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@Lemming
Narcan is only essential if the substance abuse is opiate addiction (you mentioned his teeth, if it's meth, then Narcan isn't effective), it blocks the opiod receptors and will bring a person out of an overdose crisis almost immediately. Get the nasal spray as it's easiest to use and in a life or death emergency, you don't need to try to follow complex directions. There are a lot of ways to get it free, at the drug store it's maybe $30, in the event of an overdose it saves lives. One thing to know, if you use Narcan, you must get them to a hospital afterward. The half life of a typical overdose is 4-5 hours, the half life of Narcan dose is about 45 minutes, you will need to get them to the hospital before the dose wears off. When an addict comes back they never want to go to the hospital, that's not an option, make them go.
The other thing is there are a lot of support resources available, use them, and learn to recognize the triggers and warning signs of a relapse, there will be a change in behavior a day or two before the relapse, if you see it you can help stop it.
If he's not going to meetings, see of you can get him to, the most important thing for recovery is a support network, family is huge but the addict knows you don't really understand, at the meetings he'll get the support of other addicts that he can relate to.