LSD is dropped onto paper in solution though. So to control dose is easy since you can easy halve a dose by doubling the volume of solution. Dosing a powder/crystal is much more difficult, especially if you need to get it back out of solution.
LSD is a powder/crystal (a salt). People just don't consume opioids orally, usually. There's something similar though: skin patches, since (other than LSD) fentanyl can be absorbed through the skin.
Fentanyl can be dropped onto a paper. As others said LSD is a salt, something will also dissolve fenta.