Altitude Sickness: Symptoms, Causes and Treatment
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Altitude Sickness: Symptoms, Causes and Treatment
A UK GP's guide to acute mountain sickness: the symptoms to watch for, the heights where it starts, how to prevent it and what to do if it happens.
Part of the Access Doctor Altitude Sickness Guide.
Key fact: Altitude sickness usually starts above 2,500m and is caused by going up too fast. Going up slowly is the best prevention, and going down is the best treatment.
Altitude sickness, also called acute mountain sickness (AMS), happens when you climb to a high altitude faster than your body can adjust to the thinner air. It’s common on treks such as Kilimanjaro and Everest Base Camp, and in high cities such as Cusco and La Paz.
Most cases are mild and settle with rest, but it can develop into two life-threatening conditions affecting the brain and the lungs. Knowing the signs, and acting on them early, keeps a trip safe.
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Start Altitude Sickness Consultation →What Is Altitude Sickness?
The higher you go, the lower the air pressure, so each breath takes in less oxygen. Your body adapts by breathing faster and deeper, and over days by making more red blood cells. This is called acclimatisation. Altitude sickness happens when you go up faster than you can acclimatise.
Travel health guidance calls 1,500 to 3,500m high altitude, 3,500 to 5,500m very high altitude, and above 5,500m extreme altitude.
Symptoms of Altitude Sickness
Symptoms usually start 6 to 24 hours after arriving at a new height. They often feel like a hangover:
- Headache
- Feeling or being sick, and loss of appetite
- Dizziness
- Tiredness and weakness
- Poor sleep
The three golden rules: 1. If you feel unwell at altitude, assume it’s altitude sickness until proven otherwise. 2. Never go higher to sleep while you have symptoms. 3. If you’re getting worse, go down straight away.
Severe Altitude Sickness: HACE and HAPE
Two rarer forms of altitude illness are medical emergencies. They can follow AMS or appear without warning.
HACE: swelling of the brain
High-altitude cerebral oedema. Confusion, clumsiness and unsteady walking (like being drunk), unusual behaviour, drowsiness and loss of consciousness.
HAPE: fluid on the lungs
High-altitude pulmonary oedema. Breathlessness that gets worse, including at rest or lying flat, a cough that becomes wet, chest tightness and pink or blood-stained spit.
If someone has signs of HACE or HAPE, they must go down immediately, with someone accompanying them, and get medical help. Give oxygen if it’s available. Never leave a person with altitude illness alone.
Who Is at Risk?
Anyone can get altitude sickness. Being young, fit or an experienced walker doesn’t protect you. What matters most is how high you go, how fast you get there, and whether you’ve had it before.
| Risk | Examples (if you’re not acclimatised) |
|---|---|
| Low | No previous altitude sickness and going no higher than 2,800m; or taking 2 or more days to reach 2,500–3,000m, then going up gradually |
| Moderate | Previous AMS and going to 2,500–2,800m in one day; or no previous AMS and going above 2,800m in one day |
| High | Previous HACE or HAPE; previous AMS and going above 2,800m in one day; going above 3,500m in one day; or very fast ascents, such as Kilimanjaro in under 7 days |
These categories come from the Wilderness Medical Society and are used by UK travel health services. See what they mean for popular trips in altitude sickness by destination.
How to Prevent Altitude Sickness
- Go up slowly: above 3,000m, sleep no more than 500m higher each night
- Take rest days: spend an extra night at the same height every 3 to 4 days
- Avoid big jumps: where you can, don’t travel from below 1,200m to above 3,500m in one day
- Don’t go higher with symptoms
- Consider preventive tablets if your risk is moderate or high
Acetazolamide is the medicine UK travel health services recommend when tablets are needed. It’s used off-label for this, which means outside its product licence. Read how to prevent altitude sickness, our guide to acetazolamide for altitude sickness, and altitude sickness medication compared. You can request it through our altitude sickness tablets service.
How Altitude Sickness Is Treated
Stop and rest
Don’t go any higher. Stay at the same height for 24 to 48 hours and tell the people you’re with.
Treat the symptoms
Paracetamol or ibuprofen for headache, an anti-sickness medicine if you have one, and plenty of fluids. Avoid alcohol and sleeping tablets.
Go down if you’re not improving
If you don’t feel better after a day, or you feel worse, go down by at least 500m.
Go down immediately for severe symptoms
Confusion, unsteady walking, or breathlessness at rest mean descending straight away and getting medical help.
Only go higher again once your symptoms have completely gone.
Before You Travel
- Check your itinerary: look at the sleeping height each night, not just the summit
- See a travel clinic, GP or pharmacist at least 4 to 6 weeks before you go, especially if you have a heart or lung condition, sickle cell disease or are pregnant
- Check your travel insurance covers the heights you’ll reach and helicopter evacuation
- Children: they can get altitude sickness too and may not be able to describe it; get advice before taking young children high
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Start Altitude Sickness Consultation →Frequently Asked Questions
What is altitude sickness?
Altitude sickness, or acute mountain sickness, is a set of symptoms caused by going to a high altitude faster than your body can adapt to the lower oxygen. It usually starts above 2,500m and causes headache, nausea, dizziness and tiredness.
At what height does altitude sickness start?
Usually above 2,500m (about 8,200 feet). The risk rises the higher you go and the faster you get there, especially if you fly or drive straight to a high place.
What does altitude sickness feel like?
Most people describe it as like a bad hangover: a headache with nausea, dizziness, tiredness and poor sleep, usually starting within a day of arriving at a new height.
How long does altitude sickness last?
Mild altitude sickness usually settles within 1 to 3 days if you stay at the same height and rest. If it isn't improving after a day, or it's getting worse, go down.
Can you die from altitude sickness?
Yes, rarely. Altitude sickness can develop into swelling of the brain (HACE) or fluid on the lungs (HAPE), which can be fatal within hours if the person doesn't go down. Acting early on symptoms prevents this.
Does being fit prevent altitude sickness?
No. Fitness, age and sex don't reliably predict who gets it. Fit people sometimes go up faster, which raises their risk. A previous episode is the best predictor.
Can children get altitude sickness?
Yes. Children are affected as often as adults, but young children may not be able to describe their symptoms. Get advice from a travel clinic before taking children to high altitude.
Is altitude sickness the same as acute mountain sickness?
Acute mountain sickness (AMS) is the most common form of altitude illness. The term altitude sickness is often used to mean AMS, and also covers the severe forms, HACE and HAPE.
References
- NaTHNaC, TravelHealthPro. Altitude illness. travelhealthpro.org.uk
- NHS. Altitude sickness. nhs.uk
- NHS Specialist Pharmacy Service. Using medicines for preventing acute mountain sickness. sps.nhs.uk
- Luks AM, et al. Wilderness & Environmental Medicine. Wilderness Medical Society clinical practice guidelines for the prevention, diagnosis and treatment of acute altitude illness: 2024 update. journals.sagepub.com
- Fit for Travel (Public Health Scotland). Altitude and travel. fitfortravel.nhs.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Prescription medicines require a consultation before they can be dispensed. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.
