I came across this website that has information on over 4000 prescriptions meds in both English and Japanese so I thought I'd share. It seems that even with the English version you can only search in Japanese but you can get information in both languages. They do have an alphabetical index in English.
English version: http://rad-ar.or.jp/siori/english/index.html
Japanese version: http://rad-ar.or.jp/siori/index.html