Altitude Sickness in Nepal: Recognising It and What to Do

Altitude is the reason most people who fail to finish a Nepal trek fail to finish it — not fitness, not weather, not the walking. It is also the risk most easily managed, because the mechanism is well understood and the treatment is free.

This is a practical summary, not medical advice. Talk to a travel doctor before you go, particularly if you take regular medication or have a heart or lung condition.

What is actually happening

At 5,000 m there is roughly half the oxygen in each breath that there is at sea level. Your body compensates — breathing faster, making more red blood cells, altering blood chemistry — but that takes days, and the adaptation cannot be rushed, trained for at home, or willed.

Altitude sickness is what happens when you gain height faster than the adaptation. It has nothing to do with being fit. Fit people are arguably at slightly more risk, because they can climb fast enough to outrun their own acclimatisation and strong enough to ignore the early warnings.

Acute mountain sickness: the common form

AMS usually appears 6 to 12 hours after arriving at a new altitude, and is common above 3,000 m. The defining symptom is a headache, plus at least one of:

• Nausea or loss of appetite
• Fatigue or weakness beyond what the day's walking explains
• Dizziness or light-headedness
• Difficulty sleeping

Mild AMS is not an emergency and is not unusual — a large proportion of trekkers get some of it. It is a signal to stop going up, not necessarily to go down.

What to do

Do not ascend further while you have symptoms. This is the single rule that matters. Spend another night at the same altitude, drink, eat, rest. Most mild AMS resolves in 24 to 48 hours.

If it does not resolve, or gets worse, descend. Five hundred to a thousand metres is usually enough to produce a noticeable improvement, often within hours.

The two forms that are emergencies

These are uncommon but they kill people, and both are recognisable without any equipment.

HACE — high altitude cerebral oedema

Fluid on the brain. The signs are neurological rather than painful:

• Ataxia — loss of coordination. The standard field test is to walk a straight line heel to toe. Someone with HACE cannot do it.
• Confusion, unusual behaviour, or drowsiness that is out of character
• A severe headache that does not respond to painkillers

Ataxia in someone at altitude is HACE until proven otherwise. Descend immediately, at night, in bad weather, whatever the conditions. Do not wait for morning.

HAPE — high altitude pulmonary oedema

Fluid in the lungs, and it can develop in someone with no headache at all.

• Breathlessness at rest — the key sign. Everybody is breathless walking uphill at 4,500 m; nobody should be breathless sitting still.
• A persistent cough, sometimes producing frothy or pink sputum
• Chest tightness, a very fast resting pulse, blue-tinged lips or nails

Same answer: descend immediately.

The golden rules

Rule	Why
Never ascend with symptoms of AMS	Ascending is what turns mild AMS into HACE
If symptoms worsen at rest, descend	Descent is the only treatment that always works
Never leave a symptomatic person alone	HACE impairs judgement before the person notices
Above 3,000 m, raise sleeping altitude 300–500 m a day	The standard safe rate
Take a rest day every 3–4 days or every 1,000 m	This is what acclimatisation days are for

Note that the rules concern sleeping altitude. Climbing high during the day and returning lower to sleep is not only allowed, it is the most effective thing you can do — which is why a good acclimatisation day involves a hard walk uphill rather than lying in a lodge.

On Diamox

Acetazolamide, sold as Diamox, is widely used by trekkers and speeds acclimatisation by altering blood chemistry. It is a prescription medicine and the conversation about whether to take it, and at what dose, belongs with your doctor before you travel — not with a guide, a forum or this page.

Two things worth knowing regardless. It is a sulfonamide, so it matters if you have a sulfa allergy. And it is not a shield: it reduces the chance of AMS, it does not make it safe to ascend faster, and it will not treat HACE or HAPE. Descent does that.

Guides and clinics in Nepal carry emergency drugs — dexamethasone and nifedipine — and portable hyperbaric chambers are held at some aid posts. These are for buying time while a descent or an evacuation is arranged. None of them replaces going down.

What good operators actually do

The things that reduce risk are unglamorous and mostly happen in the itinerary rather than on the mountain.

• Acclimatisation days that stay in the plan. They are the first thing cut to make a price competitive.
• A guide carrying a pulse oximeter, checking oxygen saturation morning and evening above 4,000 m. A single reading means little; a falling trend across days means a great deal.
• A turnaround culture. A guide who needs to argue with an office before ending someone's trek will hesitate, and hesitation is the dangerous part. Ours are instructed that turning a client around is a correct decision, never a failure.

The Himalayan Rescue Association runs aid posts at Pheriche in the Khumbu and Manang on the Annapurna Circuit, with free altitude talks most afternoons in season. If your itinerary passes one, go.

Insurance

A policy covering helicopter evacuation to the altitude you are actually going to is not optional above about 4,000 m. Read the exclusions rather than the summary — the common traps are an altitude cap well below your high point, and an exclusion for anything involving a rope.

Ground evacuation from the upper Khumbu or Dolpo is measured in days. That is the gap the policy exists to close.

Choosing a trek around altitude

If altitude worries you, the honest answer is not to avoid the mountains but to pick a profile that suits.

• Under 3,700 m, no real concern: the Annapurna foothill routes — Poon Hill, Muldai, Mohare Danda.
• Sleeps low, reaches high: Langtang, where you sleep under 3,900 m and touch 4,985 m as a day walk.
• Serious profile, well supported: Everest Base Camp, with two acclimatisation days built in.

Our ten things that matter covers the day-to-day habits, and each trip page lists its maximum altitude and acclimatisation days under Essential Information.