Altitude sickness is the most common reason trekkers in Nepal turn back before reaching their goal, and in rare cases it is life-threatening. Yet it is also one of the most preventable problems in the mountains. Almost every serious case we have seen in our years of guiding could have been avoided with a slower ascent, earlier attention to symptoms or a willingness to descend.
This guide explains what altitude sickness is, why it happens, how to recognise the early signs, and the simple rules that help most trekkers reach Everest Base Camp, Thorong La or Annapurna Base Camp safely. It is written from a guide's point of view, not a doctor's, so please also talk to your own doctor before travelling, especially if you have any medical conditions.
Quick answer: altitude sickness (acute mountain sickness, AMS) is caused by ascending too fast to where the air has less oxygen, usually above 2,500–3,000 m. Symptoms include headache, nausea, dizziness, fatigue and poor sleep. Prevent it by climbing slowly (no more than 300–500 m of sleeping altitude a day above 3,000 m), taking rest days, drinking plenty of water and avoiding alcohol. If symptoms worsen, stop climbing; if they become severe, descend immediately.
What causes altitude sickness?
The air at altitude contains the same percentage of oxygen as at sea level, about 21%, but the air pressure is lower, so each breath delivers less oxygen to your lungs and blood. At Namche Bazaar (3,440 m), each breath contains about two-thirds of the oxygen available at sea level. At Everest Base Camp (5,364 m) it is roughly half.
Your body can adapt to this, a process called acclimatisation. You breathe faster and deeper, your heart rate rises, your kidneys adjust the chemistry of your blood, and over days and weeks your body makes more red blood cells. But acclimatisation takes time. If you climb faster than your body can adapt, fluid can leak from blood vessels into tissues, causing the symptoms of altitude sickness.
Altitude sickness does not depend on fitness. Marathon runners get AMS; overweight trekkers in their sixties often do not. Age, gender and fitness make little difference. The main factors are how high you go, how fast you go and how your individual body responds, which you cannot predict until you are there.
The three forms of altitude illness
| Condition | What happens | Key signs | Severity |
|---|---|---|---|
| Acute Mountain Sickness (AMS) | The body struggles to adapt | Headache plus nausea, fatigue, dizziness or poor sleep | Common; usually mild if managed |
| High Altitude Cerebral Oedema (HACE) | Fluid builds up in the brain | Confusion, loss of coordination, severe headache, drowsiness | Life-threatening; descend immediately |
| High Altitude Pulmonary Oedema (HAPE) | Fluid builds up in the lungs | Breathlessness at rest, persistent cough, gurgling breath, blue lips | Life-threatening; descend immediately |
AMS is common: studies suggest a large share of trekkers who ascend to Everest Base Camp experience at least mild symptoms. HACE and HAPE are much rarer, but they can develop quickly, especially if early AMS is ignored and the person continues climbing.
Recognising the symptoms
Mild AMS
- A headache, often throbbing, that is worse in the morning or when bending over.
- Loss of appetite or mild nausea.
- Tiredness beyond what the day's walk explains.
- Dizziness or light-headedness.
- Difficulty sleeping, with frequent waking.
Many trekkers feel some of these on their first night above 3,000 m. Mild AMS usually improves within 24–48 hours if you do not go higher, rest, drink water and take simple painkillers.
Moderate to severe AMS
- A severe headache that does not improve with painkillers.
- Vomiting.
- Increasing weakness and fatigue.
- Shortness of breath with light activity.
Warning signs of HACE and HAPE
- Confusion, strange behaviour or irritability.
- Loss of balance: difficulty walking heel to toe in a straight line (the "ataxia test").
- Drowsiness or being difficult to wake.
- Breathlessness at rest, even when lying down.
- A persistent cough, sometimes with frothy or pink sputum.
- Blue or grey lips and fingernails.
Any of these warning signs means the person must descend immediately, even at night, with someone accompanying them. Do not wait to see whether it improves.
The golden rules of acclimatisation
1. Climb high, sleep low
Your sleeping altitude matters most. Above 3,000 m, try not to increase your sleeping altitude by more than 300–500 m per night. It is fine, and helpful, to walk higher during the day and come back down to sleep.
