Altitude Sickness on Nepal Treks: Prevent, Spot, Respond

Altitude sickness is the most common serious health risk on Nepal's high treks, and it is largely preventable. The rule that matters most is simple: once you sleep above 3,000 m, raise your sleeping altitude slowly, take rest days, and never go higher while you have symptoms. If symptoms get worse, go down.

Every medical statement here was checked against at least two primary sources: the Wilderness Medical Society's clinical practice guidelines, the CDC Yellow Book 2026, the NHS, the UK's TravelHealthPro and the UIAA Medical Commission. Nepal facts come from the Himalayan Rescue Association and Nepali news reports. The altitude table uses the day-by-day overnight heights in our own trek itineraries.

This is general information, not medical advice. See a travel-medicine doctor before a high trek.

What altitude sickness is

Higher up, each breath carries less oxygen, and your body takes days to adjust. Climb faster than that and you risk altitude illness, which comes in three forms.

Acute mountain sickness (AMS)

AMS is the common, milder form. The main sign is a headache, along with at least one of: poor appetite or nausea, tiredness or weakness, and dizziness. It usually starts within the first day at a new altitude, often within a few hours. It is rare below about 2,500 m.

The UIAA and TravelHealthPro both advise treating any illness at altitude as altitude illness until proven otherwise.

Mild AMS usually settles within a day or two if you stop climbing. Dropping 300 to 1,000 m usually brings quick relief.

High-altitude cerebral oedema (HACE)

HACE is swelling of the brain and a medical emergency. Doctors regard it as AMS at its most severe. Watch for confusion, odd behaviour, unusual drowsiness and loss of balance. A simple field check is to ask the person to walk heel-to-toe along a straight line; stumbling is a warning sign. Untreated, it can lead to coma.

High-altitude pulmonary oedema (HAPE)

HAPE is fluid in the lungs, and it is also an emergency. Early signs are breathlessness out of proportion to the effort, a sharp drop in walking pace, and a cough. Later comes breathlessness at rest, chest tightness and frothy or blood-stained spit. Descent is the core treatment.

The Lake Louise score, in plain words

Researchers measure AMS with the Lake Louise score. You rate four symptoms from 0 (none) to 3 (severe): headache, stomach upset, tiredness, and dizziness. Sleep problems were dropped from the score in 2018, because poor sleep at altitude is common even without AMS.

It helps you see whether you are getting better or worse, but it was built for research. Both the UIAA and the score's own authors treat AMS as a judgement made from the whole picture, not a number. A headache plus any of the other symptoms after a gain in altitude is reason to stop and rest.

How fast to go up

The Wilderness Medical Society's guidance, which the CDC, UIAA, NHS and TravelHealthPro all reflect, is:

1. Avoid going straight from low ground to sleep at 2,750 m or higher on your first night. The NHS and TravelHealthPro also say not to go from below 1,200 m to above 3,500 m in one day.
2. Once above 3,000 m, do not sleep more than 500 m higher than the night before.
3. Take a rest day at the same sleeping altitude every three or four days. The CDC puts it as one extra night for every 1,000 m of sleeping-altitude gain.

What counts is where you sleep, not the highest point you reach during the day. Some Nepal-based advice is more cautious still: the Himalayan Rescue Association suggests 300 to 400 m a night above 3,000 m.

Most guides use a rest day for a short hike a few hundred metres higher before coming back down to sleep. That is the idea behind "climb high, sleep low". Your oxygen level drops most during sleep, so the CDC notes that a day higher with a night lower is much easier on the body. The UIAA adds that it only helps if you don't wear yourself out.

The golden rules

TravelHealthPro and the CDC set these out almost word for word, and the UIAA agrees:

• Never go up to sleep higher with symptoms. Stay at the same altitude until they have gone.
• Go down if symptoms get worse, or if they are severe, even at night. Signs of HACE or HAPE mean descend now.
• Never leave someone with altitude illness alone. If they have to go down, someone goes with them.

Medicines: talk to a doctor before you go

Acetazolamide (sold as Diamox) is the preferred drug for preventing altitude sickness. The CDC and the Wilderness Medical Society recommend considering it for people at moderate or high risk. That includes people with a past history of altitude illness and anyone whose itinerary has to gain height faster than the guideline pace. It works by speeding up acclimatisation.

The dose both bodies give for prevention in adults is 125 mg every 12 hours, starting the day before you ascend. That is for recognition only, not a recommendation: acetazolamide needs a prescription, and a doctor has to check it suits you. Both TravelHealthPro and the UIAA stress that it is an addition to a slow ascent, not a replacement for one.

Myths about water, beer and fitness

• Water. Staying hydrated matters, because dehydration causes headache and tiredness that look like AMS. But forcing down extra water does not prevent altitude sickness. Drinking far too much can lower your blood sodium, which is dangerous in its own right.
• Alcohol. Alcohol slows your breathing, and faster breathing is how your body acclimatises. The CDC advises avoiding it for your first 48 hours at altitude; the NHS says to skip it while climbing. Sleeping pills that suppress breathing, and opiates, carry the same risk.
• Fitness. Being fit makes the walking easier, but the CDC and the UIAA agree it does not protect you from altitude sickness. A past history of altitude illness says far more about your risk than your fitness does.

What a pulse oximeter can and can't tell you

A pulse oximeter clips on a finger and estimates your blood oxygen. The CDC suggests one can help you follow your own acclimatisation over several days.

