I hate forbidding guests from flying because of the ticket-change fee. The era when a doctor’s note impressed the airline is long gone. It still works for travel insurers, a good reason to buy a policy.
My problem arises most often with ear pain.
Cabin pressure at cruising altitude drops only about 25 percent from sea level, but that’s significant. If you bring a bag of potato chips you’ll notice that it swells like a balloon. Air in any closed space does the same. If you have gas, you’ll have more gas. If air in your middle ear can’t escape, the ear will feel stuffy and then painful. If pressure increases still more, it may blow a hole in the eardrum. This relieves the pain, and most small perforations heal in a few weeks, but we don’t like to encourage them.
The best preventative is a chemical nasal spray (Afrin, Dristan). As you sit in the plane before takeoff, spray generously, wait five minutes for it to work, and spray again. This should send the spray far up to reach the eustachian tube opening, the only connection between your middle ear and the outside world. Do the same before the plane begins its descent, an hour before landing.
Flying doesn’t cause ear infections, so if you felt fine before boarding, it’s OK to wait if your ear hurts after landing. Pain should improve after a few days. If you see a doctor, he’ll forbid you to fly.