According to Wiki, carfentanil is ≈10,000 times more powerful than morphine. We're now talking 10s of micrograms for LD50 in humans. Whilst not as a potent a killer as botulinum toxin (a few nanograms), we're dealing with a substance that's essentially unmanageable.

As Wiki mentions, carfentanil is powerful enough to be classed as a chemical weapon. It's time this message was widely broadcast.