Taking your own leftover prescription pills

I came across a scenario and wondered what others might think.

Someone was prescribed pain medication (for a legitimate reason) - just short term to recover from an injury or something or the sort. But this person didn’t need to use all of it, so there was some leftover.

Then sometime later (weeks or months), they hurt themselves or are somehow otherwise experiencing pain and decide to take one of their leftover pain pills.

Is this “okay”? Because it’s their own prescription? Even though the prescription isn’t recent and wasn’t prescribed for that particular thing?

Extra questions:

~What if they then test positive for those opiates on a drug screen (maybe a random test at work, or a test that CPS or some other agency might ask for?) Like should that be considered a positive drug test? Or not?

~Would it be different (ok/not okay) if they do or do not have a history of substance abuse?