Like me, you might have brought medicines with you from home when you came to Japan, but eventually those run out and you will need to get more somehow. Importing medicines and drugs into Japan should not be taken lightly. Some items are perfectly acceptable. But while items containing codeine are OK in Canada and Australia, if you try to bring or ship any into Japan you will rapidly find yourself in trouble. Similarly, cold medicines containing pseudo-ephedrine, which is used in the manufacture of methamphetamine, will get you into hot water at the airport. There are of course rules for bringing prescribed medicines with you when you come and limitations on over the counter medicines as well.
For prescription medicines you can generally bring them into Japan with you without any special procedure; provided that:
Disclaimer: when traveling to Japan with prescription and non-prescription medications you should consult the Japanese Embassy, or a Japanese Consulate, in your area directly for advice. You do not want to end up in jail or deported for cold medicine. For more information read this article on Importing medicines and drugs into Japan.
Additionally, having medicines sent to you from within Japan is much more risky than importing them on your person. If you are caught with a few illegal codeine pills at the airport customs MAY simply take them from you, but having them mailed to you is a police matter, and you are much more likely to end up in court over the matter. Be careful, and research before you order!
If you are already here, and are looking for a quick fix for a headache or other general pain you can find some common ibuprofen or aspirin based products over the counter at any drug store in Japan; unlike some countries, over the counter drugs are general not sold in supermarkets or convenience stores.
Ibuprofen brands will generally have about the same dosage as western brands, but it is important to check the label to be sure as one dose may be 1 or 2 pills (to equal a foreign dosage). You can check this by looking for these kanji, 用法 or 用量, which means dosage, and錠, which means number of pills. For example: 用法 2錠 would mean the recommended dosage is 2 pills. While with Ibuprofen the dosages are similar, this does not hold true with all brands, or other medicines. You will find ibuprofen type products in the pain relief section of the drug store. Look for this: 痛み止め (i ta mi do me).
When in doubt, ask for it!
Ask an employee where to find it using this sentence;
If you are looking for pain relievers and can wait for them, you can buy some western brands using Amazon.com, and at Costco, at wildly inflated prices. Simply go to www.Amazon.co.jp and search in English for Tylenol, Advil, Bayer, etc, and you may find it right there waiting for you!