Nepal Trek Altitude Sickness: Symptoms, Prevention, and Treatment

Altitude sickness is the single most common medical reason trekkers cut their Nepal trek short. It does not discriminate by fitness level, age, or prior trekking experience. Someone who completed the Everest Base Camp trek without problems two years ago can be hit hard on the same route the following season. Understanding what it is, how it develops, and how to respond to it is not optional preparation for high-altitude trekking in Nepal. It is fundamental.

This guide covers the three forms of altitude sickness that affect trekkers in Nepal, how to recognise each one, the evidence-based steps for prevention, and what to do when symptoms appear. It is written for trekkers planning routes above 3,000 m, which includes the Annapurna region, the Everest region, Langtang, Manaslu, Mardi Himal, and all other high-altitude routes in Nepal.

This guide is for general information only. Consult a travel medicine doctor or GP before your trek to discuss medication options and any personal health factors relevant to altitude.

Trekkers at Everest Base Camp (5,364 m), where altitude sickness risk is highest without proper acclimatisation.

What Causes Altitude Sickness

At altitude, the air contains the same percentage of oxygen as at sea level, but air pressure falls as elevation increases. Lower pressure means each breath delivers fewer oxygen molecules to your lungs. Your body responds by breathing faster and producing more red blood cells over time, a process called acclimatisation. The problem is that acclimatisation takes days, and when you ascend faster than your body can adapt, the result is altitude sickness.

There is no reliable way to predict who will be affected. Cardiovascular fitness improves performance on the trail but does not protect against altitude sickness. The primary risk factors are ascent rate and individual physiology, not fitness or age.

In Nepal, altitude sickness becomes a meaningful concern above 2,500-3,000 m and increases in likelihood and severity above 3,500 m. Treks that gain altitude rapidly, particularly those that include flights directly to high-altitude starting points such as Lukla (2,840 m) or Jomsom (2,720 m), carry elevated risk compared to routes that allow gradual acclimatisation from lower elevations.

The Three Types of Altitude Sickness

Altitude sickness in Nepal falls into three distinct conditions with very different levels of severity. The table below gives an at-a-glance comparison; full descriptions follow.

Acute Mountain Sickness (AMS)

AMS is the most common and least severe form. It typically develops within 6-12 hours of arriving at a new altitude. The defining symptom is a headache. Additional symptoms include fatigue, loss of appetite, nausea, dizziness, and difficulty sleeping. Feeling generally unwell at altitude, in the absence of another explanation, is AMS until proven otherwise.

AMS is not dangerous on its own, but it is a warning that your body has not yet adjusted to the current elevation. The danger is continuing to ascend while symptomatic. AMS that is ignored and pushed through can develop into the two life-threatening forms described below.

The golden rule with AMS: never ascend with symptoms. Rest at your current altitude until symptoms resolve. If symptoms are worsening after 24 hours at the same elevation, descend.

High-Altitude Cerebral Edema (HACE)

HACE is a severe progression of AMS in which fluid accumulates around the brain. It is less common than AMS but is a medical emergency. Symptoms include a severe headache that does not respond to standard pain relief, loss of coordination (ataxia), confusion, altered consciousness, and extreme fatigue. A useful field test is asking the person to walk heel-to-toe in a straight line. Inability to do so is a strong indicator of HACE.

HACE requires immediate descent of at least 500-1,000 m. Waiting to see if it improves at the current altitude is not an acceptable response. If descent is not immediately possible, oxygen and Dexamethasone (a prescription steroid) can be used as a bridge. A Gamow bag (portable hyperbaric chamber) can replicate descent if one is available. HACE that is not treated promptly can progress to coma and death within hours.

High-Altitude Pulmonary Edema (HAPE)

HAPE occurs when fluid accumulates in the lungs. It is less common than AMS but more quickly fatal than HACE, and is the leading cause of altitude-related death among trekkers. Symptoms include shortness of breath at rest (not just on exertion), a persistent dry or wet cough, gurgling sounds when breathing, chest tightness, and pink or frothy sputum in severe cases. A resting oxygen saturation below 75% on a pulse oximeter is a warning sign.

HAPE often develops at night and can progress rapidly. Like HACE, the treatment is immediate descent. Nifedipine (a prescription calcium channel blocker) and oxygen can be used while preparing to descend. Any trekker showing signs of HAPE should be evacuated to a lower altitude hospital as quickly as possible.

Both HACE and HAPE can occur without AMS having been recognised first. They can also develop simultaneously.

Prevention

Follow the Acclimatisation Rule

Above 3,000 m, limit your net altitude gain to 300-500 m per day, and include a rest day for every 1,000 m of ascent. Rest days are not about physical recovery from walking. They are about giving your body time to produce more red blood cells and adjust to lower oxygen pressure. Use rest days to take short acclimatisation hikes above the sleeping altitude and return to sleep lower, a practice described as "climb high, sleep low."

Most well-designed Nepal trekking itineraries include built-in acclimatisation days. For the Everest Base Camp trek, standard rest days at Namche Bazaar (3,440 m) and Dingboche (4,410 m) are non-negotiable, not optional extras. Itineraries that skip these rest days to save time carry significantly elevated risk.

Nepal Trekking Guide Team Adventure plans every itinerary with proper acclimatisation as a priority. Acclimatisation days are built into every high-altitude route - EBC, Manaslu, Tsum Valley, Annapurna Circuit - because the team's guides have seen what happens when trekkers rush the ascent. The goal is reaching the highest point of your route safely, not quickly. This careful scheduling is one of the most important factors in keeping altitude sickness rates low among the team's trekkers.

