Altitude sickness is the great leveller of mountain travel: it does not care how fit you are, how many peaks you have climbed, or how well you handled altitude last time. Anyone can get it, and the only reliable cure is the same for everyone — go down. The good news is that almost all serious altitude illness is preventable with a sensible itinerary and the discipline to respect it. Here is what every trekker heading above about 2,500 metres should know.
What altitude sickness actually is
As you climb, the air pressure drops and each breath delivers less oxygen. Your body adapts — breathing faster and deeper, producing more red blood cells — but it needs time. Acute mountain sickness (AMS) is the mild, common form: headache, nausea, dizziness, fatigue and poor sleep, typically appearing 6–24 hours after a rapid gain in altitude. It is unpleasant but usually resolves with rest. The dangerous forms are high-altitude pulmonary oedema (HAPE), where fluid collects in the lungs, and high-altitude cerebral oedema (HACE), where the brain swells. Both can develop from AMS or, in the case of HAPE, sometimes without much warning — and both are medical emergencies.
The golden rules of acclimatization
Gradual ascent is the single most effective prevention, and it costs nothing. Above 2,500 metres, do not raise your sleeping altitude by more than about 300–500 metres per day. Build in a rest day (two nights at the same altitude) for roughly every 1,000 metres gained. The old mountaineer’s maxim “climb high, sleep low” works: it is fine to hike up to a viewpoint during the day as long as you descend to sleep. Avoid flying or driving straight to very high altitudes — if you must (La Paz, Lhasa and similar), rest for two or three days before any exertion. Stay well hydrated, go easy on alcohol (which dehydrates you and masks symptoms), eat enough, and never ascend while you have symptoms of AMS. Wait until they fully clear.
Recognizing AMS — and taking it seriously
The classic early signs are a headache plus one or more of: nausea or loss of appetite, dizziness, unusual fatigue, and broken sleep. Mild AMS is common and not an emergency — the correct response is to stop ascending, rest at the same altitude, hydrate, and treat the headache with whatever painkiller you normally use. Most cases settle within a day or two. What you must never do is push higher “to get it over with” while symptomatic. Communicate honestly with your companions and guides: hiding symptoms at altitude puts everyone at risk, and experienced guides would far rather lose a day than a client.
When it gets serious: HAPE and HACE
Learn the red flags. HAPE announces itself with breathlessness at rest, a persistent cough (sometimes producing frothy or blood-stained sputum), and extreme fatigue. HACE shows as worsening headache unrelieved by painkillers, loss of coordination (the classic test: can the person walk heel-to-toe in a straight line?), confusion, or unusual drowsiness. Either one demands immediate descent — at least 500–1,000 metres, as fast as safely possible — plus oxygen and evacuation if available. Do not wait for morning. Do not wait to see if it improves. Descent is the treatment, and delay is what turns these conditions fatal.
Diamox and other medication
Acetazolamide (sold as Diamox) is the most studied drug for altitude illness: it speeds up acclimatization by stimulating deeper breathing. Many travel-medicine sources cite 125 mg twice daily, started about 24 hours before ascent, for prevention — but this is prescription medication with real contraindications (notably sulfa allergy), and the right choice depends on your health history. Talk to your doctor or a travel clinic before the trip, and do a trial dose at home so you know how you react. Common side effects include tingling in the fingers, toes and lips, more frequent urination, and carbonated drinks tasting strangely flat. Understand its limits: Diamox assists acclimatization; it does not replace it, and it never justifies ignoring symptoms. Dexamethasone and nifedipine are emergency treatments for severe illness, not trekking first-aid items to self-prescribe.
Practical packing and planning
Build acclimatization into the itinerary before you book anything — a rushed schedule is the most common cause of AMS on commercial treks. Carry a simple first-aid kit with painkillers and any personal medication, plus oral rehydration salts. Make sure your travel insurance explicitly covers your maximum trekking altitude and helicopter evacuation; standard policies often exclude both. If you have had AMS before, you are more likely to get it again — plan more conservatively, not less. And remember the counterintuitive truth that surprises many fit hikers: excellent fitness helps you enjoy the walking, but it does not protect you from altitude sickness. Only a sensible ascent profile does that.
Arjun Mehta is the pen name behind Mountains Bliss — practical mountain-travel guides with routes, costs, seasons and safety notes, researched from real trekker discussions and cross-checked before publishing. No sponsored posts, ever.
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