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Altitude sickness

Altitude sickness generally refers to a condition known as acute mountain sickness (AMS). It affects people who travel to high altitude. Altitude sickness is more common, and becomes more severe, with increasing altitude.

If you ascend to an area of high altitude, you may experience some form of altitude sickness. Usually, the symptoms are mild and will improve if you either stop and rest, or descend.

Altitude sickness is most likely to affect you if you ascend quickly – especially at a rate of more than 500m per day – or if you don't allow yourself time to get used to (acclimatise to) an altitude.

AMS is the most common and mildest form of altitude sickness and can affect anyone ascending to high altitude, even if they have previously been unaffected. High-altitude pulmonary oedema (HAPE) and high-altitude cerebral oedema (HACE) are much more severe forms of altitude sickness. They are more commonly seen in climbers at very high or extreme altitude (especially if they have ascended quickly).

What counts as high altitude?

  • High altitude refers to heights that are between 1,500 and 3,500m above sea level, for example Cusco in Peru (just over 3,300m).
  • Very high altitude is from 3,500 to 5,500m above sea level, for example La Paz in Bolivia (about 3,600m) and the Inca Trail (4,198m).
  • Extreme altitude is over 5,500m above sea level, for example Mount Kilimanjaro (5,985m).
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An image of Mount Everest


  • Symptoms Symptoms of altitude sickness

    You will usually notice symptoms of altitude sickness about six to 12 hours after you have arrived at an area of high altitude, but symptoms can take up to 24 hours to develop. The time it takes for your symptoms to develop varies from person to person and will depend on the speed of your ascent. The more rapidly you ascend to high altitude, the more likely it is that AMS will develop.

    If you have AMS, your symptoms may include:

    • a headache (this is almost always present)
    • tiredness
    • sickness or vomiting
    • loss of appetite
    • dizziness
    • difficulty sleeping

    If you’re unwell and not sure if you have AMS, you should seek medical advice. If none is available, you should presume you have AMS and follow the self-help section below until you can get help.

    Symptoms of AMS usually start to ease after a day or two as your body acclimatises to high altitude, particularly if you don't ascend any further. If your symptoms don’t resolve after a day or two, the best thing you can do is descend as quickly as possible, by at least 500 to 1,000m. There are some medicines that can help ease your symptoms and treat complications, but if you have AMS, the best treatment is to descend to a lower altitude.

  • Treatment Treatment of altitude sickness


    If you have symptoms of AMS, you shouldn't ascend any higher. Rest for between 24 and 48 hours and your symptoms will often get better. You should:

    • make sure you drink enough fluid
    • take painkillers, such as paracetamol or ibuprofen, if you have a headache
    • take anti-sickness medicines, such as cinnarizine or promethazine, or other common antihistamines to ease sickness and dizziness

    You can buy painkillers and anti-sickness medicines from a chemist without a doctor's prescription. Always read the patient information that comes with your medicine and if you have any questions, ask your pharmacist for advice.

    If your symptoms improve, it’s safe for you to slowly ascend higher. If they don’t improve, or they get worse, you should descend by at least 500m.

    If someone has symptoms of HACE or HAPE, they will need immediate evacuation to a lower altitude and medical attention. These are life threatening conditions that can rapidly lead to death if not treated appropriately.


    If your symptoms become more severe, you may need to breathe bottled oxygen in via a face mask. You may also need hyperbaric treatment – this is where you enter a special, pressurised chamber to help get your oxygen levels back to normal. Hyperbaric, or Gamow bags are a special form of large pressurised bag that are only usually taken on large climbing expeditions when descent may be difficult.

    On any trip, there should be one or more people trained to administer first aid and you should have an emergency plan in place for evacuation to a medical facility. Oxygen must be given to anyone thought to have HACE or HAPE.


    If you need medicines for AMS then you should ensure that you don’t ascend any higher and descend as soon as possible. The first line of treatment is usually a medicine called acetazolamide (Diamox). If your symptoms persist or get worse (or you’re allergic to acetazolamide) the next treatment is a medicine called dexamethasone.

    If you have HAPE, you may be given a medicine called nifedipine. Sildenafil (Viagra) also helps relieve the symptoms of HAPE.

    HACE is treated with dexamethasone but may need to be given through a vein or muscle if you can’t swallow properly.

