Altitude sickness is the main health risk on the Everest Base Camp trek. The route takes you from Lukla (about 2,860 m) to Base Camp (5,364 m) and often Kala Patthar (about 5,545 m), and many trekkers get at least a mild headache along the way. Most cases are mild and settle with rest. A small number progress to high altitude pulmonary oedema (HAPE) or cerebral oedema (HACE), which can kill. A sensible itinerary, honest reporting of symptoms and a willingness to go down are what keep you safe.
The one rule to remember: never go higher to sleep while you have symptoms, and go down at once if symptoms get worse or you see any sign of HAPE or HACE.
Altitude sickness on Everest: the quick answer
| Question | Short answer |
|---|---|
| How common is it on the EBC trek? | The CDC says prevalence on the Everest Base Camp routes can approach 30% at the higher elevations, even on standard schedules. Most cases are mild AMS. |
| Where does it usually start? | Often around Namche Bazaar (3,440 m) and more commonly above 4,000 m at Dingboche, Lobuche and Gorak Shep. |
| Main symptoms | Headache plus nausea, loss of appetite, dizziness, unusual tiredness or poor sleep. |
| Emergency signs | Breathlessness at rest, a wet or frothy cough, confusion, stumbling or loss of coordination, extreme drowsiness. |
| Best prevention | A slow itinerary with acclimatisation days at Namche and Dingboche, and no more than about 500 m gain in sleeping altitude per day above 3,000 m. |
| Medication | Acetazolamide (Diamox) can help some trekkers acclimatise. Speak to your doctor or a travel clinic before you go. |
| Medical help on the trail | Himalayan Rescue Association aid post at Pheriche in season, plus helicopter evacuation if your insurance covers it and the weather allows. |
Why altitude makes you ill
The air at Everest Base Camp still contains about 21% oxygen, the same as at sea level. What changes is air pressure. At around 5,300 m the pressure is roughly half that at sea level, so every breath delivers far less oxygen to your lungs and blood.
Your body adapts by breathing faster and deeper, raising your heart rate and, over days and weeks, making more red blood cells. This process is called acclimatisation. Altitude illness happens when you climb faster than your body can adapt.
What matters most is your sleeping altitude, not the highest point you reach during the day. That is why trekkers walk high and then come back down to sleep, and why the itinerary you choose has such a big effect on your risk.
The three forms of altitude illness
Doctors divide altitude illness into three conditions. AMS is common and usually mild. HAPE and HACE are less common but are medical emergencies.
| Condition | What it is | Key signs | What to do |
|---|---|---|---|
| AMS (acute mountain sickness) | The common, usually mild form. Typically starts 6–12 hours after arriving at a new altitude, often during the first night. | Headache with one or more of: nausea or vomiting, loss of appetite, dizziness, fatigue, poor sleep. | Stop ascending. Rest at the same altitude, take simple painkillers if needed and tell your guide. Descend if it gets worse or does not improve within about a day. |
| HAPE (high altitude pulmonary oedema) | Fluid in the lungs. Usually develops after two or more nights at altitude and can appear without any AMS first. | Breathlessness out of proportion to effort, then at rest; dry cough becoming wet or frothy; chest tightness; blue lips; marked drop in performance. | Emergency. Descend at least 1,000 m if possible, give oxygen if available, minimise exertion and arrange evacuation. |
| HACE (high altitude cerebral oedema) | Swelling of the brain, usually following worsening AMS. Rare below about 4,000 m. | Confusion, unusual behaviour, severe drowsiness, staggering or inability to walk heel-to-toe in a straight line. | Emergency. Descend immediately with someone accompanying the patient. Medical treatment includes dexamethasone and oxygen. |
Both HAPE and HACE can kill within hours if the person stays at altitude. Never leave someone with these symptoms alone, and never let them descend unaccompanied.
Symptoms to watch for, day by day
Altitude illness creeps up. Symptoms often appear in the evening or overnight after a big gain in altitude, rather than while you are walking.
