[Just another Joe]

I guess some folks have a higher tolerance than others for 'addiction'.

[D-fendr]

I think so too, most definitely. It depends on the person and the substance.
Some folks seem to be able to manage withdrawals, others go schizoid.
And that's just the physical part.

From what I've seen, crack cocaine is the toughest because after the physical craving/withdrawal is over, the brain takes some time before its chemistry can recover. The former addict is very depressed for many months or a year or more.