If I take 1 grain of fentanyl powder, and sell it diluted in an entire liter of water, it might sound like homeopathy,

The standard dilution is 1:10⁶⁰. Wikipedia helpfully did the maths:

Dilution advocated by Hahnemann for most purposes: on average, this would require giving two billion doses per second to six billion people for 4 billion years to deliver a single molecule of the original material to any patient.