The short version
Altitude sickness (AMS) affects many trekkers above 3,000m. Complete guide to prevention, recognising symptoms, the Diamox debate, and the one rule that saves lives: descend.
- Altitude sickness (AMS) can affect anyone above ~3,000m regardless of fitness — the young and fit are often more at risk because they ascend too fast.
- The golden rules: climb high, sleep low, gain no more than 300–500m sleeping altitude per day above 3,000m, and take acclimatisation days.
- Know the danger signs (loss of coordination, breathlessness at rest, confusion) — and if severe, descend immediately.
- Diamox can aid acclimatisation; discuss it with a doctor before your trip.
What altitude sickness actually is
Acute Mountain Sickness (AMS) occurs when you ascend faster than your body can adapt to lower oxygen. It can affect anyone regardless of age or fitness, and commonly begins above 3,000m.
Recognising the symptoms
Mild AMS: headache, nausea, loss of appetite, fatigue, disturbed sleep — like a hangover. Stop ascending; rest until it resolves.
Severe — HACE (brain): loss of coordination (ataxia), confusion, severe headache. Test by walking heel-to-toe in a straight line.
Severe — HAPE (lungs): breathlessness at rest, a wet cough, gurgling breath, blue lips.
If symptoms are severe or worsening, descend immediately — even at night. A few hundred metres of descent can reverse symptoms dramatically. No summit, view or schedule is worth your life.
The golden rules of prevention
- Climb high, sleep low. Above 3,000m, increase sleeping altitude by no more than 300–500m per day.
- Take acclimatisation days. Every ~1,000m of gain, take a rest day with an active "climb high, sleep low" hike.
- Hydrate: 3–4 litres a day. Avoid alcohol and sleeping pills.
- Go slow. "Bistari, bistari" — slowly, slowly — is every good Nepali guide's mantra.
Acetazolamide (Diamox) helps the body acclimatise and can prevent and reduce AMS. Many trekkers take a preventative dose; some keep it for treatment. Discuss it with a doctor — it's a sulfa drug and not for everyone. It's an aid, not a substitute for sensible ascent.
Trek with a trained guide
A licensed guide recognises AMS symptoms early and makes the descent call — one of the most important safety reasons to trek guided. Pair that with proper acclimatisation built into your itinerary, and altitude becomes a manageable challenge. See our EBC guide for how the rest days work in practice.
At what altitude does altitude sickness start?
AMS commonly begins above 3,000m, though it can occur lower. The risk increases the higher and faster you ascend. Above 4,000m, careful acclimatisation becomes essential regardless of fitness.
Does being fit prevent altitude sickness?
No — fitness does not prevent AMS, and fit people are often more at risk because they ascend too quickly. Slow, staged ascent and acclimatisation days are what protect you, not fitness.
Written by
Travel Himalaya NepalPokhara-based, NMA-certified trekking guides. We’ve led 5,000+ treks across the Annapurna and Everest regions since 1998 — every word here comes from the trail. More from this author →
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