Paulh / and the Madame gave you good answers ...

Japan has a zero tolerance when it comes to recreational drugs . It happens ,but it's not common .

For you , you would be arrested & jailed . They can hold you for two weeks without even charging you .

When they do , you would go to court , be convicted , and deported . ( and banned for at least 5 years )

If it's more serious , prison time . All they will allow you in your cell is two books . No TV / Radio / or anything to play with . No pretty pictures on the walls . And you'll sleep on the floor on a thin futon .

If people see your drinking cough syrup in public they'd probably make a face and walk away .

Are you on crack ? No one goes to Japan to get F'd up . It's a cool country to visit !