I recently received the following question:
How long can you keep bottles of insulin in the fridge? I have insulin from two years ago and wanted to see how long it's safe to use.
16.HOW SUPPLIED/STORAGE AND HANDLING
"Unopened vials, cartridge systems, and the device should be stored in a refrigerator, 36°F - 46°F (2°C - 8°C). Discard after the expiration date" [which is on the vial itself]
What happens after the expiry date? Probably not much. Most likely, the insulin will very gradually lose its potency, so each milliliter of insulin will have a bit less effect on lowering BG levels. That's usually not a problem, if you are checking BG regularly, and adjusting insulin doses. Until you start a new vial -- switching from an old impotent vial to a new standard-potency vial may cause you to crash into hypoglycemia if you don't decrease the dose with the very first dose you draw up from the new vial.
Another thought about old insulin: It's possible that as time goes by, the insulin might crystallize. This was a definite problem in years gone by, if insulin was stored out-of-spec, and could occur if insulin is frozen or overheated. Crystals of insulin in a vial might appear like a layer of frost on the inside of the glass, or white clumps, or a solid white layer at the bottom of the vial. Don't use vials that show any of these effects. (And you probably can ask the manufacturer for a replacement vial if you contact them and inform them of what happened.)
I'm unaware of any other bad things that can happen to vials of unopened insulin that are stored in a fridge.
Hope this helps!