There is less oxygen in the air at high altitude than at sea level. Many people who live at sea level travel to high altitude for recreational purposes. Some people become ill shortly after arriving at high altitude. Illness at high altitude is caused by the effects of low oxygen levels on the blood vessels, lungs, and brain. About 50% of people who ascend rapidly to high altitude develop acute mountain sickness (AMS), characterized by headache, nausea, fatigue, lightheadedness, and insomnia. Four percent of people develop a life-threatening illness, high-altitude pulmonary edema (HAPE), characterized by fluid accumulation in the lungs that blocks oxygen uptake, causing shortness of breath and even death if effective therapy is not provided. Doctors treat both AMS and HAPE with extra oxygen to breathe. People with AMS are also given a cortisone-like drug known as dexamethasone, and those with HAPE are given nifedipine. More recently, researchers have suggested using tadalafil (a medicine often used to treat erectile dysfunction). Beyond treating these illnesses, it would be useful to know which medications are most effective in preventing HAPE.