Altitude sickness: who needs acetazolamide and who just needs to slow down

Acute mountain sickness is the headache, nausea, poor sleep and exhaustion that hit above roughly 2,500 m when you climb faster than your body can adjust. In its severe forms — HACE and HAPE, swelling of the brain or fluid in the lungs — it kills, and the treatment is descent.

Whether you need tablets depends on your itinerary, not your fitness. Sleeping altitude and rate of ascent are what count. A trek that takes several days to reach 3,000 m with rest days is low risk and doesn’t need medication. Flying into Cusco or La Paz, climbing Kilimanjaro in under a week, or sleeping above 3,500 m in one day is moderate or high risk, and acetazolamide is recommended.

Acetazolamide works by making the blood slightly more acidic, which drives you to breathe more — the same thing acclimatisation does, just sooner. Our pharmacists can supply it privately for adults after assessing your route against the Wilderness Medical Society risk categories: 125 mg twice daily, started a day or two before ascent, continued for two days after your highest sleeping altitude, with a trial dose at home first.

Whatever you take: ascend gradually where the route allows, keep hydrated, avoid alcohol and sleeping tablets on the climb, and treat a worsening headache with confusion or breathlessness as an emergency — go down.