Altitude illness

Altitude illness includes a number of conditions that may occur in individuals travelling to high altitude, usually above 2,500 metres (8,200 feet). Most trips to altitude can be enjoyed safely if sensible precautions are taken. High altitude is defined as an elevation above 1,500m and can be divided into the following categories: high altitude 1,500 to 3,500m, very high altitude 3,500 to 5,500m and extreme altitude above 5,500m.

If an individual ascends gradually to high altitude, their body is usually able to adjust to the reduced oxygen levels. This process is known as acclimatisation. If ascent is too swift, then acclimatisation may not occur rapidly enough and altitude illness may follow.

Altitude illness includes: acute mountain sickness (AMS), high altitude cerebral oedema (HACE) and high altitude pulmonary oedema (HAPE). Severe AMS, HACE and HAPE are life-threatening conditions that need urgent attention.

Symptoms include:

  • AMS – headache, loss of appetite, nausea, vomiting, dizziness, sleep disturbance, fatigue and weakness.
  • HACE – confusion, altered consciousness and incoordination.
  • HAPE – increasing breathlessness, breathlessness lying flat, cough (initially dry then wet), chest tightness and blood tinged sputum.


The key to preventing high altitude illness is gradual ascent with regular rest days. Medications may be used to help prevent altitude illness in certain individuals. People with pre-existing medical conditions should consult their healthcare provider prior to travel.

General advice

  • Awareness of the symptoms of altitude illness. Symptoms at altitude are caused by altitude illness until proven otherwise.
  • Never ascend to sleep at a higher altitude in the presence of symptoms of altitude illness.
  • Always attempt to descend if symptoms of altitude illness worsen at a given altitude or if symptoms are severe.
  • Never leave an individual with altitude illness alone.
  • Always trek with an experienced guide.
  • Travel insurance should adequately cover the itinerary and activities planned. The maximum altitude should be disclosed and emergency evacuation by helicopter included within the policy.
  • Where possible travel from altitudes less than 1,200m to altitudes greater than 3,500m in a single day should be avoided.
  • Above 3,000m avoid increasing sleeping elevation by more than 500m a day and ensure a rest day (at the same altitude) every three or four days.


  • Preventative medications are not necessary for low risk situations (see below) and individuals should rely on gradual ascent.
  • Preventative medications may be considered in addition to gradual ascent in moderate or high risk situations (see below).
  • Preventative medications are not a substitute for gradual ascent.
  • Acetazolamide (Diamox®) is the preferred drug (unlicensed). The recommended dose is 125mg twice daily to be commenced one to two days prior to reaching 3,500m and then continued for at least two days after reaching the highest altitude.
  • A trial dose of Diamox® for one or two days should be taken prior to travel to check for side effects which include: increased urine production (diuresis), pins and needles (paraesthesia), nausea, vomiting, headache and taste disturbance.
  • Diamox® should not be used in those with severe allergy to sulfa-based drugs and in pregnant women particularly in the first trimester.
Risk category Description
  • Individuals with no prior history of altitude illness and ascending to ≤2,800m
  • Individuals taking ≥2 days to arrive at 2,500-3,000 m with subsequent increases in sleeping elevation <500 m/day and an extra day for acclimatisation every 1,000m
  • Individuals with a history of AMS ascending to 2,500-2,800m in one day
  • Individuals with no history of AMS ascending to >2,800m in one day
  • All individuals ascending >500 m/day (in sleeping elevation) at altitudes above 3,000m but with an extra day for acclimatisation every 1,000m
  • Individuals with a history of AMS ascending to >2,800m in one day
  • All individuals with a history of HACE
  • All individuals ascending to >3,500m in one day
  • All individuals ascending >500 m/day (in sleeping elevation) above 3,000m without extra days for acclimatisation
  • Very rapid ascents (e.g. < 7 day ascents of Kilimanjaro)


  • Altitudes listed in the table refer to the altitude at which the person sleeps.
  • Altitude is assumed to start from elevations <1,200m.
  • The risk categories described above pertain to unacclimatised individuals.




The National Travel Health Network and Centre (NaTHNaC) is commissioned by Public Health England to provide health information for both healthcare professionals and travellers. Information is compiled by the NaTHNaC clinical and scientific team, and updated regularly. Further advice on health risks and disease outbreaks is available at

Date last reviewed: November 2015


MIMS Clinics

Prescribing news and resources for key therapeutic areas, collated by the MIMS editors.

Register or Subscribe to MIMS

GPs can get MIMS print & online and GPonline for free when they register online – take 2 minutes, and make sure you get your free MIMS access! If you're not a GP, you can subscribe to MIMS for full access.

Register or subscribe

MIMS bulletins

News and updates straight to your inbox.

Prescribing Update: Fortnightly news bulletin
Urgent prescribing updates
Spotlight: Disease-themed monthly round-up

Sign me up

MIMS Dermatology

Read the latest issue online exclusively on MIMS Learning.

Read MIMS Dermatology

Mobile apps

MIMS: access the full drug database and quick-reference tables on the go

MIMS Diagnosis and Management: concise information on signs and symptoms, investigations and diseases

Promo Image

Clinical calculators

Handy calculators and conversions for primary care.