The US FDA just approved over-the-counter sales of opioid overdose antidote Narcan. In the next few months, it will be available at pharmacies, convenience stores, big box stores, and hopefully many other places. If you don't want to pay for it, you can easily find it for free from local health departments, harm reduction organizations, and at some libraries, among other places. The New York Times posted a guide to using it correctly:
If used in time, Narcan, a version of the drug naloxone, which blocks the opioid's effect on the brain, can be a lifesaver for someone taking opioids, including oxycodone, heroin or fentanyl […]
How should I use it?
Gently tilt back the person's head. Insert the spray tip into one nostril until both fingers are against the nose. Push the plunger to release the full dose[…]
What do I do after I've given the spray and called 911?
Make sure the person's airways are protected and clear. Roll the person on their side, propping their hands under their head. Bend their knees to prevent them from rolling over on their stomach or back.