To use your pilot or air traffic control licence you must be certified as medically fit.
Different countries have different systems but usually the medical examination (& subsequent certification) something like:
Class 1: For professional pilot licences &/or commercial operations
Class 2: For private pilot licences & operations
Class 3: For air traffic controllers or similar occupations
In the UK an INITIAL Class 1 examination must be done with a CAA medical examiner at their HQ at Gatwick Airport.
You will undergo (I think) a Chest X-ray, Optometrist examination, general medical stuff eg blood sample, urine sample, auscultation etc etc. Under the new Joint Aviation Regulations there's probably heaps more.
It takes about a day of your time & costs a small fortune.
Renewal of a Class 1 medical can be done by a CAA approved Dr., wherever they have hung their shingle.