    Occasionally these medicines are given at location by a first aider, particularly if you’re unable to descend quickly and safely. The earlier they are taken, the greater their effectiveness.

    Ask a doctor in a travel clinic about these medicines because you can only get them on prescription. Always ask your doctor for advice and read the patient information that comes with your medicine.

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  • Causes Causes of altitude sickness

    At high altitudes, the air is at a lower pressure than it is nearer sea level. This means there is less oxygen in the air you breathe, and so less gets into your bloodstream. All cells in your body need oxygen to survive. Your body responds by increasing your breathing, heart rate and, after some time, the number of red blood cells in your blood.

    There are certain factors that may make you more likely to develop altitude sickness; for example, if you:

    • go too quickly to high altitude
    • have had altitude sickness before
    • do strenuous activity or exercise at high altitude
    • sleep at high altitude (you’re less likely to develop AMS if you climb up high in the day and return to a lower altitude to sleep)

    If you have diabetes, or a heart or lung condition, you should check with your GP before travelling to places at altitude.

  • Complications Complications of altitude sickness

    If you ascend to an area of high altitude too quickly and don't allow your body time to acclimatise, you may develop a more serious form of altitude sickness such as HAPE or HACE. These conditions are less common than AMS, but potentially fatal if you don’t descend immediately and receive medical treatment.

    HAPE occurs when fluid builds up in your lungs and makes you breathless. Symptoms usually develop between two and four days after you have ascended to high altitude. HAPE can develop after AMS, or occur without any prior altitude sickness.

    If you have HAPE, your symptoms may include:

    • a dry cough
    • breathlessness, even at rest
    • weakness/lethargy
    • having pink or bloody spit
    • having a bluish tinge to your skin, lips and nails (cyanosis)

    If you have symptoms of HAPE, you may gasp for breath and make gurgling sounds when you breathe. HAPE can occur by itself or in combination with HACE.

    HACE occurs when excessive fluid collects in your brain, causing it to swell. Around one out of 100 people who trek above 3000m develop HACE, although it has been reported at lower altitudes. HACE can happen after severe AMS, or occur without any prior altitude sickness. If you have HACE your symptoms may include:

    • a severe headache
    • vomiting
    • confusion
    • loss of coordination – for example, being unable to walk in a straight line
    • blurred or double vision
    • changes in your behaviour - for example, you may feel irritable, be unhelpful or start behaving irrationally
    • severe lethargy and drowsiness
    • hallucinations (see, hear, feel and smell odd things that aren't really there)
    • loss of consciousness or coma

    The symptoms of HACE may progress rapidly, from mild to life-threatening, within a few hours. If you have any of these symptoms, you should descend to a lower altitude immediately and seek urgent medical advice.

    Never leave a person suspected of having HACE or HAPE on their own. At night time you must stay in the same room or tent as them and assess them regularly.

    You may develop other symptoms at high altitude. For example, your face, arms or legs may swell up (peripheral oedema). This usually lasts a few days and then disappears. At altitudes of 5,000m and over, tiny blood blisters can form at the back of your eye (retinal haemorrhages), but you may not notice any symptoms and they only occasionally interfere with your vision.

  • Prevention Prevention of altitude sickness

    There are a number of steps you can take to prevent altitude sickness. Wherever possible follow the steps below.

    • Acclimatise yourself to high altitudes by slowly ascending over several days.
    • On the first night, don’t sleep higher than 2,500 to 3,000m.
    • If possible, don’t sleep at an altitude more than 300m higher than the previous night. You can go higher in the day, as long as you go down to sleep.
    • Take occasional rest days during your trip, where you don’t sleep higher than the previous night.
    • Make sure you drink enough fluids and don’t drink caffeine or alcohol.
    • Eat small meals that are high in carbohydrate, such as pasta.
    • Don't do any strenuous exercise or activity for the first few days after you arrive at a high altitude and have rest days planned if you’re ascending further.
    • Don’t smoke.
    • Don’t take sedative medicines, such as sleeping tablets.

    It’s possible to use medicines to prevent the symptoms of altitude sickness, including acetazolamide and dexamethasone. However, ascending slowly is the best way to reduce the chances of developing AMS. For more advice, speak to your GP.

  • FAQs FAQs

    Can altitude sickness affect my sleep?


    Yes. Altitude sickness may keep some people awake at night but other factors can also affect your sleep at altitude.


