> even safe dosages of the substance they're using? Can anyone, even experienced and properly educated users, truly know the safe dosages when the potency (and

> even safe dosages of the substance they're using? Can anyone, even experienced and properly educated users, truly know the safe dosages when the potency (and potentially even the substance itself -- see, e.g., the fentanyl analogues used in heroin) of the drug in question are unknown?