What is the best treatment for altitude illness?

Acetazolamide: First get off the mountain. Most climbers at higher altitudes carry a supply of acetazolamide which can cushion some of the symptoms. The most common are dizziness and disorientation, pulmonary problems and hemorrhages of the retina. These can be serious and might prevent further climbing.

Safe descent: The best treatment for altitude illness or altitude sickness is to decrease one's elevation by at least 1000 ft (quickly and safely.) if symptoms do not improve, one needs to descend another 1000 ft. If one has symptoms of hace (high altitude cerebral edema) or hape (high altitude pulmonary edema) contact search and rescue and arrange for emergent air evacuation.