It has limits. The CDC notes that someone with AMS often reads normal for the altitude, or only slightly low, so a reassuring number does not rule AMS out. Readings also vary widely between healthy people at the same altitude, which is why altitude doctors warn against any single cut-off. Trust symptoms over the screen: a headache and nausea with a "good" reading are still AMS.

Our treks: the days that need care

The table uses the overnight altitudes in our own sample itineraries. It lists every night where sleeping altitude rises by more than 500 m and ends above 3,000 m, including days that cross the 3,000 m line. These are not reasons to avoid a trek. They are the days to walk slowly, ask about an extra night and watch for symptoms. Heights are rounded; published figures for some villages differ by 50 to 150 m.

Every Everest trek has one big day into Namche, which is why the rest day there is standard. Annapurna Base Camp gains about 1,800 m of sleeping altitude in two days with no rest day. Its top is lower than Everest, but the pace is the steepest in the table. Gosainkunda climbs about 1,400 m on its first walking day.

For the bigger picture by region, see Everest, Annapurna, Manaslu and Langtang. Our month-by-month guide shows when each route is at its best.

Rescue, insurance and aid posts

Helicopter evacuation and insurance

On most high routes, a serious case of HACE or HAPE means getting down fast, and the fastest way is often a helicopter. Rescue flights in Nepal are expensive.

So your travel insurance must cover emergency helicopter evacuation up to the highest altitude on your trek. The UIAA and TravelHealthPro both say this: disclose your maximum altitude to the insurer, and make sure search, rescue and helicopter evacuation are included. The UIAA warns that some policies exclude search and rescue altogether, so check the wording. For Everest Base Camp or the Annapurna Circuit, that means cover above 5,400 m.

One more thing. In January 2026 Nepal Police's Central Investigation Bureau arrested rescue-company operators over fake helicopter evacuations billed to foreign insurers. In March 2026 prosecutors charged 32 people. Reports describe trekkers with mild symptoms being pushed into flights they didn't need. A genuine emergency always justifies a helicopter. Mild AMS usually needs rest or a walk down, not a flight. If someone urges an evacuation, ask what has changed, and call your insurer's assistance line if you can.

Himalayan Rescue Association aid posts

The Himalayan Rescue Association (HRA) is a Nepali non-profit that runs two seasonal high-altitude aid posts staffed by volunteer doctors:

• Pheriche, on the Everest Base Camp route below Lobuche. Our EBC itinerary spends a night there on the way down.
• Manang, on the Annapurna Circuit, where our itinerary's acclimatisation day falls.

Both posts treat patients and give talks on altitude illness, which are well worth an hour of a rest day. They open for the two trekking seasons. In 2026 the HRA sent teams to Pheriche, Manang and its Everest ER clinic at base camp in late March for spring. Autumn teams for Pheriche and Manang deployed in early October, offering 24-hour emergency care until the end of November. The base camp clinic serves the spring climbing season. Outside those windows, don't count on a doctor being there.

How a good guide manages altitude

• Pace. A good guide sets a slow, even pace from day one, especially on the big days in the table, and keeps the group together.
• Rest days. They keep the acclimatisation days in the plan, use them for a short climb-high, sleep-low walk, and will add a night when someone is struggling.
• Watching people. They ask each person how they slept, how their head feels and whether they're eating, every morning and evening. Porters and guides can get altitude sickness too, and a good guide watches the team as well.
• Turnaround decisions. They agree a rule before the trek: no one goes higher with symptoms, and anyone who gets worse goes down with a companion.

The UIAA points out that many commercial itineraries move faster than the guidelines. Before you book, ask any guide or agency how their schedule compares with the 500 m rule.

Finding a guide

Every guide on xTrekka has had their trekking licence and citizenship document checked by our team before their profile goes live. Browse guides by region and language, read reviews from trekkers they have led, then write to the guide directly. Ask how they plan acclimatisation days on your route, what they do on the big days in the table above, and when they turn back. You agree the route, dates and price with them, and xTrekka takes no cut of a trek you arrange directly with a guide.

If you're still deciding whether you need a guide at all, read do you need a guide to trek in Nepal. For permits, see our Nepal trekking permits guide.

Sources

• Wilderness Medical Society: Clinical practice guidelines for acute altitude illness, 2024 update (Luks et al., Wilderness & Environmental Medicine), PubMed record; appraisal and summary of the 2024 guidelines (PMC); American Family Physician summary of the 2019 guidelines
• CDC Yellow Book 2026: High-altitude travel and altitude illness (Hackett and Shlim)
• NHS: Altitude sickness
• TravelHealthPro (NaTHNaC): Altitude illness
• UIAA Medical Commission: Consensus statement no. 2, field management of AMS, HAPE and HACE; Standard no. 7, checking the quality of an organised trek; Altitude sickness factsheet
• Lake Louise score and oximetry: Roach et al., The 2018 Lake Louise Acute Mountain Sickness Score (PubMed); Basnyat, pulse oximetry and AMS, High Altitude Medicine & Biology (PMC)
• Himalayan Rescue Association: altitude advice; Pheriche aid post; Manang aid post; spring 2025 season report
• HRA 2026 deployments: B360 Nepal, spring 2026 teams; The Rising Nepal, autumn 2026 teams; The Annapurna Express, autumn 2026 teams
• Fake-rescue investigation: The Kathmandu Post, Nepal charges 32 in fake rescue scam (March 2026); The Himalayan Times, CIB arrests six rescue operators (January 2026)
• Trek altitudes: xTrekka trek itineraries (overnight altitudes per day)

Last checked 20 October 2026.