I think it depends on the individual...and the substance. I quit smoking cigarettes 25+ years ago and still get cravings. I would love to casually enjoy a smoke from time to time, but I know I would get right back on the smoking train and be hooked again. I also quit drinking caffeine a few years ago, but on occasion I'll have a Coke or a latte and it's not a problem for me. Depending on the substance, I think if you were previously addicted you have to weigh what the consequences of you trying that substance again could be. In most cases, I'm guessing the risks aren't worth it.