Altitude Sickness in Nepal: The Complete Guide

Overview

Important: this guide is general information to help you plan and stay safe, not medical advice. Altitude illness can be life-threatening. Always consult a doctor or travel clinic before a high-altitude trek, and in the mountains follow the advice of a qualified guide and descend and seek medical help if symptoms are serious. When in doubt, go down.

Altitude sickness, more precisely Acute Mountain Sickness (AMS) and its severe forms, is the body's reaction to the thin air of high mountains. As you climb, air pressure falls and each breath delivers less oxygen; given time the body adapts, a process called acclimatisation, but climb too fast and it cannot keep up. On Nepal's classic high routes, this, not cold, distance or difficulty, is what most often ends a trek early or, rarely, turns dangerous.

The reassuring news is that altitude sickness is largely preventable and manageable. Tens of thousands of trekkers reach places like Everest Base Camp (5,364 m) and the Thorong La pass (5,416 m) safely every year by ascending slowly, recognising the warning signs, and acting on them. This guide explains what altitude sickness is, how to spot AMS, HAPE and HACE, the golden rules that prevent it, the role of Diamox, what to do if it strikes, and which Nepal treks carry the most risk.

How Altitude Affects the Body

At sea level the air is about 21% oxygen, and that does not change with height, but the air pressure does. The higher you go, the lower the pressure, so the oxygen molecules spread further apart and you take in significantly less oxygen with every breath. At 5,000 m you are breathing roughly half the oxygen available at sea level.

Your body responds by breathing faster and deeper and, over days, by making more red blood cells and other changes that help it cope, this is acclimatisation. Given enough time it works remarkably well. The problem comes when you gain height faster than your body can adjust, leaving your tissues starved of oxygen and triggering altitude illness.

Altitude illness generally becomes a real concern above about 2,500–3,000 m, and the risk rises the higher and faster you go. It is important to understand that fitness offers no protection: young, strong, marathon-fit trekkers get altitude sickness just as often as anyone else, and sometimes push on dangerously because they feel they "should" be able to. Susceptibility varies from person to person and is only loosely predictable, though a history of altitude illness raises your risk on future trips.

Symptoms of AMS (Mild Altitude Sickness)

Acute Mountain Sickness is the mild, common form, and often feels like a bad hangover. It usually appears within a few hours to a day of arriving at a new, higher altitude. The cardinal symptom is a headache, combined with one or more of the following:

• Headache (the key warning sign)
• Nausea or loss of appetite
• Fatigue or unusual weakness
• Dizziness or light-headedness
• Difficulty sleeping

Mild AMS is common and, handled correctly, not dangerous. The essential rule is simple: do not go higher until the symptoms have gone. Rest at the same altitude, hydrate, take simple painkillers such as paracetamol or ibuprofen for the headache, and give your body time. Most mild AMS settles within a day or two of rest at the same height.

The danger is not mild AMS itself but ignoring it and climbing on. AMS that worsens instead of improving, or is accompanied by the warning signs below, can progress to the two severe, life-threatening forms, HAPE and HACE.

HAPE & HACE — the Dangerous Forms

The two severe forms of altitude illness are medical emergencies. Both can develop from untreated AMS, sometimes quickly, and both require immediate descent and medical help. Learn to recognise them, in yourself and your trekking companions.

HAPE — High Altitude Pulmonary Edema (fluid in the lungs)

In HAPE, fluid leaks into the lungs, so the body cannot take in oxygen. Warning signs include:

• Breathlessness even at rest, or far more than the altitude should cause
• A persistent cough, sometimes producing pink or frothy spit
• Chest tightness or congestion, and a rattling sound when breathing
• Extreme fatigue and blue-tinged lips or fingernails

HACE — High Altitude Cerebral Edema (fluid on the brain)

In HACE, fluid causes the brain to swell. It is rarer but extremely dangerous, and can be fatal within hours. Warning signs include:

• Severe headache that painkillers do not touch
• Loss of coordination (ataxia) — the classic test is being unable to walk heel-to-toe in a straight line
• Confusion, strange behaviour, drowsiness or difficulty staying awake
• Hallucinations, and in the worst cases, unconsciousness

If you or anyone in your group shows signs of HAPE or HACE, treat it as an emergency: descend immediately, do not wait for morning, and get to medical care and, where available, supplemental oxygen. A descent of even a few hundred metres can be life-saving. These conditions kill trekkers on Nepal's high routes almost every year, and the victims are very often the ones who felt "too fit" to stop, or too committed to their schedule to turn back.

The Golden Rules of Prevention

Almost all altitude illness is preventable by giving your body time to adapt. These are the widely accepted golden rules for trekking safely at altitude:

• Ascend slowly. Once above about 3,000 m, do not increase your sleeping altitude by more than roughly 300–500 m per day, even if you feel great. It is where you sleep, not where you walk, that matters most.
• Climb high, sleep low. On acclimatisation days, hike up to a higher point during the day, then come back down to sleep. This exposes your body to thin air while letting it recover overnight.
• Take rest days. Build in acclimatisation days, roughly one for every 1,000 m of ascent above 3,000 m (for example, the standard rest days at Namche Bazaar and Dingboche on the Everest trek).
• Never ascend with symptoms. If you have AMS, stay put until it clears. If it gets worse, go down.
• Stay hydrated and eat well. Drink plenty of water and keep eating carbohydrates, even when your appetite fades; do not overhydrate to excess.
• Avoid alcohol and sleeping pills. Both suppress your breathing, exactly what you do not want when your body is short of oxygen.
• Don't trek alone at altitude. A companion or guide can spot changes, like confusion or unsteadiness, that you may not notice in yourself.

