As far as the Japanese legal system is concerned, marijuana use is the
equivalent of shooting heroin.  Be wary.  Everyone here has a story of
someone they know or a friend of a friend getting jail time.  But like
another poster said, stuff like meth, pills and stuff seem to leak
through the cracks for some reason.  If you`re looking to get high, do
it in the comfort of your own house, with people you trust, and try
and get it off a Japanese friend.  NEVER get pot sent by mail, even
under an assumed name.  Narita customs are ruthless.