Japanese police have extraordinary powers. They can detain a suspect for up to three days without charging them; after this time a prosecutor can decide to extend this period for another 20 days. Police can also choose whether to allow a suspect to phone their embassy or lawyer, though if you find yourself in police custody you should insist that you will not cooperate in any way until allowed to make such a call. Your embassy is the first place you should call if given the chance.
Police will speak almost no English; insist that a tsūyakusha (interpreter) be summoned (police are legally bound to provide one before proceeding with any questioning). Even if you do speak Japanese, it's best to deny it and stay with your native language.
Note that it is a legal requirement to have your passport (or, if you are staying longer than 90 days, your resident card) on you at all times. Though checks are not common, if you are stopped by police and caught without it, you could be hauled off to a police station to wait until someone fetches it for you.
Japan takes a hard-line approach to narcotics possession, with long sentences and fines even for first-time offenders.