Altitude Sickness and High-Altitude Travel: Clinical Risk, Prevention and the Role of Acetazolamide

Reviewed by GPhC Pharmacist at Priory Pharmacy, York

High-altitude trips are more popular than ever. Trekking in Nepal, climbing Kilimanjaro, hiking the Inca Trail, skiing at altitude, visiting cities like Cusco, La Paz, or Lhasa — these are now common trips, even for people who wouldn’t call themselves serious mountaineers.

Altitude sickness deserves real attention. It’s common. And in a small number of cases, it can turn dangerous very quickly.

Understanding Altitude Sickness: Three Levels

Altitude sickness happens because there’s less oxygen available in the air as you climb higher.

• At sea level, the air has plenty of oxygen.
• At 3,000 metres, oxygen availability drops by about a third.
• At 5,000 metres, it drops by about half.

Your body adjusts to this over time. If you climb too fast, it can’t keep up — and that’s when illness develops. There are three levels, from mild to severe:

1. Acute Mountain Sickness (AMS) the common, mild form

• The most common type of altitude illness.
• Usually starts above 2,500–3,000 metres.
• Main symptom: headache plus at least one of: nausea, vomiting, tiredness, dizziness, or poor sleep.
• Usually appears 6–12 hours after arriving at altitude.
• In most cases, it settles with rest and no further climbing.

2. High-Altitude Cerebral Oedema (HACE) a medical emergency

• The most serious brain-related form of altitude illness.
• Symptoms: severe headache, loss of coordination (stumbling, unable to walk in a straight line), confusion.
• Left untreated, it can lead to coma and death.
• Treat this as a medical emergency. Descend immediately.

3. High-Altitude Pulmonary Oedema (HAPE) fluid in the lungs

• The most common cause of death from altitude illness.
• Caused by fluid building up in the lungs.
• Symptoms: breathlessness even at rest, a cough (dry at first, later with pink frothy phlegm), reduced ability to exercise, bluish lips or fingertips.
• Needs immediate descent and oxygen.
• Can happen without any warning signs of AMS beforehand and can appear even on a second night at the same altitude.

Who’s at Risk?

The single biggest factor is how fast you climb. Climbing too quickly doesn’t give your body time to adjust.

Other risk factors:

• Climbing above 2,500m too fast, without proper acclimatisation
• A previous episode of altitude illness this is the strongest predictor of future risk
• Sleeping above 3,500m
• Strenuous exercise in your first 24–48 hours at altitude

Important myth to bust: fitness does not protect you. A very fit athlete and a less active person, climbing at the same speed, have a similar risk of altitude sickness. Being fit lets you push harder physically but it doesn’t help your body acclimatise any faster.

The Best Prevention: Climb Slowly

The single most effective way to avoid altitude sickness is a gradual ascent, giving your body time to adjust.

General guidance for treks above 3,000m:

• Don’t increase your sleeping altitude by more than 300–500 metres per day
• Take a rest day (no altitude gain overnight) for every 1,000 metres climbed

“Climb high, sleep low”:

• Hike to a higher point during the day.
• Return to a lower altitude to sleep at night.
• This helps your body adjust while reducing overnight strain.

Well-planned treks (like standard Everest Base Camp itineraries) build this in — which is exactly why properly paced treks have fewer AMS cases than rushed ones, even reaching the same final height.

Acetazolamide (Diamox): Prevention You Can Take

Acetazolamide is a prescription medicine that helps your body acclimatise faster.

How it works (in plain terms):

• It changes how your kidneys handle acid in your blood.
• This encourages you to breathe faster and deeper.
• That improves how much oxygen gets into your blood speeding up your natural acclimatisation process.

How well it works:

• Trials show it cuts your risk of AMS by about 50%, compared to not taking anything.

Typical dose: 125–250mg, twice a day, starting 24–48 hours before you climb, continued for 2 days once you reach your highest point.

Who it’s usually recommended for:

• Travellers on a fast ascent like compressed Kilimanjaro climbs, or flying straight into high-altitude cities like Cusco or La Paz
• Anyone with a previous history of altitude illness
• Anyone whose itinerary can’t be adjusted to climb more slowly

Important: This is a prescription-only medicine you’ll need a clinical assessment first.

Who shouldn’t take it:

• Anyone allergic to sulfonamide medicines (acetazolamide contains a related ingredient)
• Anyone with severe liver or kidney problems
• Anyone on certain other medications, including some blood thinners — this needs checking with a clinician

Common, harmless side effects:

• Tingling in fingers and toes
• Needing to urinate more often
• Fizzy drinks taste flat an odd but harmless effect worth knowing about in advance

The one rule that matters most: if you get worse, go down.

If you develop loss of coordination, confusion, breathlessness at rest, or your AMS symptoms get worse despite resting descend immediately. Even going down 300–500 metres can bring real improvement in HACE and HAPE.

Don’t delay descent for darkness, bad weather, or logistics if symptoms are serious.

Oxygen and portable pressure bags (Gamow bags) can help while you’re getting down — but they’re a bridge to descent, not a substitute for it.

Emergency medicines (dexamethasone for HACE, nifedipine for HAPE) are sometimes carried on expeditions — but only use these with specific guidance from a clinician beforehand.

This article is for general information only. It’s not personal medical advice. For altitude sickness prevention advice and an acetazolamide prescription, please book a travel health consultation at Priory Pharmacy in York.