I have long wondered about this question and did search for an answer. I must say that I have a feeling (but no proof) that there is a conspiracy against this class of drugs by the medical/ pharmaceutical industry and the hidden reasons are related to profitability of selling these "old" drugs which compete with the l-dopa and dopamine agonists. I know that this statement sounds weird, irresponsible and stupid so I repeat that it is only a hunch and you may take it as a joke which is not funny !!

The following recent article is a recent example of research which provide a ‘may be / may be not’ answer.

http://www.guardian.co.uk/lifeandsty...blems-in-older

Imad