Read up on stereoisomers (enantiomers and diastereomers) and
thalidoamide for an example where R and S have different properties.

Well there are a few drugs which are simply isomers of older drugs, a
couple that come to mind are Celexa vs Lexapro and Prilosec vs Nexium.
Without getting into a discussion about how pharma sells new drugs and
keeps making profits by extending patents in this manner, the use of
Lexapro (the purified enantiomer) does yield in a noticeable difference
for many people.  The reports of less side effects are true and very
real for them, so in that case the drug would be justified.

For Nexium however, I have heard of many people complaining of side
effects from Prilosec, so the automatic switch over that the doctors did
when it first came out was, I feel, a waste since Prilosec went generic
and was cheaper.  But then some insurance plans started not paying for
the generic of prilosec because it went OTC, at which point when people
were being switched over for that reason, I guess you can't do anything
else.

Xopenex vs Albuterol is another pair of drugs that are enantiomers.
Although Xopenex has been purported to not have the same increase in
heartrate as albuterol does, it really hasn't panned out that way.  But
I haven't seen Xopenex fly off the shelves, so I am guessing the
prescriber community knows how different that drug really is from albuterol.