Will narcan (naloxone) still reverse opiate overdose once cardiac arrest has occurred?

Yes: If the Narcan (naloxone) can reach the circulation, it will work. CPR can provide a reduced blood flow that is adequate to reverse the opiate. Injecting the Narcan (naloxone) into the central circulation (jugular or subclavian vein) will further improve its efficacy.

Unclear: Usually breathing stops with opiate overdose and then cardiac arrest occurs. Reversing with Narcan (naloxone) may allow breathing to restart, but the cardiac arrest would still have to be treated with CPR and usual measures. Depending on the duration of cardiac arrest attempts may or may not be successful, thus the uncertainty.