For users of particular substances it's a coin toss if what they decide to take is:
* what they think it is
* as potent as they think it is
* more well-traveled than they are, passing through N hands
Point being: they can know exactly down to the microgram how much they're taking, it changes nothing. Every day may be the day.
Trying to not be rude, I feel you're attributing far too much rationality to this. Their situation has no lack of uncertainty.
It's the furthest thing from their mind. They've deliberately and repeatedly signed up for much worse odds/stakes.
Rationality long left. Something is compelling, exceeding, these hypothetical deterrents. These people aren't choosing in their rational/best interest.