2. Take a rest day every 1,000 m
For every 1,000 m you gain above 3,000 m, or every three to four days, spend an extra night at the same altitude. On the Everest Base Camp trek, these are the rest days in Namche Bazaar and Dingboche. They are not wasted days: use them for a short acclimatisation hike to a higher viewpoint.
3. Walk slowly
Nepali guides have a phrase: bistarai, bistarai, "slowly, slowly". If you cannot talk comfortably while walking uphill, you are going too fast. A slow, steady pace uses less oxygen and helps you arrive at the lodge with energy left.
4. Drink plenty of water
The dry, cold air makes you lose water rapidly through breathing. Dehydration does not cause AMS, but its symptoms are similar and it makes everything feel worse. Aim for 3–4 litres a day, including tea and soup. Clear, pale urine is a good sign.
5. Avoid alcohol and sleeping pills
Alcohol dehydrates you and depresses breathing during sleep, and many sleeping pills do the same. Save the celebratory beer for Namche on the way down.
6. Eat well
Carbohydrates are easier to digest at altitude and provide energy. Even if your appetite drops, keep eating: soup, dal bhat, noodles, porridge and garlic soup (a trekkers' favourite, though its benefits are more traditional than scientific).
7. Listen to your body and tell your guide
This is the most important rule. Report any headache or other symptom to your guide early. Early AMS is easy to manage; late AMS is not. Good guides would much rather hear about a mild headache on the first evening in Namche than discover a trekker who has hidden symptoms for two days.
Acclimatisation on popular Nepal treks
| Trek | Highest sleeping altitude | Highest point | Recommended acclimatisation stops |
|---|---|---|---|
| Everest Base Camp | 5,164 m (Gorak Shep) | 5,545 m (Kala Patthar) | Namche Bazaar, Dingboche |
| Gokyo Lakes | 4,790 m (Gokyo) | 5,357 m (Gokyo Ri) | Namche Bazaar, Machhermo |
| Annapurna Circuit | 4,450 m (Thorong Phedi/High Camp) | 5,416 m (Thorong La) | Manang |
| Manaslu Circuit | 4,460 m (Dharamsala) | 5,106 m (Larkya La) | Samagaon |
| Annapurna Base Camp | 4,130 m (ABC) | 4,130 m | Usually none needed with a gradual ascent |
| Langtang Valley | 3,870 m (Kyanjin Gompa) | 4,984 m (Tserko Ri) | Kyanjin Gompa |
How to spend an acclimatisation day
Many trekkers see acclimatisation days as "rest days" and spend them in bed. That is a missed opportunity. The best way to use the day is to follow the principle of climb high, sleep low: after breakfast, walk slowly for two to four hours to a point 300–500 m above your lodge, spend some time there, then return to sleep at the same altitude as the night before.
In Namche Bazaar, the classic hike is to the Everest View Hotel (3,880 m) and the villages of Khumjung and Khunde. In Dingboche, trekkers climb the ridge behind the village towards Nangkartshang Peak, reaching around 5,000 m. In Manang on the Annapurna Circuit, the choice is the Gangapurna Lake viewpoint or the higher Ice Lake at around 4,600 m. In Samagaon on the Manaslu Circuit, the hike to Manaslu Base Camp or Birendra Lake does the job. Afterwards, eat a good lunch, drink plenty, wash some clothes, visit the local monastery and get an early night.
Flying and driving to altitude
Some Nepal itineraries start high, which changes the altitude picture. Flying to Lukla (2,840 m) is not a problem in itself, but many trekkers then walk to Namche Bazaar (3,440 m) on their second day, a big gain for bodies straight off a long-haul flight. That is why the rest day in Namche matters so much.
New roads have also made it possible to drive to high places quickly: to Manang (3,540 m) on the Annapurna Circuit, to Muktinath (3,800 m) from Jomsom, or into the Langtang region. Driving saves days of walking, but it removes the natural acclimatisation that comes from walking up gradually. If you drive or fly high, plan an extra night before climbing further. Helicopter tours to Everest or Annapurna Base Camp land only briefly for the same reason.
