Do drugs stop working if you know they are little better than a sugar pill? And do cultural factors, like our collective faith in a treatment, have a measurable effect on the benefits?
The response to placebo has increased significantly in recent years (as has the response to medication): perhaps our expectations of those drugs have increased.
The Wikipedia page on Placebos is pretty excellent too.
“Our minds create the medicine, and that is pretty freakin’ weird.”