That's the magic of drugs...you just never know!
It only takes once....sorry for the cleshea but it's true, there's probably been people who hadn't used drugs since high-school, but then during their mid-life crisis, they might remember the good ol' days and how they used to sniff speedballs off a stripper's behind, they accidentally find a good connect, and BAM they're addicted worse than they ever were as a kid.
It's just life, man, you can schedule it, jot it all down in your journal as much as you want, but at the end of the day a narcotic-use leger won't keep anybody from doing what they want to...(except the extensive one they keep on the gotdamn pharmacy computer!! Damn em'!)
I'm an incredibly MODERATE drinker (like 1 beer/night once or twice a month), why? Because I'm not crazy about booze. However, when my opiate supply dries up(knock on wood, God forbid) It's probably quite likely I'll drink 5 nights a week...who knows.
The only real way to truly PREVENT addiction is abstinence, (although personally I find it MUCh easier to abstain when I keep a fair amount of good quality narcotics on hand...a nice big variety)