It isn't easy to start having faith in people especially if they've hurt or betrayed you in the past. You might forgive them but you may find it very hard to trust them again. I suppose though if you really want to have faith in them you'd need to embrace the golden rule. Do to unto others what you expect them to do unto you. Whether we use drugs or not, we all make mistakes. You'd expect someone you've wronged to forgive you and probably trust you if you apologize and try your best to earn their trust once again. Why don't you do the same thing for them?