Hi - so it depends on two things really - first is how much you’re planning to bring, and second is if any are restricted. You’re allowed to bring in up to a month’s worth of prescription medicine without any paperwork, (and two month’s of over the counter stuff). If you have more than a month’s worth of the former,you need a Yakkan Shomei, which is a form you fill in along with a stamped letter from your doctor showing it is prescribed to you, with the specific amount listed on it. This is sent off and then if approved, they send you back the paperwork to have with you on your entry to Japan. Depending on which country you’re coming from you can find guidance online like here for America: http://www.mhlw.go.jp/english/policy/health-medical/pharmaceuticals/dl/qa1.pdf
Even if you have less than a month’s worth, it’s advisable to have a letter from your doctor listing your prescriptions and what they are for so you can prove they are for personal use.
Medicine regulations are pretty weirdly strict around certain areas like stimulants (vicks inhalers can be banned, for example). So try to find your country’s up to date guidelines, but some details are included in the link above.
This all depends on them actually checking you though, so it may all amount to nothing! I had to do it in the past for stuff like contact lenses, but have never actually been checked - it’s luck of the draw, or you can voluntarily declare it at customs.