I think @the_doc is right. Fentanyl. I wonder if also that would make some pre-fentanyl abusers think before they use it?
Good point. The lethality certainly should give narcotics addicts pause before knowingly using it for a high. (Unfortunately, a lot of weaker narcotics are laced with fentanyl. A narcotics-naïve user--for example, someone with only a little experience with Oxycontin--will die from fentanyl-laced Oxy.
Caveat emptor. [I think that's Latin for "know your pusher and be sure you never make him mad."])
As I recall, the lethal dose for fentanyl is frightfully close to the effective dose for achieving significant pain control from it. By the same token, the lethal dose is probably just above the euphoria-inducing dose for addicts. I suspect that fentanyl is one of the main causes of the 200 or so narcotics overdose deaths we have every day in the USA.
(As an aside, it occurs to me that some folks might have discarded the fentanyl idea for executions because death is likely not instantaneous, even in absolutely lethal doses. [Then again, so what if that's true?])