Yeah the nasal spray should work within a couple of minutes. But even IM (5 minutes) is going to be pretty brutal.

You imagine going from a numb high... to feeling everything you were blocking out with opiates. Many of these people have legitimate pain. You'd probably not be too happy.

I'd certainly be wary about administering it to a stranger, without their consent and without knowing what their reaction would likely be. It's legally sketchy, especially if the drug is —as it is— not generally available (it's prescription-only in many places). Still, don't let me put you off saving somebody's life from an accidental overdose.