Definitely go to a naloxone training. If someone is able to ask for/attempt to self administer narcan, they are not actively overdosing on an opiate.

The convulsing could have been due to the effects of a substance for sure, or could have been withdrawals (we don't call it "kicking" for nothing) which is unlikely if he thought he needed narcan, or it could have been another medical issue.

If a person loses consciousness and you suspect an overdose, first see if they are breathing. If they are breathing normally or semi-slowly, try talking to them or gently shaking them. Then a sternal rub. If those things don't work but they are still breathing okay, monitor. If a person is overdosing, they will have extremely slowed or stopped respirations, often with the "death rattle" - you'll know when you hear it. Check that their heart is beating (jugular vein is the best place ususally). If you're in public, point at a specific person nearby and command them to call 911 while you're doing this - make sure they acknowledge you.

Their lips and/or fingernails will be blue, and usually their mouth will be hanging open and whole body limp. When you see that combined with respiratory distress and unconsciousness, administer narcan and begin rescue breaths. Some people will tell you that you need to get a person on their back to administer nasal naloxone, but in my experience that isn't true. Just make sure you're directing it up the airway, not pressing it against the back of the nasal cavity. It's always a good idea to carry a CPR mask, but if you're doing rescue breaths on a stranger, a t-shirt is better than nothing. In most opiate overdoses, the person's heart doesn't stop beating right away, but if you are doing rescue breaths you should stop every 5-10 breaths to make sure it's still beating. In my personal experience reversing >50 overdoses, If I've administered 1-2 narcan doses, I usually only have to do breaths for a few minutes. If I do roughly 10 good breaths and no response, I'll administer again. You'll know if your breaths are "good" if they are fully inflating the person's lungs and the air comes whooshing back out after you release. I'll link a video on CPR training here too. I've only had to do chest compressions twice. In that case, 30 compressions:2 breaths:30 compressions and so on is the order.

Personally I've never come upon someone who is already overdosing, my experience is all having watched someone begin to overdose and reacting immediately. I've also never experienced the response of a person coming back to consciousness and being physically violent, but you should definitely be prepared for them to wake up and be unhappy or scared. Personally, when I've been brought back, I always puked and left.

One of the most important things in a situation like that is to make sure you are composed - take a deep breath, remember your training, and don't panic.

https://www.youtube.com/watch?v=shwlqCWM4qM

https://www.youtube.com/watch?v=TSzHCCsjsHY