Mild AMS
- A headache that eases with paracetamol or ibuprofen and rest
- Reduced appetite or mild nausea
- Feeling more tired than the walk explains
- Broken sleep, or waking up feeling short of breath (periodic breathing at night is common and on its own is not AMS)
Moderate to severe AMS
- A headache that does not respond to painkillers
- Vomiting, or being unable to eat
- Marked weakness or exhaustion
Red flags that mean go down now
- Breathlessness while resting, or breathing that does not settle after a few minutes’ rest
- A cough with frothy, pink or bloody sputum
- Confusion, slurred speech or strange behaviour
- Stumbling, poor balance or clumsy hands
- Being very sleepy or hard to wake
The UK’s TravelHealthPro guidance puts it simply: symptoms at altitude are caused by altitude illness until proven otherwise. Stomach bugs, colds and dehydration are common on the trail too, but do not let them become an excuse to keep climbing.
The EBC altitude profile and acclimatisation schedule
The standard route flies into Lukla and climbs steadily up the Dudh Koshi and Imja valleys. The table below shows a typical sleeping schedule and how much higher you sleep each night. Altitudes vary slightly between maps.
| Day | Sleep at | Approx. altitude | Change in sleeping altitude | Notes |
|---|---|---|---|---|
| 1 | Phakding | 2,610 m | Fly to Lukla (2,860 m), walk down | Gentle first day |
| 2 | Namche Bazaar | 3,440 m | +830 m | The biggest jump of the trek |
| 3 | Namche Bazaar | 3,440 m | 0 | Acclimatisation day: walk up towards Syangboche or Khumjung, sleep low |
| 4 | Tengboche or Phortse | 3,810–3,860 m | about +400 m | Within guidelines |
| 5 | Dingboche | 4,410 m | about +550 m | First night above 4,000 m |
| 6 | Dingboche | 4,410 m | 0 | Acclimatisation day: hike up Nangkartshang ridge, sleep low |
| 7 | Lobuche | 4,940 m | about +530 m | Where many trekkers first feel unwell |
| 8 | Gorak Shep | 5,164 m | about +220 m | Visit Everest Base Camp (5,364 m) the same afternoon |
| 9 | Pheriche or Dingboche | 4,250–4,410 m | Descent | Climb Kala Patthar (about 5,545 m) at dawn, then descend |
Two days on this route exceed the 500 m rule: Phakding to Namche and Tengboche to Dingboche. The rest days at Namche and Dingboche exist to compensate. Any itinerary that skips them, or that reaches Base Camp in fewer than about eight walking days, raises your risk significantly.
Our 16-day Everest Base Camp trek goes via Phortse and keeps full acclimatisation days at both Namche and Dingboche. For more on how the route and days fit together, see our guide to Everest Base Camp trek difficulty.
How to prevent altitude sickness on the EBC trek
The Wilderness Medical Society (WMS), CDC and NHS agree on the basics. Gradual ascent prevents far more altitude illness than any tablet.
- Choose a gradual itinerary. Above 3,000 m, aim to increase your sleeping altitude by no more than about 500 m a night, with a rest day every three to four days or for every 1,000 m gained. Build in a spare day for illness or bad weather.
- Climb high, sleep low. Use acclimatisation days to walk a few hundred metres higher, then return to sleep at the same altitude as the night before.
- Walk slowly. You should be able to hold a conversation while walking uphill. Guides call this “bistarai, bistarai” (slowly, slowly).
- Drink enough, not too much. Dehydration mimics AMS, but forcing down huge volumes of water does not prevent it and can be dangerous. Drink to thirst and keep your urine pale. Our guide to staying hydrated when hiking has practical tips.
- Avoid alcohol and sleeping pills. Alcohol and sedatives can depress breathing during sleep, which is when oxygen levels drop most. Save the celebration beer for the walk back down.
- Eat well. Carbohydrate-rich meals such as dal bhat, porridge and noodle soups are easy to eat when appetite drops.
- Keep warm. Cold adds stress and makes it harder to judge how you feel. Pack properly for nights well below freezing; see our Everest Base Camp packing list.
- Be honest. Tell your guide about every headache, every missed meal and every bad night. Early reporting is what lets a mild case stay mild.
Fitness helps you enjoy the trek and recover each day, but it does not protect you from altitude sickness. Very fit people often walk too fast. Age is also no guarantee either way. Training still matters for the trek itself: see how to train for Everest Base Camp.
