Ambulance. Call one immediately if they are struggling to breathe or are unconscious.
While you wait
Check their responsiveness. Shake them gently and shout their name. If they don't respond, the muscle between the shoulder and the neck (trapeze) firmly.
Stay with them. If they're awake, keep them calm through reassurance and by encouraging deep breaths.
Breathing. If they don't respond to any of the above, they are most likely unconscious → watch their breathing.
Airways. If they're unconscious, make sure nothing is obstructing their breathing.
Recovery position. If they're unconscious but breathing, flip them on the side so they don't choke if they vomit.
CPR. If they're unconscious and not breathing, it's time to start basic life support → 30 chest compressions (to the beat of "Staying alive") then two rescue breaths (make sure to pinch the nose).
Once help has arrived
Cooperate with medics. You are guilty of nothing, they simply want to help.
What drug and how much. If you know this, it's crucial to tell the medical team so that they can give the correct care and potentially save their life!
Hand it over. If you don't know what the person has been using, consider collecting the pills or powder and give it to the medical team. The more they know the better.