Altitude sickness is the one Himalayan risk that every trekker should understand before they leave home. It does not care how fit you are, how many marathons you have run or how well you felt on your last trek. What it does respond to is time, sensible pacing and an honest response to symptoms.
This guide covers what happens to your body at altitude, how to acclimatise, the warning signs of acute mountain sickness and its dangerous complications, and what to do if they appear. It is general information, not medical advice: talk to a doctor or travel clinic before any high-altitude trip.
What happens to your body at altitude
Air at 5,000 metres contains roughly half the oxygen available at sea level in each breath. Your body adapts by breathing faster and deeper, raising your heart rate and, over days and weeks, producing more red blood cells. This process is acclimatisation, and it cannot be rushed.
Acute mountain sickness (AMS) is what happens when you climb faster than your body adapts. It is common above 2,500 metres and very common on treks that gain height quickly. Most cases are mild and resolve with rest. A small number progress to high-altitude cerebral oedema (HACE) or high-altitude pulmonary oedema (HAPE), both of which can be fatal without descent.
Who gets AMS is hard to predict. Age, gender and fitness are poor guides; the strongest predictors are how fast you climb and whether you have had altitude sickness before. Previous trips are useful, but a good experience last time is no guarantee this time.
Altitude sickness is not a test of toughness. The trekkers who get into trouble are rarely the unfit ones; they are the ones who ignore a headache and keep climbing.
How to acclimatise properly
The best protection is a well-designed itinerary. Our Everest Base Camp Trek includes acclimatisation days at Namche Bazaar and Dingboche, and the Annapurna Circuit builds in time at Manang before Thorong La. These are not rest days in the lazy sense: you hike higher and sleep lower.
- Above 3,000 metres, aim to increase your sleeping altitude by no more than 300–500 metres a day.
- Take a rest day for every 1,000 metres gained, or every three to four days.
- “Climb high, sleep low”: day hikes to a higher viewpoint help your body adapt.
- Walk slowly enough to hold a conversation. Your guide’s pace is deliberate.
- Drink enough to keep your urine pale, and avoid alcohol and sleeping pills at altitude.
Flying straight to a high airstrip removes the gradual approach that walking gives you. Lukla sits at 2,860 metres, so the Everest itinerary starts gently with a short first day down to Phakding before climbing to Namche. On the Annapurna side, the road now reaches higher than it used to, so be careful not to drive too high, too fast.
Recognising the symptoms
Doctors and guides commonly use the Lake Louise score to assess AMS. In its current form it rates four symptoms (headache, nausea or loss of appetite, fatigue or weakness, and dizziness) from zero to three. Someone who has recently gained altitude, has a headache and scores three or more in total is considered to have AMS.
The key point is that symptoms appearing after a gain in altitude should be treated as altitude sickness until proven otherwise. It is tempting to blame a headache on dehydration or a long day, but assuming the worst and resting is always safer.
Mild to moderate AMS
- Headache, often worse in the morning or when lying down
- Loss of appetite or nausea
- Unusual tiredness, weakness or lethargy
- Light-headedness or dizziness
- Disturbed sleep, though this is also common in healthy trekkers at altitude
Danger signs: HACE and HAPE
- HACE: confusion, unusual behaviour, extreme drowsiness, or loss of coordination (unable to walk heel to toe in a straight line).
- HAPE: breathlessness at rest, a persistent cough (sometimes frothy or pink sputum), chest tightness and a sharp drop in exercise tolerance.
- Either condition is a medical emergency. Descend immediately, with assistance, and do not wait for daylight if it can be done safely.
What to do if you feel unwell
The rules are simple and they save lives. Experienced guides apply them without hesitation, and many also use a pulse oximeter each evening to track oxygen saturation alongside how you say you feel.
- Never ascend with symptoms of AMS. Stay at the same altitude until they resolve.
- If symptoms get worse despite a rest day, descend. Even 300–500 metres often makes a dramatic difference.
- Descend immediately at any sign of HACE or HAPE. Helicopter evacuation may be needed from remote areas.
- Never leave someone with altitude symptoms alone, and never let them descend alone.
Mild AMS is usually treated with rest at the same altitude, fluids and simple painkillers for the headache. Most people feel better within a day or two and continue the trek. Supplementary oxygen and portable pressure bags are used for more serious cases while descent is arranged.
u003cstrongu003eTell your guide earlyu003c/strongu003ernrnA mild headache reported at dinner is easy to manage. The same headache hidden until morning, after another night higher up, can end a trek. Honest reporting is the best safety tool we have.
Diamox, insurance and other questions
People with existing heart or lung conditions, those who are pregnant and anyone who has had HACE or HAPE before should seek specialist advice before trekking at altitude. Many can still trek safely with the right plan.
Acetazolamide (sold as Diamox) can speed acclimatisation and is widely used on Himalayan treks, both as prevention and as part of treatment. It is a prescription medicine, it has side effects (tingling fingers, more frequent urination and fizzy drinks tasting flat are common) and it is not suitable for everyone, including some people with sulfa allergies. Discuss it with your doctor before travel rather than buying it on the trail.
Make sure your travel insurance explicitly covers trekking to the maximum altitude on your itinerary, including helicopter rescue. Many standard policies stop at 3,000 or 4,000 metres. Read the small print, and carry your policy number and the insurer’s emergency number where your guide can find them. If altitude worries you, a lower route such as the Poon Hill Trek is a superb introduction to the Himalaya, and our seasonal guide will help you choose when to go.