Eating and sleeping at altitude
Loss of appetite is one of the first effects of altitude, and many trekkers eat less just when their bodies need more energy. Try to eat something at every meal even if you are not hungry. Carbohydrate-rich foods such as rice, potatoes, noodles, porridge, bread and pasta are easiest to digest and provide energy efficiently. Soups count towards your fluid intake, and hot drinks help you stay warm and hydrated. Snacks such as nuts, chocolate and dried fruit keep you going between meals.
Sleep is often poor above 3,500 m. You may wake frequently, have vivid dreams or notice your breathing changing during the night. Keep your sleeping bag warm, drink a hot drink before bed, keep a bottle of water by your bed and a head torch within reach, and do not take sleeping pills without medical advice, as many can depress breathing at altitude. If you wake with a severe headache, confusion or breathlessness, wake your guide immediately.
Diamox (acetazolamide): should you take it?
Acetazolamide, sold as Diamox, is the most widely used medicine for preventing and treating AMS. It works by making your blood slightly more acidic, which stimulates faster breathing and speeds up acclimatisation. Many travel doctors prescribe it for high-altitude treks in Nepal.
- It is usually taken as a low dose, starting a day before reaching about 3,000 m, and continued until you start descending. Your doctor will advise the right dose for you.
- Common side effects include tingling in the fingers and toes, more frequent urination and a strange taste in fizzy drinks.
- It is a sulfonamide-type drug, so people with sulfa allergies should discuss alternatives with their doctor.
- It does not mask symptoms: if you develop AMS while taking it, you still need to stop climbing.
- It is not a substitute for a sensible itinerary.
Diamox is available in Kathmandu pharmacies without a prescription, but we strongly advise getting medical advice before you travel rather than self-prescribing on the trail.
For severe altitude illness, doctors and rescue teams may use other medicines, such as dexamethasone for HACE or nifedipine for HAPE, and supplemental oxygen or a portable hyperbaric chamber (Gamow bag). These are emergency treatments to support descent, not reasons to keep climbing.
What to do if you get altitude sickness
| Situation | Action |
|---|---|
| Mild headache, otherwise well | Rest, drink water, take a painkiller; do not climb higher until it clears |
| Mild AMS symptoms | Stay at the same altitude for a day; continue only if symptoms have gone |
| Symptoms getting worse despite rest | Descend 300–1,000 m; symptoms often improve quickly |
| Signs of HACE or HAPE | Descend immediately with support; use oxygen if available; call for evacuation |
The golden rule is simple: never ascend with symptoms of AMS, and descend if symptoms worsen. Descent is the most effective treatment for every form of altitude illness. Often, dropping just a few hundred metres is enough to feel much better.
How our guides monitor altitude
On our treks, guides check every trekker each evening and morning above 3,000 m. We ask how you slept, whether you have a headache, how your appetite is, and we measure your oxygen saturation and pulse with a pulse oximeter. Numbers alone do not diagnose AMS, since oxygen saturation drops naturally at altitude, but trends and symptoms together help us make good decisions.
If a trekker needs to rest or descend, a member of our team goes with them while the rest of the group continues, and our office in Kathmandu coordinates any evacuation with your insurance company. We carry first-aid kits on every trek and, on high routes, access to oxygen through lodges and rescue services. Our guides are trained in first aid and altitude awareness.
Helicopter evacuation and insurance
If someone becomes seriously ill at altitude, the fastest way down is often by helicopter. Rescue helicopters operate across the main trekking regions in good weather and can reach most trails within hours. A rescue flight costs several thousand US dollars, and helicopter companies will ask for confirmation of payment or insurance before flying.
That is why your travel insurance must cover emergency helicopter evacuation at the maximum altitude of your trek. Check your policy carefully: many standard policies exclude trekking above 3,000 or 4,000 m. Carry your policy number and the insurer's 24-hour emergency number, and give copies to your trekking company.
Who is at higher risk?
- People who have had altitude sickness before.
- People who ascend quickly, for example by flying directly to a high airstrip such as Lukla and walking up fast, or by driving to high altitude.
- People who over-exert themselves in the first days.
- People with certain heart or lung conditions, who should get medical advice before travelling.
Fitness does not protect you, but it does make the trek more enjoyable and helps you recover from each day's walking. Children and older trekkers can acclimatise well, but they may be less able to describe their symptoms, so watch them carefully.
