> similar in several important ways (e.g. the legal framework around them
You are mistaking noise for signal here.
> the fact that they should both be avoided for similar reasons
In no sense is this a "fact." Many people suggest LSD use. Many people regard themselves as better for having used LSD. I don't think LSD is something to be avoided at all - it's utterly benign and very, very interesting.
> They're certainly more similar to each other than they are to antibiotics or chemotherapy drugs, both of which are also labeled with the term "drug".
In which ways? LSD is dissimilar to just about anything else on the planet, and has little in common, as far as I can tell, with opioids, antibiotics, or chemotherapy drugs.
And again, let me bring up refined sugar. Sugar is a refined white powder, made by solvent extraction form plant matter, and is consumed solely for its chemical effects on the nervous system. It seems far closer to heroin than either of them are to LSD or MDMA or psilocybin. Yet these three, along with heroin, are "drugs?"
I can't imagine a consistent taxonomy which casts such a wide net on behalf of the term "drug" as to include both heroin and LSD but exclude sugar.