From what I recall, it mostly boils down to two ingredients: pseudoephedrine and codeine. While both are psychoactive substances, they commonly appear in OTC drugs in many countries: the first one is used in nasal decongestants, while the other most often shows up in cough medicines. In Japan, however, they're treated as narcotics, so make sure you aren't bringing any of the two. That being said, if you're a typical traveler, it's unlikely you'll even have your bags checked.