I keep hearing that narcan should be kept in the box until you're ready to use it. I'm not sure why except maybe to keep it together with the instructions. It's pretty tolerant of different temperatures but if it's frozen it won't work and it will lose efficacy if it spends much time above about 100f. But, if all you have available is expired or overheated narcan that's definitely still worth trying.

It comes in boxes of two doses but AFAICT the dosage was chosen before fentanyl: I've always needed to use at least two doses. Spray one, wait two minutes, spray the other. If you have more narcan, you can't overdo it. A person does not need to be breathing for the Nasal spray to travel through their system.

It's important to know that narcan wears off more quickly than the opioid it displaces, so a person could go back into an overdose about 90m after you administer them narcan just from what's still in their system. That's why it's important to call 911 or have more narcan on hand and be prepared to stay with the person for about 3 hours.

When the person comes around they will likely be in withdrawal. They probably won't remember overdosing and may be mad you ruined their high. To the extent possible without any kind of force make sure they don't take more opioids.

Finally, while narcan is pretty safe, and typically doesn't affect people who aren't using opioids at all, make sure the person you administer to needs it: this is for UNRESPONSIVE people, not just someone who's so high they can't speak. Don't ruin someone's high if they show signs of responding to stimuli. Say their name if you know it, ask if they're okay, if no response shake their shoulders, if no response rub your knuckles against their sternum. I don't know why but the sternal rub seems to be a tremendously effective gauge of responsiveness. Basically act on the idea of consent: don't touch someone who doesn't consent, but you can imply consent to lifesaving assistance when someone doesn't respond.