The story of the drug Thalidomide is heartbreaking, but it illustrates why we pay so much attention to stereochemistry. The compound was identified in the 1950s as a neurologically active molecule that had the ability to quell morning sickness in pregnant women. After the drug began to be marketed, children born to women taking it displayed horrific birth defects--often extremely shortened arms and hands that were not functional. Less obvious abnormalities were defects in children's eyes and hearts, deformed alimentary canals and urinary tracts. The drug was pulled from the market after these problems, along with death rates approaching 50%, were reported. This disaster helped lead to the creation of the modern drug testing and approval regime in the United States and Europe.

The part of the story that pertains to stereochemistry is that the original drug was made and sold as a mixture of 2 forms shown above. These are mirror images of each other, as you can see; they are not identical. Further research revealed that only the form on the right (the "R" form) was therapeutically active; the one on the left (the "S" form) was not only ineffective, it was the source of the birth defects!

Thalidomide is still used (rarely) for a variety of conditions. It is possible to make the compound in only one of the two forms, but because there is always some small proportion of the S-isomer, great pains are taken to avoid exposing women of childbearing age to it.

Many pharmaceuticals are "chiral" like Thalidomide, and almost always only one of the two mirror images is effective. At best the mirror image is processed by the body and excreted, but there is always the question--and the risk--that the wrong form will do harm.