Is it legal to use legitimately prescribed pain meds for fun? (See details)

Now I know that while Rush Limbaugh originally got prescriptions to Oxycontin quite legitimately for severe back pain, he clearly stepped over a legal line when he started doctor shopping in order to get an ever increasing supply. But excluding behavior like that, is a person breaking any laws if they have a legitimate prescription to one of the opiate or synthetic opiate painkillers and, let’s say that it’s for some sort of recurrent pain, but the pain doesn’t flare up for months. The prescription is about to go stale dated. They can get a refill if needed. So they decide to—knowing the risks and what to avoid to prevent addiction—have a little fun. Are prescription drugs only legal to use as long as they make you feel worse, or can you legally get high on them providing you are willing to sensibly manage the risks of addiction and/or overdose?

I am not asking if such a practice is wise. I jolly well know it isn’t. I’m just wondering about how the law would view it. Of course, operating a motor vehicle while high or using dangerous machinery of any kind would definitely be a legal issue, so let’s set those behaviors aside. For purposes of this question, assume our unwise partier does this in a setting where at the very worst, they aren’t going to hurt anybody but themselves.