Admin UserAltitude sickness is the biggest medical risk on high-altitude treks in Nepal. It can affect anyone, regardless of age or fitness, but it is largely preventable with sensible planning and honest self-monitoring. Understanding the symptoms, the rules of acclimatisation and what to do if things go wrong could save your trek, or your life. This guide explains what altitude illness is, who is at risk, how to prevent it, how to recognise warning signs and when to descend.
Medical note: This article is general information, not medical advice. Consult a travel-medicine doctor before your trip, especially if you have heart, lung or other chronic conditions.
What is altitude sickness?
Altitude sickness, also called acute mountain sickness (AMS), develops when your body cannot adjust quickly enough to the lower oxygen levels at height. It usually begins above 2,500 m, and risk rises sharply above 3,500 m. Air pressure falls with altitude, so each breath contains less oxygen. Your body adapts by breathing faster, increasing heart rate and over days making more red blood cells, but this process takes time. When you ascend faster than your body can adapt, symptoms appear.
Video: How To Prevent Altitude Sickness in Nepal (Women Want Adventure on YouTube)
Who is at risk?
Anyone can get altitude sickness, and fitness does not protect you. Risk factors include:
- Ascending too quickly, especially sleeping altitude gains of more than 500 m per night above 3,000 m
- Previous altitude illness
- Flying directly to a high altitude (for example, Lukla at 2,860 m)
- Dehydration, alcohol and sleeping pills
- Exertion without rest
Children and older adults can be affected as well, but age alone is not a strong predictor.
Symptoms to watch for
Mild AMS
Headache plus one or more of: nausea, dizziness, fatigue, loss of appetite and poor sleep. Many trekkers experience mild symptoms at some point.
Severe warning signs
- HACE (high-altitude cerebral oedema): confusion, loss of coordination (ataxia), drowsiness, severe headache unrelieved by medication. A simple test is to ask the person to walk heel to toe in a straight line.
- HAPE (high-altitude pulmonary oedema): breathlessness at rest, persistent cough, pink or frothy sputum, extreme fatigue, blue lips.
Severe symptoms require immediate descent and medical help. Do not wait until morning.
The golden rules of acclimatisation
- Climb high, sleep low. Do day hikes higher than where you sleep.
- Do not ascend more than 300–500 m of sleeping altitude per day above 3,000 m.
- Take a rest day every 3–4 days or every 1,000 m of ascent.
- Never go higher with symptoms. If symptoms worsen, descend.
- Hydrate (3–4 litres daily) and eat enough carbohydrates.
- Avoid alcohol and sleeping pills at altitude.
- Tell your guide how you feel, every day, honestly.
What the Lake Louise score is, and why guides use it
Many guides and doctors use a simple symptom checklist, the Lake Louise Score, to measure the severity of AMS. It rates headache, gastrointestinal symptoms, fatigue and dizziness on a scale of 0–3 each. A low score with a mild headache may allow you to continue with caution, while a rising score or any severe symptom means rest or descent. You do not need to memorise it, but it helps to know that your guide's questions are structured and meant to catch problems early.
Pulse oximeters: useful, not magic
Guides often use a finger pulse oximeter to check oxygen saturation. At 4,000 m, readings in the 80s are common in healthy people; readings drop with altitude and during sleep. The number alone does not diagnose AMS, but a sudden fall, or a low reading combined with symptoms, is a warning sign. Your guide will look at the whole picture: your symptoms, your behaviour, your heart rate and your oxygen levels.
Diamox (acetazolamide)
Many trekkers take acetazolamide as a preventative, but it should be discussed with your doctor beforehand. It speeds up acclimatisation by helping you breathe more deeply, and it does not replace proper acclimatisation. Common side effects include tingling hands and feet, increased urination and a metallic taste in carbonated drinks. People allergic to sulfa drugs should not take it. Do not take unfamiliar medication from other trekkers.
What to do if you feel sick
- Mild symptoms: stop ascending, rest, drink water, eat and monitor. If symptoms improve in 24 hours, you can continue slowly; if they worsen, descend.
- Moderate symptoms: descend at least 300–500 m and rest.
- Severe symptoms: descend immediately with assistance, use supplemental oxygen if available and arrange evacuation.
Never leave a person with severe symptoms alone, and never ignore symptoms to keep to an itinerary.
