yes, narcon is used to counteract heroin overdoses. it can be fairly easily obtained in some places (like needle exchanges) so that you may keep it with you when you are shooting up, but it is what you would get if you had to go to the hospital for on OD. the only dangers with narcon that im aware of is once you take it you will feel sober and want to do more drugs. mistake. don't do it. i think every junkie should have narcon on hand for emergencies. of course i never did, but it might have saved me a trip to the ER if i had... ah, well, live and learn.
-peace