Perceived reputation, color and packaging plays a big role in perceived drug efficacy[0], I wonder if a double blinded trial would show the same results you have seen anecdotally.
I'd be interested to see results from such a trial.
Closer to home, we twice found out that the pharmacy gave her a generic when she developed quirks 4-5 days after she started taking a new bottle. Sort of the reverse of a placebo effect -- she thought she was on the brand all along, and she didn't think to check until things didn't feel right.
[0] http://www.theatlantic.com/health/archive/2014/10/the-power-...