Many insurance plans won't let you refill early. I have tried to help my 70 year old Mother set up some emergency plans, including a small stock pile of her meds, but her insurance company will keep track right down to the day, and will not let her refill until just in time for the new pills to arrive as the old ones are running out.

She's had a few times when the mail was late, that she actually missed meds because she ran out before the new ones arrived.