You must self-certify that you’re medically fit to fly but must not fly if you have any known medical conditions that you know would disqualify you. Epilepsy, fainting spells, type 1 diabetes, sky-high blood pressure, etc. would be among them. But what if you doubt whether you could pass an FAA physical but are still legal to drive? For example, what if your blood pressure is slightly over the limits (but not ‘walking heart-attack’ high)? Or if you doubt your vision is good enough to pass an FAA physical (but still good enough to legally drive)? Essentially you’re relatively healthy with no major medical issues, but you doubt if you could pass a physical.
