Who it affects, which is not who people expect
Acute mountain sickness is caused by thin air, not by poor conditioning. Marathon runners get it; their sedentary companions sometimes do not. The factors that matter are how high you go, how fast you get there, how high you sleep, and your own individual susceptibility - which you only discover by going.
It starts as a headache with nausea, poor sleep, loss of appetite and a general flatness, usually six to twelve hours after arriving at altitude. At that stage it is unpleasant and manageable. Its two severe forms - fluid on the lungs and swelling of the brain - are life-threatening and can develop within hours.
Where this comes up
The Everest and Annapurna trekking routes in Nepal; Cusco and the Inca Trail in Peru, where the city itself is already at altitude; Kilimanjaro, which is climbed fast enough that a substantial proportion of trekkers are affected; La Paz, Quito and Bogotá, where you arrive by plane with no acclimatisation at all; and ski resorts in the higher Alps and Rockies.
Flying straight into a high city is the pattern that catches people out, because there is no gradual ascent to notice. If your itinerary begins at three thousand metres, plan the first forty-eight hours as rest rather than sightseeing.
Ascending slowly is the actual treatment
No medicine replaces a sensible ascent profile. Above three thousand metres, increasing your sleeping altitude by no more than three to five hundred metres a day, with a rest day every thousand metres, is what prevents this - climb high, sleep low.
And one rule above all the others: if symptoms appear, do not go higher. Descending is the only reliable treatment and it works quickly. People get into serious trouble by pushing on because the summit is close and they have paid for the trip.
- Above 3,000m, raise your sleeping altitude by no more than 300 to 500m a day
- Take a rest day for every 1,000m gained
- Climb high, sleep low
- Do not ascend further with symptoms - wait, or go down
- Descend for severe symptoms. Immediately, at night, in bad weather, whatever it costs
- Avoid alcohol and sleeping tablets while acclimatising
Where tablets fit in
Acetazolamide speeds acclimatisation and reduces the chance of acute mountain sickness. It is worth considering where the ascent profile cannot be made gentle - a fixed trekking itinerary, a flight into a high city, a previous bad experience at altitude.
It has real side effects: tingling in the fingers and toes, needing to pass urine far more often, and fizzy drinks tasting strange. It contains a sulfonamide group, so it matters if you have reacted to sulfa drugs. Our pharmacists assess whether it suits you and, where it does, advise taking a trial dose at home rather than discovering the side effects on a mountain.
Nothing in a packet makes it safe to ignore symptoms. The tablets reduce the odds; descent is what treats the illness.