Diamox (acetazolamide): what you need to know
Acetazolamide, sold as Diamox, speeds up acclimatisation by making you breathe more deeply, especially at night. It is the main medicine recommended for preventing AMS by the WMS and the CDC. It is a prescription medicine, and the decision to take it should be made with your doctor or a travel health clinic.
- Who might use it: people who have had AMS before, people on fast itineraries, and some trekkers on standard itineraries after discussion with a clinician.
- Typical guideline dose: WMS and CDC guidance gives 125 mg twice a day for adult prevention, usually starting the day before ascent. Your prescriber will set the dose for you; do not adjust it yourself.
- Common side effects: tingling in the fingers, toes and face, needing to urinate more often, and fizzy drinks tasting flat.
- Who should not take it, or needs specialist advice: anyone who is pregnant, and people with a history of severe sulfonamide allergy. Tell your doctor about kidney, liver or other conditions and all medicines you take.
- In the UK: TravelHealthPro notes acetazolamide is used unlicensed for altitude illness, so it is usually prescribed privately through a travel clinic or GP.
Diamox does not mask the symptoms of AMS, HAPE or HACE. If you feel ill while taking it, you are ill and the same rules apply: stop, rest, and descend if it gets worse. It is also no substitute for a gradual itinerary.
Other drugs, including dexamethasone and nifedipine, are used to treat severe altitude illness or prevent HAPE in people with a history of it. These are for medical professionals and guides trained in their use, under medical advice.
What to do if you or a teammate feels unwell
- Tell your guide straight away. Do not hide symptoms to stay with the group.
- Stop going up. Stay at the same altitude, rest, and treat a headache with paracetamol or ibuprofen. Eat and drink if you can.
- Reassess. If symptoms clear, you can usually continue the next day once you feel well. The NHS notes mild symptoms often improve within one to three days.
- Descend if things do not improve or get worse. Even 300–500 m of descent often makes a big difference. From Lobuche or Gorak Shep, that means walking down to Pheriche or Dingboche.
- Treat red flags as an emergency. With any sign of HAPE or HACE, descend immediately, even at night if it is safe, with someone alongside. Give oxygen if available and arrange evacuation through your guide and insurer.
A portable hyperbaric chamber (often called a Gamow or PAC bag) can buy time where descent is impossible. Some lodges, aid posts and expedition teams carry them, but they do not replace going down.
Pulse oximeters: useful, but not a diagnosis
Many guides check oxygen saturation (SpO2) each evening with a fingertip oximeter. At 4,000–5,000 m, readings in the 80s or even high 70s can be normal for a healthy, acclimatising trekker.
The number alone cannot tell you whether you have AMS. A trend of falling readings combined with symptoms is meaningful; a single low number with no symptoms usually is not. Cold fingers, nail polish and movement all affect readings. How you feel, behave and walk matters more.
Medical help on the trail: the Himalayan Rescue Association
The Himalayan Rescue Association (HRA) is a Nepali non-profit formed in 1973 to reduce deaths from altitude illness. It runs seasonal aid posts staffed by volunteer doctors.
- Pheriche aid post (about 4,250–4,370 m): on the main EBC route below Dingboche, open during the spring and autumn trekking seasons.
- HRA Everest ER (about 5,350 m): a clinic at Everest Base Camp during the spring climbing season.
- Manang aid post (about 3,550 m): on the Annapurna Circuit at Manang, also open in season.
During the season, HRA doctors give a free talk on altitude sickness most afternoons. If you are passing Pheriche or acclimatising in Dingboche nearby, it is well worth attending. Consultations and treatment are charged, so carry some cash. Opening dates vary year to year, so ask your agency before you go.
Lower down, Khunde Hospital near Namche and small health posts in larger villages can treat minor problems. For anything serious, evacuation to Kathmandu is the norm.
Helicopter rescue and insurance
Helicopters fly to the Khumbu from Lukla and Kathmandu and can reach most villages on the EBC route in good weather. A rescue is not instant: cloud, wind and darkness can ground flights for hours or days, so walking down remains the first treatment.
- Get the right cover. Your policy must cover trekking to at least 5,545 m (most trekkers choose cover to 6,000 m), including helicopter evacuation and medical treatment. Declare your maximum altitude to the insurer.
