In short: Altitude sickness (altitude illness) can affect anyone who goes too high too quickly, usually above 2,500 metres. The most effective prevention is a gradual ascent: above 3,000 metres, avoid raising your sleeping altitude by more than about 500 metres a day and build in a rest day every three or four days. Mild symptoms mean stop climbing and rest; worsening symptoms, confusion, unsteadiness or breathlessness at rest mean descend urgently.

This guide, written by a Ready Health prescribing pharmacist and based on UK travel health guidance from NaTHNaC (TravelHealthPro), explains how to recognise, prevent and manage altitude illness.

What is altitude sickness?

At high altitude the air pressure is lower, so each breath delivers less oxygen. Given time, the body adjusts, a process called acclimatisation. If you climb faster than your body can adjust, you can develop altitude illness. There are three main forms:

Acute mountain sickness (AMS): the most common and usually mild form.
High altitude cerebral oedema (HACE): swelling of the brain, often following AMS. It is life-threatening.
High altitude pulmonary oedema (HAPE): fluid in the lungs, which can occur without AMS. It is life-threatening.

Where is altitude sickness a risk?

Popular destinations where UK travellers may be at risk include treks in Nepal such as Everest Base Camp, Mount Kilimanjaro in Tanzania, the Inca Trail and Cusco in Peru, La Paz in Bolivia, Quito in Ecuador and parts of Tibet, as well as high Alpine and Rocky Mountain routes. Flying or driving directly to a high city, or a fast-paced trek, increases the risk.

Who is most at risk?

people who ascend quickly or sleep at much higher altitudes from one day to the next
anyone who has had altitude illness before (the best predictor of future risk)
people who normally live near sea level
people who over-exert themselves soon after arriving

Being physically fit does not protect you. Some heart and lung conditions, and other health problems, can make altitude more risky, so seek advice before booking.

What are the symptoms of altitude sickness?

AMS usually starts 6 to 10 hours after arriving above 2,500 metres. The main symptom is headache, together with one or more of:

loss of appetite, nausea or vomiting
dizziness or light-headedness
tiredness or weakness

Warning signs of HACE: confusion, unusual behaviour, drowsiness, loss of coordination or being unable to walk in a straight line.

Warning signs of HAPE: breathlessness out of proportion to effort, then breathlessness at rest, a cough that may bring up frothy or bloodstained sputum, chest tightness and reduced ability to exercise.

At altitude, any new symptoms should be treated as altitude illness until proven otherwise.

How can I prevent altitude sickness?

Where possible, avoid going from below 1,200 metres to above 3,500 metres in a single day.
Above 3,000 metres, increase your sleeping altitude by no more than about 500 metres a day.
Plan a rest day at the same altitude every three or four days.
If you have to fly or drive straight to a high city, take it easy for the first few days and add extra rest days.
Choose itineraries that allow time to acclimatise; for example, very fast ascents of Kilimanjaro carry a higher risk.
Avoid heavy exertion and alcohol for the first day or two at a new altitude.
Travel with an experienced guide, never leave someone with symptoms alone, and make sure your travel insurance covers your maximum altitude and helicopter evacuation.

Is there medication to prevent altitude sickness?

For most low-risk trips, medication is not needed and a gradual ascent is enough. For people at moderate or high risk, for example those who must ascend quickly or who have had altitude illness before, a prescriber may consider a medicine called acetazolamide alongside, never instead of, a sensible ascent plan.

It is a prescription-only medicine and its use to prevent altitude illness is outside its UK licence, so a prescriber must assess whether it is suitable for you.
Common side effects include tingling in the fingers and toes, needing to pass urine more often, nausea and a change in the taste of fizzy drinks. A short trial before travel is often advised.
It is not suitable for everyone, including people who are pregnant and those with a severe allergy to sulphonamide (sulfa) medicines, and it needs caution with some kidney and liver conditions.

Herbal products such as ginkgo and coca are not recommended for prevention, because the evidence is lacking or inconclusive.

What should I do if I get altitude sickness?

Mild AMS: do not go any higher. Rest, drink enough fluids and use simple pain relief such as paracetamol or ibuprofen, if suitable. Symptoms often settle within one to three days.
Symptoms worsening or not improving: descend.
Any signs of HACE or HAPE: descend immediately, even at night, and seek urgent medical help. Oxygen and emergency medicines may be used by guides or medics to support descent, but descent is the priority.

Frequently asked questions

At what altitude does altitude sickness start?

It is most common above 2,500 metres, but some susceptible people notice symptoms lower than this.

Can fit people get altitude sickness?

Yes. Fitness does not protect you, and very fit people may climb too fast.

How long does it take to acclimatise?

It varies, but most people adapt over a few days at each new altitude. That is why rest days and a gradual increase in sleeping altitude matter.

Can children go to high altitude?

Yes, with careful planning, but young children may not be able to describe symptoms. Watch for fussiness, poor feeding, unusual sleepiness or vomiting, and seek specialist advice before travel.

Key takeaways

Altitude illness usually occurs above 2,500 metres and can affect anyone, however fit.
Gradual ascent with rest days is the most effective prevention.
Never go higher with symptoms; descend if symptoms worsen or if there are signs of HACE or HAPE.
Preventive medication is only for some higher-risk trips and needs individual assessment by a prescriber.

Sources and further reading

Last reviewed: October 2026 by Shamir Patel, Independent Prescribing Pharmacist, Ready Health. This article is general information and does not replace advice from your own clinician.

Planning a trek or a trip to a high-altitude city? A travel health consultation at Ready Health in Standish, serving Wigan, Chorley and Bolton, can cover your itinerary, vaccines and altitude plan; the prescriber will advise whether any medicine is suitable for you. You can also book online.

Altitude Sickness: Symptoms, Prevention and Treatment