1.53 × 10¹⁵ molecules per 1mcg. Which is 15 orders of magnitude larger than a single molecule. If 2mg of fentanyl is considered potentially lethal that would place carfentanyl at 20mcg, though I’ve heard conflicting reports on potency with some placing it at 10x not 100x. So roughly 16-17 orders of magnitudes.

I don’t take fentanyl but I regularly take signaling peptides where the doses are typically in the mcg range and still rather potent - though with signaling peptides the response curve typically flattens out so bigger doses don’t necessarily mean stronger effects - they can be saturated. Depends on what you’re signaling though.

I had correctly predicted my hypersensitivity to semaglutide (ozempic) so started using it for treating autoimmune issues at a dose of 25mcg and I still ended up with temporary gastroparesis.