I've been puzzled for a while as to why fentanyl has been killing so many people. Okay, gram for gram it's much more potent than other opiates -- something like a hundredfold. But drug dealers know this and dilute it accordingly. Junkies can be quite careless with their lives, but usually don't try to kill themselves intentionally. So what explains the tens of thousands of deaths?
From some sources I won't cite because I don't entirely trust them, they don't unambiguously say this, and it'd be a bother to look them up again, I think I have an explanation: it's in the mixing. Fentanyl is normally mixed with other substances to dilute it, and the difficulty of mixing solid particles into a perfectly even mixture is highly underappreciated. It's not just that mixing solid particles takes a while and that diffusion won't finish the job for you (as it does for liquids). It's also that particles of different sizes, shapes, and densities tend to separate themselves even as you try to mix them. Even among particles of the same density, larger particles like to move to the top of the mixture when it's vibrated. (Yes, counterintuitively, it's the larger particles that end up at the top.)
The pharmaceutical industry, of course, has machines and techniques for doing the job thoroughly. But you can't expect a back-alley drug dealer to adopt those. And the task is particularly hard with a drug that has to be diluted a hundred to one: it's easy to end up with a small pocket of material that's only diluted ten to one and thus is lethal.