I don’t think it is possible to "scare" them into submission on the drugs.  I think it could even backfire.

Elaine has the right idea about sharing with friends/roommates what to do in an emergency.  If nothing else, get them to make sure their closest friends know that if something appears to be wrong, it’s okay to call 911.  Best would be knowing how to jab that needle into their thigh but at minimum, don’t be afraid to call for help.

I think people get intimidated by that needle and the process of giving an injection in case of emergency but ultimately, it really is just a matter of shoving the needle into a muscle and pushing that plunger down.  Of course, it does help wipe with an alcohol pad but in all honesty, if it were me and there was not a pad handy, I’d rather risk the infection--it’s not like you are going into a vein or anything.  If necessary, they could even practice on an orange to get the feel of it.

I think much of it also has to do with having respectful doctors.  That is, if the doc they see there treats them like adults rather than overgrown children, they will be better off and be willing to entrust their care to him or her.