Japanese drug prosecution is about as strict as American child porn prosecution, with the same para-judicial stigma about possession of it. If you are caught with drugs in Japan, you are going to be doing hard time, and even when you get out of jail people will not want to associate with you. In the rare case where the police prosecute you without evidence, you're still going to wind up spending months in a holding cell waiting for exoneration, and have to explain to everyone in the meantime that you were falsely accused of drug possession.
(The inverse is also true: Japan is rather lax about CSAM possession, about to the same degree as America is lax about weed possession. It's technically a crime but most people caught will be let off with a fine and a warning.)
