An often cited example of the importance of stereochemistry relates to the thalidomide disaster. Thalidomide is a pharmaceutical drug , first prepared in 1957 in Germany, prescribed for treating morning sickness in pregnant women. The drug was discovered to be teratogenic , causing serious genetic damage to early embryonic growth and development, leading to limb deformation in babies. Some of the several proposed mechanisms of teratogenecity involve a different biological function for the ( R )- and the ( S )-thalidomide enantiomers.  In the human body however, thalidomide undergoes racemization : even if only one of the two enantiomers is administered as a drug, the other enantiomer is produced as a result of metabolism.  Accordingly, it is incorrect to state that one of the stereoisomer is safe while the other is teratogenic.  Thalidomide is currently used for the treatment of other diseases, notably cancer and leprosy . Strict regulations and controls have been enabled to avoid its use by pregnant women and prevent developmental deformations. This disaster was a driving force behind requiring strict testing of drugs before making them available to the public.
The Diels-Alder reaction between cyclopentadiene and maleic anhydride can produce two possible products, the 'endo' and the 'exo' adducts. This is because although the hydrogens of the maleic anhydride must be cis in the product, there are two possible arrangements where this is true. The actual product formed is the 'endo' adduct. This diastereoisomer is the less stable one, but is formed because it is the kinetic product. There is also a bonding interaction between the carbonyl groups of the dienophile, and the developing π bond at the back of the diene.