Will Narcan reverse opiate overdose once cardiac arrest has occurred?

That person was dead wrong.

Ok, first of all, allow me to explain Narcan...it's drug that blocks the bodys absorbtion of narcotics...simple as that. You could shoot anyone with Narcan, if they aren't on the drug, it won't do anything to them, if they are they will lose their high as the drug will no longer affect them.

Now, about your question: Narcan WILL cancel out what's left of the drug, HOWEVER if the heart has stopped, and the drug cannot circulate, it may not work as well...but CPR can fix that.
Here's the problem: MOST overdoses go into cardiac arrest not because the heart stops, but because the person stops breathing...and when you stop breathing everything else shuts down.

Here's the bottom line: If they haven't been down too long, they can be revived, but Narcan alone cannot save someone who is already in cardiac arrest. If they've been down too long there will be too much damage to reverse, even with Narcan and an effort to bring them back.

NO no no!!! Narcan is only to be used if the person under situations where a person has received morphine in a clinical setting and they are having a reaction to it. Not for overdose; they will need advanced cardiac life saving with a defibrillator and ACLS drugs.

The paramedic is right. IF the ambulance gets there soon enough after the person arrests, then there is a CHANCE of survival.

Narcan will knock out the opiate, but damage to the heart, brain, and kidneys may have already occurred.