What is the exact reason why, when you purchase insulin, its only supposed to be good for 28 days? Why do people use it once and somehow it goes bad on them?

Insulin will keep stable in an acid solution at domestic refrigerator temperatures almost indefinitely. Insulin, in any of the many forms approved for human administration, is vulnerable to freezing as well as to excessively high temperatures All insulins have been tested extensively over the package insert range and a little beyond. Surprisingly, perhaps contamination from frequent vial aspiration is not an issue. Twenty-eight days is also, in some measure, an arbitrary period in which most users are likely to have used the whole vial and, at the same time, there has been a minimum opportunity for loss of potency. Pen cartridges have further reduced this risk. I think the most likely cause of a sudden loss of potency would be exposure of the vial beyond the safe temperature range.

DOB