[Edited out] (read something wrong)
Back in HS I kinda binged on heroin for about 4-5 months, had a friend who was severely addicted and I wanted to try everything. Never injected or smoked it, just sniffed, but I also had no problems with withdrawal or relapse. I feel I just didn't care about it enough, wasn't my thing. Same with any downer or pain killer. Took a bunch of OC's, Vic's, Xanax, Liquid Codeine(sometimes with tylenol and others with promethazine[R.I.P. DJ Screw and Pimp C]) a bunch of times, Liquid Hydrocodone a few times, some perc's. Most of that was done in high amounts and close together. Not as much as I did the heroin though(pretty much every day from after school till I went to sleep). I just never really had a problem with downers, but at the same time I keep cocaine away from me most of the time because I can easily slip into a downward spiral. So when I do it, I get a one nights amount and wont let myself see it again for at least a month. No longer friends with the dealers, now just friends of friends. Used to be the same with ecstasy when I was 18-19 but I have chilled on that more, still do it just no longer fiend. Love the other psychs but I've always tried to respect them so I haven't had much of a problem, except for a few occasions. Alcohol is also probably my most addictive substance. Love weed but can chill out if I want to lower my tolerance. Have actually started doing that now as I'm low on money and I smoke too much when I have it.
I'm no expert in addiction, not even close, but I know people who are addicted to everything. Some who will do everything heavily and not get addicted at all, others that will do everything and only get addicted to a few certain things (me, thank God I know this). I do know some people are more susceptible to certain chemicals, or certain feelings. I get addicted to the feelings. That's why I haven't had much of a problem with withdrawal, no chemical dependence. Except with alcohol, I only like alcohol in smaller amounts but when I was younger I couldn't keep myself under what I wanted and would get horribly drunk. Now I either just buy less at a store, or bring less money to a bar. But when a bottle is being passed around, I have to focus on being strong.
Kind of went on a rant there, sorry. But my main point is, everyone has there vices, everyone has there own different strengths, always be careful because you may not know you're addicted until you're in very deep. If you do something a lot for a week or two, then chill out for the next week or two, or month and see how much you fiend for it. Even if it's not a lot keep it in moderation. Also, I realize I got lucky that I had no, or very little, chemical dependence on heroin. Others generally are not that lucky