It's a bit funny how this whole thing is handwaved as "oh, but it's only illegal because the king decreed it, so it's fine". My man, all illegal substances are illegal because the government decreed it, and you're in a monarchy. The drug impacting birth rates in a medieval society's a pretty big deal, and it's not like the gray/black market sales of it going on right now are just going towards medical-esque uses like the MC dreams about with the old widower example.