    It's common for people who have recently arrived at altitude to have a poor night's sleep. Your sleep may be disturbed by a combination of the low temperature, an uncomfortable bed and altitude sickness. You may find you need to get up more during the night to urinate because your body will naturally make more urine as you acclimatise.

    Your breathing will become quicker at high altitude because it's harder for your body to take up oxygen from the air you breathe when the air pressure is low. For some people this can cause periodic breathing at night (deep breaths followed by shallow breaths or even a brief pause in breathing), which can wake you or others up.

    There are a number of things you can do at altitude to help get a better night's sleep.

    • Keep yourself warm by using a well insulated and comfortable sleeping bag.
    • Wear earplugs at night to keep out background noise.
    • Try to reduce your caffeine intake, especially before bedtime.
    • Don’t drink alcohol, smoke tobacco or take sleeping pills.
    • Only go to bed when you're feeling really sleepy.

    If you still have difficulty sleeping after a few nights at altitude, don't go any higher and consider descending so you can spend more time acclimatising.

    Can I have a reaction to acetazolamide for altitude sickness?


    Yes, some people can have unwanted side-effects from acetazolamide.


    Acetazolamide is sometimes used as a preventive medicine to decrease the symptoms of altitude sickness and in the treatment of acute mountain sickness (AMS). It works by speeding up acclimatisation but will not stop you getting altitude sickness.

    After taking acetazolamide, some people experience a tingling sensation (especially in their legs, hands and face) and need to urinate more often. Other possible side-effects include vomiting, headache, dizziness and diarrhoea. Acetazolamide may also alter the taste of fizzy drinks and occasionally people develop rashes.

    Your GP may recommend you take acetazolamide as a trial several weeks before you go away. If you don't have any unpleasant side-effects, then he or she will advise you to take between 125mg and 250mg twice a day, beginning one day before your ascent and continuing for two or three days until you have acclimatised at high altitude.

    Can children get altitude sickness?


    Yes. It's quite common for children to suffer from altitude sickness.


    People of all ages can experience altitude sickness when travelling to altitudes above 2,500m. Young children are less likely to say they have a problem because they may not understand what it is or tell you how they feel.

    The symptoms of altitude sickness are mostly the same for children as they are for adults. These can include vomiting, loss of appetite and difficulty sleeping. Your child may also not seem as playful as usual. It's important that you monitor your child for any changes in his or her health and behaviour.

    There are a number of steps you can take to prevent your child getting altitude sickness.

    • Move slowly and ascend over several days so your child can acclimatise to the high altitudes. You should ascend at a rate of 300m per day above 2500m and rest every 1000m.
    • Make sure your child drinks enough fluids to prevent dehydration.
    • Don't let your child do any strenuous activity for the first few days at high altitude.
    • Give your child frequent small meals rich in carbohydrates.

    Your GP may prescribe children weighing under 40kg preventive medicines in small doses to reduce the symptoms of altitude sickness. It's important that you discuss your travel plans with a doctor from a travel clinic who has experience in this area before you go and carry a card with your child’s weight, medicines and doses on.

  • Resources Resources

    Further information


    • Altitude sickness. National Travel Health Network and Centre., published July 2011
    • Altitude illness. Merck Online Medical Library., published February 2009
    • Altitude sickness. published February 2010
    • Travel at high altitude. MEDEX., published 2008
    • Clarke C. Acute mountain sickness: medical problems associated with acute and subacute exposure to hypobaric hypoxia. Postgrad Med J 2006; 82(973):748–53
    • Bartsch P, Gibbs JSR. Effect of altitude on the heart and the lungs. Circ 2007; 116:2191–202. doi:10.1161/CIRCULATIONAHA.106.650796
    • Maggiorini M. High altitude-induced pulmonary oedema. Cardiovasc Res 2006; 72:41–50
    • Joint Formulary Committee, British National Formulary. 64th ed. London: British Medical Association and Royal Pharmaceutical Society of Great Britain, 2012
    • Altitude sickness. BMJ Clinical Evidence., published 18 March 2010
    • Consensus statement of the UIAA Medical Commission vol: 9, Children at Altitude. International Mountaineering and Climbing Federation, 2008.
    • Personal communication, Dr Daniel Martin, Director, UCL Centre for Altitude, Space and Extreme Environment Medicine, May 2012
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