How altitude sickness shows up on Nepal treks, how to tell mild AMS from life-threatening HACE and HAPE, the acclimatisation rules our itineraries follow, what to ask your doctor about acetazolamide, and what happens if you need to go down or be flown out.
14 min read
Altitude sickness is the most common serious health risk on a Nepal trek, and it can affect anyone above about 2,500 m, however fit they are. It usually starts as acute mountain sickness (AMS): a headache, often with nausea, tiredness or dizziness. Mild AMS gets better with rest at the same altitude. If you ignore it and keep climbing, it can progress to high-altitude cerebral oedema (HACE) or high-altitude pulmonary oedema (HAPE). Both can kill.
Three rules prevent most problems. Above 3,000 m, don't raise your sleeping altitude by more than about 500 m a day. Build in rest days. Never go higher with symptoms. If symptoms get worse, go down. This guide explains how to recognise each condition, what the Wilderness Medical Society (WMS), the CDC and the NHS advise, what our guides check every evening, and what happens if someone has to be evacuated.
Altitude sickness is the body's reaction to the lower oxygen pressure at high altitude when you climb faster than you can adjust. It takes three forms: acute mountain sickness (AMS), which is common and usually mild; high-altitude cerebral oedema (HACE), which is swelling of the brain; and high-altitude pulmonary oedema (HAPE), which is fluid in the lungs. HACE and HAPE are emergencies.
AMS usually occurs above about 2,500 m, and some people notice symptoms from around 2,000 m. HAPE usually occurs above 2,500–3,000 m. On Nepal treks, the risk becomes real at places like Namche Bazaar (3,440 m), Manang (3,540 m), Samagaon (3,530 m) and Kyanjin Gompa (3,870 m), and it rises with each night higher.
The NHS sets the threshold at about 2,500 m. The WMS guideline notes that HAPE generally appears above 2,500–3,000 m. On lower treks such as the Ghorepani Poon Hill Trek, where you sleep at 2,874 m and climb to 3,210 m for sunrise, mild headaches are possible but serious illness is uncommon.
The main symptom of AMS is a headache, together with at least one of: loss of appetite or nausea, tiredness or weakness, and dizziness. Symptoms usually begin 6 to 10 hours after arriving at a new altitude (the CDC gives a range of 2 to 12 hours). They usually improve within 1 to 3 days if you don't climb higher.
Poor sleep and mild breathlessness when walking uphill are normal at altitude and are not AMS on their own. What matters is the pattern: a headache that does not ease with rest, fluids and a simple painkiller, or symptoms that get worse overnight.
The CDC adds a useful rule of thumb. Symptoms that first appear after three days at the same altitude, without climbing higher, are probably not AMS, so your guide will look for another cause.
HACE is swelling of the brain. Look for confusion, drowsiness, odd behaviour and loss of balance or coordination. HAPE is fluid in the lungs. Look for unusual breathlessness, reduced ability to walk uphill, a cough, and later breathlessness at rest and frothy or bloody spit. Both need immediate descent and medical help.
AMS | HACE | HAPE | |
|---|---|---|---|
What it is | The body struggling to adjust | Brain swelling, usually developing from AMS | Fluid in the lungs; can occur without AMS |
Key signs | Headache plus nausea, fatigue or dizziness | Confusion, drowsiness, stumbling (ataxia), altered behaviour | Breathless on effort far more than companions, cough, chest tightness, then breathless at rest, frothy or bloody spit, blue lips |
Danger | Uncomfortable; can progress |
A simple test for HACE is to ask the person to walk heel-to-toe along a straight line. Someone with HACE often can't do it. They sway or step off the line as if drunk. Our guides use this check whenever a trekker seems confused or unsteady. Failing it means going down, not waiting until morning.
The Lake Louise score is the standard questionnaire for diagnosing AMS. You score four symptoms from 0 (none) to 3 (severe): headache, gastrointestinal symptoms, fatigue or weakness, and dizziness. A total of 3 or more, including at least 1 point for headache, after at least six hours at altitude means AMS.
