I think genarr4 is correct on this one.
I used to have 75 grams of methandrostenolone (Dianabol) in powder form in a bulk bag. I got myself a small capsulator and an accurate measuring device for making 10mg tabs for me and a couple buds.
I wasn't sure how long it would last, I emailed Bill Roberts and he said shelf life was 'probably decades', that assumed it was in a cool dark room. I know this is my only source, but this guy knows his gear, and you probably agree. That's why I think this is true for now.
Moving on, if manufacturers say 6 months, then it most likely is an issue of sterility, because it sure isn't the hormone itself.
~Silex