Altitude sickness

About altitude sickness

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).

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.

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.

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.

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.

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.


Produced by Rebecca Canvin, Bupa Health Information Team, November 2012.

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