Symptom | 0 | 1 | 2 | 3 |
|---|---|---|---|---|
Headache | None | Mild | Moderate | Severe, incapacitating |
Stomach | Good appetite | Poor appetite or nausea | Moderate nausea or vomiting | Severe nausea and vomiting, incapacitating |
The 2018 revision dropped sleep quality from the score, because poor sleep at altitude is common and not closely linked to AMS. Its authors suggest mild AMS at 3–5 points, moderate at 6–9 and severe at 10–12, while noting that these bands have not been validated. On the trail, any score of 3 or more with a headache means you don't go higher that day.
Climb slowly. Above 3,000 m, raise your sleeping altitude by no more than about 500 m a day, add an extra acclimatisation night for every 1,000 m gained, and never climb higher with symptoms. Avoid alcohol for the first days at altitude, drink enough to avoid dehydration, and ask your doctor about acetazolamide.
The guidelines agree on the essentials:
Sleeping altitude: above 3,000 m, no more than about 500 m higher than the previous night (CDC, NHS, WMS).
Rest days: the CDC advises an extra day for every 1,000 m of sleeping altitude gained. The NHS advises a rest day every 3 to 4 days above 3,000 m.
Avoid big first jumps: the CDC advises against going from low altitude straight to a sleeping altitude of 2,750 m or more in one day. The NHS warns against going from below 1,200 m to above 3,500 m in one day.
"Climb high, sleep low": on rest days, walk a few hundred metres higher and come back down to sleep. Our acclimatisation days work this way, with hikes above Namche, Dingboche, Manang and Kyanjin Gompa.
Every high trek we run has acclimatisation days built in where the route goes above 4,000 m. The Everest Base Camp Trek has two, at Namche Bazaar (3,440 m) and Dingboche (4,410 m). The Annapurna Circuit Trek rests in Manang (3,540 m) before Thorong La.
Trek | Days | Highest point | Acclimatisation nights |
|---|---|---|---|
15 | Kala Patthar 5,545 m | Namche Bazaar (day 4), Dingboche (day 7) | |
14 | Gokyo Ri 5,357 m | Namche Bazaar (day 4), Gokyo (day 8) | |
Treks such as the Annapurna Base Camp Trek don't include a rest day. You spend only two nights above 3,000 m, at Deurali (3,200 m) and Annapurna Base Camp (4,130 m), then descend the next day. The climb from Deurali to base camp is larger than the 500 m guideline, so your guide watches closely that evening, and nobody goes up from Deurali with symptoms. Our Everest Base Camp vs Annapurna Base Camp comparison looks at how the altitude profiles differ.
Acetazolamide is a prescription medicine that speeds up acclimatisation and is effective at preventing AMS. Whether you should take it is a decision for your doctor, based on your health, your itinerary and any previous altitude illness. It does not replace slow ascent, and it is not a reason to keep climbing with symptoms.
For reference, these are the guideline doses for adults. Do not take any of them without medical advice.
Prevention (WMS 2024 and CDC): 125 mg every 12 hours, starting the day before ascent. The CDC gives 250 mg twice a day for people over 100 kg. The CDC advises continuing for the first two days at your highest altitude, or longer while you are still climbing.
Treatment of AMS (WMS and CDC): 250 mg every 12 hours.
Children (CDC): 1.25 mg/kg every 12 hours for prevention, up to 125 mg per dose.
Things to know before your appointment:
Tingling in the fingers, toes or face is a common side effect. The CDC notes that lower doses reduce it.
It makes fizzy drinks taste flat or metallic.
It is a sulphonamide-type drug, but the CDC states that no cross-reaction with sulpha antibiotics has been reported. Take extra care if you have had anaphylaxis to any medicine.
Your doctor may suggest a short trial at home to check for side effects.
