Nepal Altitude Sickness Guide 2026: Prevention, Symptoms & Treatment

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.

The air at Everest Base Camp holds roughly half the oxygen of sea level; at the top of a 5,000m pass, closer to 55%. Your body copes by breathing faster and, over days, making changes to your blood — but that adaptation takes time you cannot rush. AMS is what happens in the gap between how fast you climbed and how fast your body caught up. This is why fitness is no shield: a fit trekker who races to Namche in two days is at more risk than an unfit one who takes five. On the classic Nepal routes — Everest Base Camp, Annapurna Base Camp, Manaslu — the itinerary is your single most important piece of safety equipment.

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.

The three faces of altitude illness — and what each one means you do

Condition	Onset altitude	Key signs	What to do
Mild AMS	~2,500–3,000m+	Headache, nausea, poor appetite, broken sleep, tiredness	Stop ascending. Rest, hydrate, treat the headache. Only go higher once symptoms fully clear.
HACE (brain swelling)	Usually >3,500m	Loss of coordination, confusion, drowsiness, crushing headache, vomiting	Medical emergency. Descend immediately, even at night. Guide will arrange evacuation.
HAPE (fluid in lungs)	Usually >3,000m	Breathlessness at rest, wet/crackly cough, gurgling breath, blue lips, chest tightness	Medical emergency. Descend immediately. Both HACE and HAPE can kill within hours if ignored.

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.

What safe ascent looks like on the trail

The rules are easy to recite and easy to break the moment a good weather window tempts you to push on. Here is what disciplined ascent actually looks like on a typical high-altitude Nepal trek:

A sensible high-altitude ascent pattern (illustrative)

Sleeping altitude	Rule in play	What to do that day
Below 3,000m	Baseline	Walk normally; start hydrating and pacing habits early.
3,000–3,500m	First acclimatisation stop	Build a rest day here (e.g. Namche on EBC). Hike higher by day, sleep low.
3,500–4,500m	300–500m/day cap	Keep sleeping-altitude gains small. Add a second rest day around 4,000–4,500m.
Above 4,500m	Highest vigilance	Short days, constant symptom checks, be ready to turn back. This is where HACE/HAPE live.

The unprofitable truth about turning back

Here is the thing an honest guide has to say: we would far rather refund the last few days of your trek because you sensibly stopped at Dingboche than carry you down from Lobuche on a stretcher. Reaching Base Camp is not the measure of a successful trek — coming home healthy is. A good guide will call the descent before you want to hear it, and the right response is to trust that call, not to argue for one more day. We build our itineraries with the extra acclimatisation day already in, even though a shorter trip would be cheaper to sell, because the shortcut is where people get hurt.

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. Our guides carry a pulse oximeter and a first-aid kit, know where the nearest help is on each route, and — on the higher treks — can arrange a helicopter evacuation when descent on foot is not fast enough.

Who should think twice before going high

• Anyone with unmanaged heart or lung disease — get cleared by a doctor first; high altitude stresses both.
• Trekkers on a rigid, no-buffer schedule. If your flights leave no room to add an acclimatisation day or descend and retry, you have designed pressure into your trip — and pressure is what pushes people past sensible limits.
• People who "don't do slow." If waiting a day at Namche while you feel fine sounds unbearable, a high trek will test your patience before it tests your legs. Consider a lower trek like Langtang Valley (max 3,870m) or Poon Hill first.
• Sulfa-allergic trekkers planning to rely on Diamox — talk to your doctor about alternatives well before you fly.