Stay Hydrated

Dehydration worsens altitude sickness symptoms. Drink 3-4 litres of water per day on the trail. Avoid alcohol in the first few days at a new altitude, as it impairs the hypoxic ventilatory response and worsens dehydration.

Recognise Symptoms Early

The sooner you recognise AMS, the more options you have. Monitor how you feel at each new sleeping altitude. A mild headache after a long day of climbing is common; a headache that persists through the next morning, combined with nausea and fatigue, is AMS.

A pulse oximeter is a useful tool at altitude. While oxygen saturation readings vary significantly between individuals and are not a standalone diagnostic, a reading that falls sharply compared to your baseline or drops below 85-90% at altitude, combined with symptoms, warrants taking seriously.

Acetazolamide (Diamox)

Acetazolamide (sold as Diamox) is a prescription medication that speeds up acclimatisation by stimulating breathing. It is the most commonly used pharmaceutical prevention for AMS. It does not mask symptoms; it works by accelerating the physiological adjustment process.

• Common side effects: Increased urination, tingling in hands and feet, altered taste of carbonated drinks
• Who should not take it: People with sulfa allergies (it is sulfa-based)
• When to start: 1-2 days before reaching altitude; continue for at least 2 days after reaching your highest sleeping altitude
• Important: Discuss with a doctor before your trek - it interacts with certain other medications

Some trekkers choose to take it preventively throughout the ascent. Others carry it as an emergency option. Neither approach is wrong, but neither replaces proper acclimatisation.

Recognising Severity: The Lake Louise Scoring System

The Lake Louise Scoring System is the standard field tool for assessing AMS severity. Score each symptom from 0 (none) to 3 (severe):

Interpreting the total:

• 3-5: Mild AMS. Rest at current altitude, do not ascend.
• 6 or above: Severe AMS. Descend and seek medical attention.

The scoring is less important than the principle behind it: if symptoms are present and worsening, descent is required. When in doubt, treat it as serious. The consequences of incorrectly treating mild AMS as severe are minimal. The consequences of the reverse are potentially fatal.

Which Nepal Treks Carry Highest Altitude Risk

Everest Base Camp: The most popular high-altitude trek in Nepal. Extended time above 4,000 m and large numbers of trekkers who push their pace make it the most common setting for serious altitude illness. Properly run EBC itineraries of 12-14 days with acclimatisation days have a strong safety record, but 8-10 day "fast track" versions are where problems cluster.

A high-altitude pass crossing on the Annapurna Circuit: big altitude gains in a single day demand careful acclimatisation.

Annapurna Circuit via Thorong La Pass: The Thorong La crossing requires a very early morning push to altitude from Thorong Phedi (4,450 m) with a long descent to Muktinath (3,760 m) on the same day. Trekkers who arrive at Thorong Phedi already symptomatic from Manang (3,519 m) should not attempt the pass.

Manaslu Circuit: A restricted area trek with fewer medical post options along the route. Rescue times are longer. A conservative acclimatisation schedule and carrying emergency medication are more important here than on more trafficked routes.

Trekkers at Larke Pass on the Manaslu Circuit, a restricted-area route with longer rescue times.

When to Descend

Descend immediately if any of the following apply:

• Symptoms of HACE or HAPE are present
• AMS symptoms are worsening after 24 hours at the same elevation
• A trekking companion is showing signs of confusion, loss of coordination, or extreme breathlessness at rest
• Oxygen saturation consistently below 80% on a pulse oximeter, alongside symptoms

Do not wait until morning. Do not wait to see if sleep helps. Descent works faster than any drug available to trekkers and is the definitive treatment for all three forms of altitude sickness. Even a descent of 300-500 m can produce noticeable improvement within hours.

Helicopter evacuation is available from most points on major Nepal trekking routes and is covered by standard travel insurance with helicopter rescue clauses. Ensure your insurance explicitly covers altitude-related medical evacuation before you trek.

Frequently Asked Questions

Can I trek in Nepal if I have had altitude sickness before?

A previous episode of altitude sickness, including AMS, HACE, or HAPE, is a risk factor for future episodes. It does not disqualify you from trekking, but it means you should discuss your history with a travel medicine doctor before the trip, consider Acetazolamide, and plan a conservative itinerary with extra acclimatisation days. Being familiar with your own symptoms is also an advantage.

Does fitness level protect against altitude sickness?

No. Cardiovascular fitness determines how well you perform on the trail, not whether you will develop altitude sickness. High-altitude mountaineers and elite runners have been severely affected. Fitness is no substitute for acclimatisation.

How do I know if my headache is altitude sickness or dehydration?

Dehydration headaches typically improve with fluid intake within 1-2 hours and are often positional (worse when standing). AMS headaches persist despite hydration and rest, often worsen through the night, and are accompanied by other symptoms. When in doubt, drink water, rest, and monitor. If the headache persists into the next morning, treat it as AMS.

Should I carry a pulse oximeter on my trek?

A pulse oximeter is a useful supplementary tool and costs little to add to your kit. It provides data to track trends rather than make absolute diagnoses. A single low reading without symptoms is not necessarily cause for alarm; a consistently falling trend alongside worsening symptoms should be taken seriously.

What is the difference between altitude sickness and regular tiredness from trekking?

Normal trekking fatigue improves after rest and a good night's sleep. Altitude sickness fatigue does not resolve with rest and is accompanied by one or more of: headache, nausea, dizziness, or shortness of breath at rest. If you wake up feeling worse than when you went to sleep at a new altitude, that is altitude sickness, not normal tiredness.