Every time you refill a prescription take 3 days worth out and stash it. Add to it with your monthly refills. Before you know it you’ll have 3-4 months worth.
Dry pills will last just about forever. They won’t go bad when they expire, the worst that might happen is they will lose some effectiveness. I have some serious pain pills that I got when my daughter was born, (she is now thirteen). I had some back pain where nothing otc worked, so I took one and got relief. Always fill any pain prescriptions that you get after surgery or whatever, and save them in a safe place, but TELL NO ONE.