Recommended itineraries
Ask your agency for a schedule with built-in rest days. Our Everest Base Camp 14-day itinerary follows these principles, and the Annapurna Circuit and Thorong La guide explain how to handle the crossing. Lower-altitude options are covered in our short treks guide.
What guides and lodges can do
Reputable guides carry pulse oximeters, a first-aid kit and know when to descend. The Himalayan Rescue Association runs aid posts at Pheriche (Everest) and Manang (Annapurna) that give free daily altitude lectures; see the HRA website. Lodges can provide warm rooms, hot drinks and a place to rest, but they cannot provide medical care for severe illness.
Altitude at a glance for popular places
For context, Kathmandu sits at about 1,400 m, Lukla at 2,860 m, Namche Bazaar at 3,440 m, Manang at 3,540 m, Dingboche at 4,410 m, Thorong La at 5,416 m and Everest Base Camp at 5,364 m. Knowing the altitude of each stop helps you judge how fast you are climbing and where to add a rest day. Ask your guide to share the plan with you each evening so you understand the next day's gain.
Preparing before you go
Build your fitness (training plan), see a travel doctor, check your medication, plan an itinerary with acclimatisation days and buy insurance that covers helicopter evacuation up to your maximum altitude. If you have a history of altitude illness, discuss prevention strategies with a doctor before booking.
A sample acclimatisation day
On a rest day at 3,500–4,500 m, wake up and have breakfast as usual, drink plenty of fluids and walk to a viewpoint 300–500 m higher than where you slept. Take your time, rest at the top and return to the same altitude to sleep. Spend the afternoon resting, eating and drinking. This "climb high, sleep low" pattern gives your body a taste of altitude without a big jump in sleeping height, and it is the best-supported method for preventing AMS. Do not treat rest days as lazy days: gentle movement helps circulation and acclimatisation more than staying in bed.
Sleeping at altitude
Poor sleep is common above 3,500 m, with periodic breathing (cycles of fast and slow breaths) causing awakenings. It is not dangerous in itself, but it can be unsettling. Keep warm, avoid alcohol and sleeping pills, drink water before bed (but not so much that you wake repeatedly), and sleep with your head slightly raised. Tell your guide if you wake with a severe headache or breathlessness.
Eating and drinking for altitude
Appetite often falls at altitude, but your body needs energy. Eat small, frequent carbohydrate-rich meals such as dal bhat, pasta, porridge and soup. Sip fluids throughout the day: aim for pale yellow urine. Warm drinks help, and ginger tea is a popular local remedy for nausea. Avoid alcohol until after the highest point, because it dehydrates and interferes with sleep and acclimatisation.
Myths about altitude sickness
- "Fit people don't get it." Fitness does not protect you from AMS.
- "Drinking more water cures it." Hydration helps but does not treat AMS.
- "It only happens above 5,000 m." Symptoms can appear from about 2,500 m.
- "You will know when you're in trouble." Confusion and poor judgement are part of severe illness, so rely on your group.
- "Drugs mean you can skip acclimatisation." Medication supports acclimatisation, but does not replace it.
Special situations: children, older trekkers and pre-existing conditions
Children can get altitude illness, and symptoms are harder to spot, so keep sleeping altitudes lower and rest more. Older trekkers are not necessarily at higher risk, but they should check their heart and lung health with a doctor and allow extra time. People with conditions such as asthma, diabetes, heart disease or sickle-cell trait should get specialist advice before going high, and should tell their guide and insurer. Pregnant travellers should consult an obstetrician and usually avoid sleeping above about 3,000 m.
Insurance and evacuation
Make sure your policy covers trekking to your maximum altitude and helicopter evacuation. Read our travel insurance guide for what to check. For further public health advice, consult the CDC high-altitude guidance.
FAQ
At what altitude does altitude sickness start?
Typically above 2,500 m, with higher risk above 3,500 m.
How do I prevent altitude sickness?
Ascend gradually, hydrate, rest regularly and descend if symptoms develop.
Does fitness protect against altitude sickness?
No. Fit people can still get AMS, so pace and acclimatisation matter more than fitness.
Should I take Diamox?
Ask your doctor. It can help prevent symptoms for some trekkers but is not a substitute for a good itinerary.
What is the difference between AMS, HAPE and HACE?
AMS is the mild form; HAPE affects the lungs and HACE affects the brain. HAPE and HACE are medical emergencies requiring immediate descent.
