It's not easy to get medicine in Japan.
First, there's the language problem. A lot of medicines don't have English labels.
Next, drug stores in Japan are mostly-filled with Japanese medications. There are only a handful of well-known international medicine brand.
Lastly, over-the-counter-drugs in Japan have lower dosages when compared to other countries. You have to see a doctor and ask for a prescription. That's another difficulty.
I've been sick in Japan a couple of times. I tried going to the pharmacy and self-medicate but I still end up at the doctor's office asking for prescription. So, I suggest that when you're unwell and don't really know what Japanese medicine to take, do not self-medicate. Just go to the doctor's office. It will save you lots of time.
But there's one medicine that I need to take almost every month. It's Ibuprofen. Simple language: Pain killers. I usually suffer from menstrual cramps. I can't use that as an excuse for missing work so pain killers it is.