The CDC's risk table recommends considering acetazolamide when your first night is at 2,750–3,400 m, or when you climb more than 500 m a day above 3,000 m with extra acclimatisation days. It strongly encourages acetazolamide when your first night is above 3,400 m, when you climb that fast without extra days, or when you have had HAPE or HACE before. On a Lukla flight, you fly to 2,860 m and sleep lower at Phakding (2,610 m), which keeps the first night within the CDC's low-risk band. Your doctor may also prescribe dexamethasone to carry for emergencies.
Drinking enough to avoid dehydration matters, but drinking extra water does not prevent AMS. Dehydration causes headaches and tiredness that look like AMS and make you feel worse. The NHS advises drinking enough to stay hydrated. In the cold, dry air above 4,000 m, we suggest around three to four litres a day of boiled, filtered or treated water, tea or soup.
Avoid alcohol for at least the first 48 hours at a new altitude, and ideally until you are back down. On our Everest treks we suggest none until you are back below Namche Bazaar. The CDC advises against alcohol in the first 48 hours, and against alcohol or sleeping tablets to help you sleep, because they depress breathing at night. The NHS advises avoiding alcohol altogether while climbing.
No. Fitness does not protect against altitude sickness, and fit trekkers sometimes get it because they walk too fast. The best predictor is your own history: people who have had altitude illness before are more likely to get it again. Fitness does make each day less tiring and helps you recover overnight.
Train for the walking, then let your guide set a slow pace above 3,000 m. Our guide to the best time to trek in Nepal and our beginner treks guide help you match a trek to your experience.
Every evening, your guide checks each trekker for altitude symptoms, using the Lake Louise questions and a pulse oximeter reading of oxygen saturation and heart rate. They follow a firm rule of not climbing higher with symptoms. If someone needs to descend, the guide arranges it and a member of the team goes with them.
Our guides carry a first aid kit and a pulse oximeter on every trek. A few points about the oximeter:
It is a guide, not a diagnosis. The CDC notes that oxygen saturation in AMS is often normal for the altitude, or only slightly lower. Symptoms decide.
The trend matters. A reading that falls night after night, or one well below the rest of the group, prompts a closer look.
HAPE shows clearly. The CDC notes that saturation in HAPE is commonly 50–70%, at least 10 points below healthy people at the same height.
Your guide also changes the day's plan for weather, trail conditions or the health of the group. Our trust and safety page sets out how we work.
Tell your guide straight away and don't climb higher. With mild AMS, rest at the same altitude, drink, and take paracetamol or ibuprofen for the headache. If symptoms haven't improved after a day, or get worse at any point, descend. With any sign of HACE or HAPE, descend immediately, even at night if it is safe to move.
How far to go down:
AMS: the CDC notes that descending at least 300 m usually brings improvement. The NHS suggests 300 to 1,000 m if symptoms haven't improved after a day.
HACE and HAPE: keep going down until the person clearly improves, with oxygen if available. The WMS guideline recommends supplemental oxygen and, for HACE, dexamethasone.
If descent is impossible because of weather or terrain, a portable hyperbaric bag can stand in for a descent of roughly 1,500–1,800 m (CDC) while you wait for evacuation.
Ibuprofen and paracetamol treat the headache only, not AMS itself. Feeling better after a painkiller is not a reason to climb higher.
The Himalayan Rescue Association (HRA) runs seasonal aid posts at Pheriche in the Khumbu and at Manang on the Annapurna Circuit, open in spring and autumn. Volunteer doctors treat trekkers and give talks on altitude sickness. Our Everest Base Camp itinerary stops at Pheriche on the way down, and the Annapurna Circuit rest day is in Manang.
Elsewhere, care means health posts in larger villages and evacuation to hospitals in Kathmandu or Pokhara.
A helicopter is used when someone with HACE, HAPE or a serious injury needs hospital care faster than walking down allows, or can't walk. Above Namche, or on remote routes like Manaslu, Dolpo and Kanchenjunga, it is often the only fast way out. Helicopters need daylight and flyable weather, so descent on foot starts first.