Above all, treat your itinerary as flexible. Weather, illness and acclimatisation can all force changes, so build in buffer days and be willing to slow down or turn back. The mountain does not care about your flight home.

Diamox & Medication

Speak to a doctor before using any medication for altitude, including the drugs below. This is general information, not a prescription.

Diamox (acetazolamide) is the medication most commonly used to help prevent and treat mild AMS. It works by speeding up acclimatisation rather than masking symptoms. Many trekkers on high routes take it prophylactically, typically a low dose starting a day before serious altitude gain, but the decision, dose and timing should be made with a doctor. People with a sulfa (sulfonamide) allergy should not take it, and common, harmless side effects include tingling in the fingers and toes, more frequent urination, and a flat taste to fizzy drinks. Diamox is a helper, not a licence to ascend faster, the golden rules still apply.

Two further drugs are used in emergencies to buy time while descending, and are usually carried and administered by guides, expedition medics or doctors: dexamethasone for HACE (and sometimes severe AMS), and nifedipine for HAPE. These are prescription medicines with real risks and are not a substitute for descent, they help you get down, they do not let you stay up. Do not self-medicate with them without training.

For a simple headache, ordinary paracetamol or ibuprofen is fine. Carry a small personal first-aid kit, and if you are on an organised trek, ask your operator what altitude medication and oxygen the guide carries.

What to Do If It Strikes

Recognising altitude sickness is only useful if you act on it. The response is straightforward and the same principle runs through all of it, go down:

• Mild AMS: stop ascending. Rest at the same altitude, hydrate, take a simple painkiller, and wait for symptoms to clear before going any higher. Do not sleep higher that night.
• Symptoms not improving, or getting worse: descend to a lower altitude, even a few hundred metres and one night lower often makes a dramatic difference.
• Any sign of HAPE or HACE (breathlessness at rest, cough with frothy spit, loss of coordination, confusion): this is an emergency. Descend immediately, day or night, with a companion, and get to medical care and oxygen. Arrange evacuation if needed.

The definitive treatment for serious altitude illness is descent. Supplemental oxygen and a portable pressure bag (a Gamow bag), where available at aid posts or with expeditions, can stabilise a casualty, but they are a bridge to getting down, not a reason to stay up. In the Everest and Annapurna regions, seasonal aid posts such as the Himalayan Rescue Association clinics at Manang and Pheriche can assess and advise, but do not count on them being open or nearby, prevention and early descent are what keep you safe.

Nepal Treks & Altitude Risk

Altitude sickness matters on any Nepal trek that climbs above roughly 3,000–3,500 m, which includes most of the famous high routes. The risk is driven by how high you sleep and how fast you get there, not by the trek's overall difficulty. Some of the routes where acclimatisation is most important:

• Everest Base Camp — sleeping up to around 5,140 m at Gorak Shep, with viewpoints at Kala Patthar (5,545 m); built-in rest days at Namche and Dingboche are essential.
• Annapurna Circuit — crosses the Thorong La pass at 5,416 m, one of the highest-risk points on any teahouse trek; the acclimatisation stop at Manang matters hugely.
• Manaslu Circuit — over the Larkya La pass (around 5,100 m) in a remote setting.
• Gokyo Lakes and the Everest Three Passes — high lakes and passes above 5,300 m.
• Higher base-camp and pass treks such as Kanchenjunga, Makalu and Tilicho Lake.

Lower and shorter treks carry much less risk. The Ghorepani Poon Hill trek tops out around 3,210 m, and gentler routes stay lower still, though even these deserve respect if you ascend quickly. If you are new to altitude, starting with a lower trek before attempting a 5,000 m route is a sensible way to learn how your body responds. Every trek guide on BriefNepal lists its maximum altitude and a safety note, check it when you plan.

How to Prepare

• See a doctor or travel clinic before a high trek, especially if you have heart, lung or blood-pressure conditions or have had altitude illness before, and discuss whether Diamox is appropriate for you.
• Plan a realistic itinerary with proper acclimatisation days and buffer days, and resist operators or schedules that ascend too fast to save time.
• Buy travel insurance that explicitly covers high-altitude trekking and helicopter evacuation up to the maximum altitude of your route. This is non-negotiable, a rescue flight can cost thousands of dollars, and standard policies often exclude altitude.
• Trek with a knowledgeable guide. Since 2023, a licensed guide is required in many of Nepal's national parks and conservation areas; a good guide monitors the group for early symptoms and manages the pace and the descent decision.
• Train for endurance, not to "beat" altitude. Fitness makes long trekking days more comfortable but does not prevent AMS, so never let being fit tempt you to skip acclimatisation.
• Know the plan for going down. Agree with your guide, before you need it, what you will do and where you will descend to if someone gets sick.