You have made it to 4,500 metres with only a few hundred metres before Everest Base Camp. Suddenly you feel a severe headache and nausea. Like countless other mountaineers and trekkers, you are experiencing acute mountain sickness (AMS).

AMS is a common affliction for people who climb above an altitude of 2500m. Although it is best to prevent AMS by gradual ascent without using any drugs, this may not always be an option in many settings. Rescuers may need to go up rapidly to high altitudes or owing to a lack of camp site, it may not be possible for trekkers to spend the night at an optimal altitude.

Furthermore, airports in places like Lhasa, Tibet (3,490m) and La Paz, Bolivia (4,058m) may cause travellers to arrive at high altitude without the opportunity to acclimatise en route. Many trekkers specially pilgrims, often disregard advice about gradual ascent in their single-minded determination to reach the sacred site.

In addition, sudden military deployment at high altitudes, such as the Hindu Kush Mountains in Afghanistan, may necessitate drug prophylaxis for the prevention of AMS. Finally there is a fast-growing population of climbers in pursuit of records who are incorrectly being advised by physicians to use prophylactic medicine to both improve performance and summit successfully.

Up-to-date knowledge about the most commonly used drug Diamox (acetazolamide) for the prevention of AMS is helpful. Recently there has been a significant change in its dose recommendation. In a publication in the British Medical Journal in 2000, Dumont and colleagues had arbitrarily shown that only 750 mg/day of diamox (acetazolamide) would prevent AMS.

Recent review articles, which now include studies post 2000 (many of which were conducted by us in the Himalayas), have clearly shown that a much lower dose (250 mg/day) of diamox is adequate for prevention. Other drugs including ginko biloba and ibuprofen have been tested in the prevention of AMS. But diamox continues to be the superior drug due to its proven efficacy over the years in a large number of trials with an acceptable side-effect profile.

Another important use of diamox in the mountains is in the prevention of periodic breathing (literally, trying to 'catch your breath' while sleeping) at high altitudes. This is a very common problem which sometimes triggers anxiety attacks. Diamox decreases the hypoxemic (low oxygen) spells during sleep and successfully treats this problem in most instances.

Climbers ascending high altitude need to be encouraged to go up gradually without the use of drugs, including diamox to enhance acclimatisation. However, in certain instances, chemoprophylaxis may be useful. In those cases, low dose (250 mg/day) diamox is the drug of choice.