aren't healthy, sure, but if you bought a packet of fags, and found they were 10% tobacco
and 90% horseshit
, you'd have the usual consumer rights
to sort it out.
The same goes for illegal drugs, like, say, heroin. Since heroin supply is underground, an addict can't stop his supplier from cutting the dose with Ajax.
He might one day get a purer supply than usual, and give himself a massive overdose, or he may inject himself with some other poison.
In response to imago's followup writeup: In the case of someone dying of lung cancer, the medical professionals caring for him can be fully aware of what is wrong with this person, and what steps can be taken to cure them, or at least make them more comfortable.
In the case of a dying drug taker, all they know is that he ingested a white powder (say), and they must waste time identifying the substance before treatment can begin.