When a trekker is seriously ill or injured, your guide arranges the evacuation, by helicopter where needed, to a hospital in Pokhara or Kathmandu, and contacts your insurer. Evacuation is billed to your travel insurance. That is why insurance covering helicopter rescue is a condition of booking.
Your policy must cover trekking and helicopter evacuation up to at least the highest altitude on your itinerary. Many standard travel policies stop at much lower heights or exclude trekking. Check the altitude limit in writing, and carry your policy number and the insurer's 24-hour emergency number. Your guide keeps a copy.
Trek | Evacuation cover needed up to |
|---|---|
6,000 m | |
Gokyo Lakes, Annapurna Circuit, Tilicho Lake, Manaslu Circuit | 5,500 m |
Our guide to what's included in a Nepal trekking package explains which costs sit with insurance and which with us.
Not from the flight itself, as you only spend a short time at 2,860 m in Lukla before walking down to Phakding at 2,610 m. That first night is lower than Lukla and within the CDC's low-risk range. The real test comes from Namche Bazaar onwards, which is why we spend two nights there.
No. It speeds up acclimatisation rather than masking symptoms, so if you develop AMS while taking it, treat it as AMS. The rule of not going higher with symptoms still applies.
Children get altitude sickness at similar rates to adults but may not describe it clearly, so watch for fussiness, poor appetite and unusual tiredness. Lower treks such as Poon Hill or Australian Camp suit families well. Talk to your doctor before taking a child above 3,000 m.
Yes. Previous altitude illness is the strongest predictor, and the CDC places anyone who has had HAPE or HACE in the high-risk group. Tell us and your doctor about any past episodes so the itinerary and medicine plan can allow for it.
Packing and preparation
A Nepal trekking packing list by season and altitude: porter weight limits, what we lend you, what to rent in Thamel or Lakeside, layering, boots, sleeping bag ratings, water, cash, first aid and what to leave at home.
Can lead to coma within 24 hours
Life-threatening |
What to do | Stop climbing, rest; descend if worse | Descend now; oxygen and medicine; evacuation | Descend now; oxygen; evacuation |
Fatigue or weakness
None |
Mild |
Moderate |
Severe, incapacitating |
Dizziness or light-headedness | None | Mild | Moderate | Severe, incapacitating |
20 |
Kala Patthar 5,545 m |
Namche (day 4), Dingboche (day 7), Gokyo (day 14) |
10 | Thorong La 5,416 m | Manang (day 5) |
12 | Thorong La 5,416 m | Manang (day 4) |
12 | Larkya La 5,106 m | Samagaon (day 7) |
8 | Tserko Ri 4,984 m | Kyanjin Gompa (day 5) |
8 | Khayer Lake 4,660 m | Khopra Danda (day 5) |
12 | Marang La 4,280 m | Lo Manthang (day 7) |
8 | Hotel Everest View 3,880 m | Namche Bazaar (day 3) |
5,000 m |
4,500 m |
3,500 m |
Garlic soup is a teahouse tradition and a good warm drink, but there is no evidence it prevents AMS. The WMS 2024 guideline specifically recommends against ginkgo biloba for prevention. Slow ascent and, where your doctor advises, acetazolamide are what work.
Your guide will look at the reading alongside how you feel, your heart rate and how others in the group compare. A low reading with no symptoms is common at altitude. A low reading that keeps falling, or one with symptoms, means resting or going down.
Many people with well-controlled conditions trek safely, but you need to talk to your doctor before booking. Tell us about any condition when you enquire, so we can suggest an itinerary with gentler gains and make sure your guide knows.
Mild AMS usually settles within one to three days if you stop climbing. If it doesn't improve after a day at the same height, descending 300 m or more usually brings quick relief. HACE and HAPE need descent and medical treatment, and recovery takes longer.