Thalidomide is probably the most well known of these compounds. It exists in a right-handed (R) form and a left-handed (S) form. The currently accepted theory is that the S form is responsible for the therapeutic benefits (of which there are many) and the R form responsible for the side effects (including the birth defects for which the drug is most widely known). Naturally, thalidomide occurs as a combination of the two (a racemic mix), but can be separated. However, if a pure R form (for example) is created, over time it becomes a racemic mix (especially in the acid environment of the human stomach. Synthetic versions are being created which do not allow this interconversion to occur.

And that's the biology lesson for today, folks. As for that organic poison, did you really expect me to stay on topic?