Common myths about altitude
- "I'm fit, so I won't get it." Fitness has little effect on AMS. Fit people often walk too fast.
- "I've been to altitude before without problems, so I'll be fine." Past experience helps, but your response can vary from trip to trip.
- "Diamox will let me go faster." It helps acclimatisation, but it is not a licence to skip rest days.
- "A headache is normal; I'll push on." A headache is a warning. Rest until it clears before climbing higher.
- "Oxygen at the lodge means I'm safe." Oxygen helps in an emergency but does not replace descent.
Altitude and helicopter tours
On an Everest Base Camp helicopter tour, you fly from Kathmandu (1,400 m) to Kala Patthar (about 5,545 m) in less than an hour, with no chance to acclimatise. Landings at the highest points are kept short, typically 10–15 minutes, and passengers are advised to move slowly and avoid exertion. Most healthy people are fine, but those with heart or lung conditions should consult a doctor first.
How altitude feels day to day
Understanding what is normal helps you stay calm. At altitude, almost everyone experiences:
- Breathlessness on uphill sections, which settles when you rest.
- A faster pulse.
- Disturbed sleep and vivid dreams.
- Periodic breathing at night: breathing slows or pauses briefly, then speeds up. It can wake you with a feeling of suffocation. It is common and usually harmless, and Diamox often helps.
- More frequent urination, as your body adjusts.
- A dry cough from breathing cold, dry air (the "Khumbu cough"). A buff over your mouth and throat lozenges help.
What is not normal is breathlessness at rest, confusion, a headache that does not go away, vomiting or inability to walk straight.
A story from the trail
A few seasons ago, one of our groups reached Dingboche on day six of their Everest Base Camp trek. Everyone felt fine except one trekker, a strong runner in her thirties, who had a mild headache she put down to the sun. She did not mention it until the evening, when our guide noticed she had barely touched her dinner. Her oxygen saturation was lower than the rest of the group, and she admitted the headache had been there since lunchtime.
The group stayed in Dingboche for an extra day. She rested, drank water and slept, and the next morning she felt much better. She went on to reach Everest Base Camp and Kala Patthar two days later than planned, happy and healthy. Had she walked on to Lobuche that morning, as she wanted to, the outcome could have been very different. It is a typical example: mild AMS caught early is an inconvenience; ignored, it can end a trek or worse.
Preparing before you leave home
Altitude preparation starts before your flight to Kathmandu. Book a consultation with a travel clinic or your doctor six to eight weeks before departure to discuss altitude, vaccinations and whether Diamox is right for you. If you live near mountains, spending a few nights at 2,000–3,000 m in the weeks before your trip can help your body respond, although the effect fades after a couple of weeks. Get fit, so that the walking itself takes less out of you and you have energy left to adapt. And choose your itinerary carefully: a trek that looks two days shorter on paper may cost you the summit if it skips acclimatisation.
Altitude and your trekking insurance
When you buy insurance, tell the insurer the maximum altitude of your trek, not just the country. A policy that covers "trekking in Nepal" may still exclude altitudes above 4,000 m. If you plan a trekking peak, you will need specialist mountaineering cover. Keep the policy number and emergency phone number in your phone and on paper, and share them with your guide, so evacuation can be arranged quickly if needed.
A simple altitude checklist
- Choose an itinerary with proper acclimatisation days.
- See your doctor about altitude and Diamox before you travel.
- Buy insurance that covers helicopter evacuation at your trek's altitude.
- Walk slowly and drink 3–4 litres of fluid a day.
- Avoid alcohol above 3,000 m.
- Tell your guide about any symptoms, however mild.
- Never go higher with symptoms; descend if they get worse.
Finally, remember that turning back is never a failure. The mountains will still be there next season, and the trekkers we admire most are the ones who listen to their bodies and make good decisions, even when the summit or base camp is only a few hours away.
Altitude should not put you off trekking in Nepal. Every season, thousands of people of all ages safely reach Everest Base Camp, Thorong La and the Annapurna Sanctuary. They do it by respecting the mountains, taking their time and trusting their guides. If you would like to talk about a trek with a gentle altitude profile, contact our team, or read our Everest Base Camp guide for a well-acclimatised itinerary.