The majority of airlines do not require specific authorisation (check with your airline to confirm).
Airlines recommend flying only with medical authorisation.
Airlines recommend you should not fly. If you need to travel, you should consult your doctor and travel with obligatory authorisation.
In the majority of cases, you will need to sign the airline’s Waiver of Liability in relation to any problems that could potentially arise as a result of your condition during the flight.
Contact your airline to check the measures that apply in these situations.