I am currently on assignment near the Heroin capital of the US (Wierdly a small place called Chimayo NM) The cops carry Narcan here and can administer it IM, as they usually arrive first on the scene, in cases where IV access is difficult, it, along with supporting respiratory function works very very well.
IV access is of course more appropriate, but IM narcan can and does work. In fact they are now making Drug rescue kits for addicts that includes some sort of inhaled Narcan.
The only benefit of IM narcan I could see in a Cardiac Arrest patient would be if the arrest were due to OD, and there was no way to gain venous access.
We always prefer to give it IV, but there are some cases where the addict has no veins, and has been injecting themselves actually in the neck so even the neck is all messed up.
I've only usually given it IM once, immediately and then started the IV. It may seem like a lot, but lately the standard narcan dose has not been reversing our patients, they are requiring somewhat more.
And yes,I am looking forward to my next assignment, somewhere else.