It is known that although only the (S)-enantiomer of the infamous sedative thalidomide possesses teratogenic properties, it is not very useful to administer the pure (R)-enantiomer since it is racemized within the body. Are there other known examples of drugs that are stereoconverted in vivo? How does the body perform the conversion?
Although I can't think of any drug examples other than thalidomide, here's information on thalidomide's mechanism:
The chiral carbon of thalidomide can tautomerize in basic conditions into an enol, which is achiral. A reversal back to the ketone results in a mix of (R) and (S) enantiomers.
In the body, this tautomerization is generally catalyzed by basic amino acids. Specifically, albumin is the main catalyst in humans.
While this is beyond the scope of your question, the reason that only (S)-thalidomide causes birth defects is that it can insert itself into DNA and suppresses certain genes necessary for embryonic development.