative thalidomide, which was sold in various countries around the world between 1957 and 1961. It was withdrawn from the market when it was discovered that it caused birth defects. An enantiomer determined the desired sedative effects, while the other, inevitably present in equal amounts, caused birth defects.
The mecoprop herbicide is a racemic mixture with the enantiomer (R) – (+) (“Mecoprop-P”, “Duplosan KV”) with herbicidal activity.
Another example is the antidepressants escitalopram and citalopram. Citalopram is a racemic mixture (1: 1 of (S) -citalopram and (R) -citalopram); escitalopram [(S) -citalopram] is a pure enantiomer. The dose for escitalopram is typically 1/2 of those for citalopram.