Yea like a lot of other people have said, this really is not a story made to be some 1-D anti drug PSA. The drugs are a vehicle to explore a deteriorating relationship and a woman who needed help before the drugs, as shown by the cutting scars on her arms. And her girlfriend who is not equipped to handle any of that so follows her into madness. Exploring is a stretch bc it's only 30 pages but i digress.
Ending in a tragedy which again, this is a one shot, ofc none of the characters are reaally developed and many things are implied. Just taking "drugs bad" from this story is a very blunt take that i feel ignores the rest of the story. Especially with how generic and vague the drugs are spoken about. To me the real tragedy is how mental illness irl is not taken seriously or lacks accessible avenues for people to pursue help and instead Tsukushi went and bought random drugs made with human horrors for help. Bringing down her gf and ultimately each other.
I also would not be surprised if JP publishers were extremely strict about how drugs are portrayed so the author had to make many concessions. JP government wanting to snipe someone on site for holding weed or smth. Like an actor had to quit acting for being accused of drugs while never doing it bc it's that harsh. As another commenter pointed out, it is darkly logical when we look back at history at how drugs in East Asia were used as an avenue of colonization from the West which the West has done inside itself as well against minorities.