Don't trust over-the-counter drugs overseas. Be sure to carry basic OTC drugs like antacids, cough and cold medicine, motion sickness medicines and antihistamines.
Take more prescription medicine than you think you will need and keep it in your carry-on. You're much less likely to lose it that way.
Create a simple "medical information form" to carry in your wallet or with your passport. Include important information like emergency contacts, current known food and drug allergies.
Remember, Medicare will not cover you outside of the United States, Canada, or Mexico. Even in the United States, medicare will only pay to get you to the nearest hospital - no matter how inadequate that
hospital maybe. If you need to be evacuated to a more appropriate facility, it's at your expense.