Seoul has medical-care standards equal to those of other developed countries. You need a doctor’s prescription to buy most medications, and it may be difficult to find the exact medication you use at home, so take extra. A letter from your physician outlining your medical condition and a list of your medications (using generic names) could be useful.
Hospitals normally require cash upfront, which you should be able to claim back from your insurance company, if you have appropriate cover.
Almost all pharmacies stock at least some Western medicines. Pharmacists often know some English but it may help them if you write down your symptoms or the medicine you want on a piece of paper. If you have a language problem and a mobile phone, dial 1330, explain what you want in English, and ask the interpreter to explain in Korean to the pharmacist.