It looks like Viagra might be effective against pulmonary edema (fluid leakage in the lungs caused by high blood pressure).

High Altitude Pulmonary Edema (HAPE) and High Altitude Cerebral Edema (HACE) are serious and potentially lethal risks for mountain climbers at elevations starting above 10,000'.

I've never had any problems with Acute Mountain Sickness (AMS), much less anything more serious like HAPE or HACE, as high as 13,000', but they say it can strike anyone anytime. (Kilimanjaro, my next mountain, is 19,340'--by far the tallest I've ever attempted.)

Now the question is this: Do you really want your mountain climbing buddy, sleeping in a two man tent, at high altitude, days away from the nearest female companionship, popping Viagra?