It's all in your head and has EVERYTHING to do with what character you have (strong/weak). Before my kid was born I used to be a weekly a regular drug user for ages (alcohol, fags, weed, shrooms, pills, lsd, coke, mescaline, you name it...) I mostly combined 4 things or so too when we went out, yet I never got addicted and when the time to give it all up was there (I don't think drugs and kids go together

), I just ended it, even threw my ciggies away, and that was that. Same goes for the missus, she was a heavy drinker (could outdrink any guy I knew), sleeping pills, anti depressants, smoked profusely, but when she got pregnant, she just quit all that shit in one week.
I still drink every now and then (once or twice a month), but that's it.
It was fun while it lasted, but I wouldn't recommend any of that shit to anyone, coz most folks simply can't cope with it and are hooked after a few months...and let's not kid ourselves, it's a nasty unhealthy habit...
Just my 2 cents