Japan doesn't have a blanket rule for all drugs. Some drugs are more easily accessible in Japan than in the West, and vice-versa.
For example, Japan usually doesn't care about depressant drugs because they're not drugs of crime and are linked to few deaths (except alcohol). That's why you can order Tramadol or Codeine freely and get it delivered to your door legally without prescription. Benzos are also prescribed very freely for most psychological issues. If you're not stupid enough to illegally obtain restricted drugs like morphin, then you can be a user with peace of mind.
For psychedelics and cannabinoids, while the main substances are illegal, because there is no analog law in Japan, there are variants of LSD or THC that are sold and pretty much no one care because they're not linked to crime or death. You can buy THCM vapes legally on Rakuten (Amazon competitor). As long as you don't grow or own pure weed plants, you're fine legally speaking.
Now, when it comes to stimulants, Japan will fuck you up if you get anywhere close to them. Because of the
history that Japan has with meth (they invented it, legalized it, commercialized it, it became part of the culture, and it didn't end well), the laws specific for "stimulating substances" are so old and harsh that you'll get up to decades in prison for being positive to meth when you pee. There isn't much you can do in Japan legally in terms of uppers except from coffee. This is to the big chagrin of big pharma that would love to sell amphetamine pills for "ADHD" to 100 millions people that are in a very competitive and performance-related society. But sadly for them ADHD isn't real and the successful CBT therapies here show that perfectly well, so the government has no push from the people to legalize any sort of amphetamine, even for medical use.
In short, if you know the laws and categorization of drugs (which Hector is strangely aware of), you have a playing field if you like substance use. The mistake is going to Japan without knowing anything, having some adderall in your bagage, or thinking you can buy some coke in the back alley of a karaoke in Shinjuku without getting in a fuckton of trouble.