- Carry your policy details. Give your guide the policy number and the insurer’s 24-hour assistance number before the trek starts.
- Be wary of pressure to fly out. Nepal Police have investigated fraud involving unnecessary helicopter rescues of trekkers, and the government has tightened oversight of rescue flights. Book with a registered agency and follow medical advice, not sales pressure.
Our guide to Everest Base Camp travel insurance explains what to look for in a policy.
Common myths about altitude sickness
| Myth | Reality |
|---|---|
| “I’m fit, so I won’t get it.” | Fitness does not protect you. Fit trekkers often walk too fast. |
| “Drinking loads of water prevents AMS.” | Good hydration helps you feel better, but over-drinking does not prevent AMS and can cause low blood sodium. |
| “Garlic soup cures altitude sickness.” | It is a tasty local tradition, not a treatment. |
| “A headache at altitude is normal, so push on.” | A headache at altitude may be AMS. Do not go higher until it has gone. |
| “Diamox hides the symptoms.” | It aids acclimatisation; it does not mask illness. Symptoms on Diamox still count. |
| “I’ve trekked high before without problems, so I’m safe.” | Previous experience is useful evidence but no guarantee. Susceptibility can vary between trips. |
If you have had altitude sickness before
A previous bout of mild AMS does not rule out the EBC trek. It does mean you should choose a slower itinerary, talk to your doctor about acetazolamide and plan extra days.
If you have had HAPE or HACE, or have heart or lung disease, sickle cell disease, significant anaemia, sleep apnoea or are pregnant, speak to a doctor with expertise in altitude medicine before you book. Many people with medical conditions trek safely with the right advice and planning.
Next step: plan a safe EBC trek
Altitude sickness is manageable when the itinerary is slow, the guide is experienced and everyone is honest about how they feel. Read our step-by-step guide to planning an Everest Base Camp trek, check our safety tips for trekking in Nepal, or look at the acclimatisation built into our 16-day Everest Base Camp trek.
This article is general information, not medical advice. Speak to your doctor or a travel health clinic about your own health before trekking at altitude.
Frequently asked questions
At what altitude does altitude sickness start on the Everest Base Camp trek?
AMS can start from about 2,500 m, but on the EBC trek most people first notice symptoms around Namche Bazaar (3,440 m) or, more commonly, above 4,000 m at Dingboche, Lobuche and Gorak Shep. Risk depends on how fast your sleeping altitude rises.
Can you die from altitude sickness on the EBC trek?
Yes, although it is rare. Deaths on the trek are usually caused by HAPE or HACE in people who kept climbing or did not descend quickly enough. Recognising symptoms early and descending are the most important safety measures.
Should I take Diamox for Everest Base Camp?
It depends on your history, itinerary and health. Acetazolamide can help some trekkers acclimatise, but it is a prescription medicine with side effects and some contraindications. Discuss it with your doctor or a travel health clinic before you go.
Can I keep trekking with mild altitude sickness?
You can usually continue once symptoms have completely gone, but you should not go higher to sleep while you still have them. Rest at the same altitude, tell your guide and descend if they get worse or do not improve within about a day.
How many acclimatisation days do you need for Everest Base Camp?
Most well-designed itineraries include at least two: one at Namche Bazaar and one at Dingboche (or Pheriche). Many add a spare day for illness or weather, which also reduces pressure to keep climbing when unwell.
Is a helicopter rescue covered by travel insurance?
Only if your policy covers trekking to the altitude you will reach and includes emergency helicopter evacuation. Check the altitude limit, declare your itinerary to the insurer and carry the 24-hour assistance number.
Does bottled oxygen help on the Everest Base Camp trek?
Oxygen is used to treat serious altitude illness, often while descending or waiting for evacuation. Trekkers do not use it routinely to reach Base Camp, and the WMS does not recommend short bursts from small canisters for prevention.
Is there a doctor on the Everest Base Camp route?
In the spring and autumn seasons the Himalayan Rescue Association runs an aid post in Pheriche staffed by volunteer doctors, and in spring a clinic at Everest Base Camp. Khunde Hospital near Namche also offers basic care.
