Altitude Sickness: A Traveller's Guide to Prevention and Diamox

Some of travel's greatest destinations sit high enough to make you ill just by arriving — Cusco, La Paz, the Everest trekking routes, and even a city break in Mexico City or Bogotá. Altitude sickness has nothing to do with fitness and everything to do with how fast you go up. The good news is that it's largely preventable once you understand the rules, and those rules are the same whether you're sightseeing in the Andes or trekking in the Himalaya.

This guide covers the three forms of altitude illness, the golden rules of ascent that prevent most cases, where the medication Diamox (acetazolamide) fits, and the warning signs that mean it's time to go down. It's based on CDC and Wilderness Medical Society guidance.

Why it happens

As you climb, the air thins and each breath delivers less oxygen. Your body can adapt — that's acclimatisation — but it needs time. Push up faster than your body can adjust, especially while sleeping at increasing heights, and you risk altitude illness. It typically becomes a concern above around 2,500 m (8,000 ft), which is why so many iconic high-altitude trips are affected. Cusco sits near 3,400 m; La Paz higher still; Everest Base Camp treks push past 5,000 m.

Crucially, being young or fit offers no protection — sometimes the opposite, because fit travellers ascend faster. Your best predictor of trouble is your own past history at altitude.

The three forms

Acute mountain sickness (AMS) is the common, milder form — think of it as a bad hangover at altitude: headache plus some combination of nausea or loss of appetite, fatigue, dizziness and poor sleep. It usually appears within hours of arriving high and eases with rest and time — as long as you don't keep climbing.

High-altitude cerebral edema (HACE) is AMS's dangerous progression: swelling of the brain. The hallmark signs are neurological — loss of coordination (a stumbling, drunk-like walk), confusion, or a change in mental state. HACE is a medical emergency.

High-altitude pulmonary edema (HAPE) is fluid in the lungs, and can occur with or without AMS. Signs include breathlessness at rest, a cough, chest tightness, and a sharp drop in exercise tolerance. HAPE is also a medical emergency, and is a leading cause of altitude deaths.

The single most important treatment for the serious forms is simple and non-negotiable: descend.

The golden rules of ascent

Most altitude illness is prevented by controlling how fast you sleep higher, not how high you go during the day. The core rules:

• "Climb high, sleep low." It's fine to go higher during the day; what matters is the altitude you sleep at.
• Above 3,000 m, don't increase your sleeping altitude by more than about 500 m per night, and build in a rest day every 3–4 days or so.
• If you develop AMS symptoms, do not go higher until they resolve. This one rule prevents most tragedies.
• If symptoms are worsening despite rest — or you see any sign of HACE or HAPE — descend immediately, ideally by 300–1,000 m, and seek help.
• Sleep low where the itinerary allows. In Peru, many travellers sleep in the Sacred Valley (~2,800 m) rather than Cusco (~3,400 m) for the first nights — a simple, effective tactic. Our Peru & Machu Picchu guide covers this in detail.
• Go easy at first: avoid alcohol for the first 48 hours, take it gently, and stay hydrated.

Where an itinerary allows a gradual or staged ascent, medication often isn't needed at all.

Diamox (acetazolamide): where it fits

Acetazolamide, sold as Diamox, is the medication most associated with altitude. It's genuinely useful because it doesn't just mask symptoms — it speeds acclimatisation, helping your body adapt faster. It can be taken to prevent AMS before and during ascent, and also helps recovery if symptoms have already started.

A few practical points travellers should know:

• It's a prescription medication — discuss it with a travel-health clinic or doctor before your trip. A commonly used preventive dose is modest (often around 125 mg twice daily), started the day before ascent, but your prescriber will advise the right regimen for you.
• Side effects are usually minor: tingling in the fingers and toes, increased urination, and a flat or metallic taste — famously making fizzy drinks and beer taste odd.
• Acetazolamide is a sulfonamide, so flag any sulfa drug allergy to your doctor; it's also not used in pregnancy.
• It is not a substitute for sensible ascent. It reduces risk; it does not license racing up a mountain, and it does not remove the need to descend if serious symptoms appear.

Other drugs exist for specific situations — dexamethasone (a steroid used more for treatment, which masks rather than aids acclimatisation) and nifedipine (reserved for people with a history of HAPE) — but these are specialist choices your doctor would discuss if relevant. Over-the-counter ibuprofen has some evidence for reducing AMS headaches.

Who should plan most carefully

Take extra care — and talk to a travel doctor well ahead — if you have a history of altitude illness, a heart or lung condition, are pregnant, or your itinerary forces a rapid ascent (for example flying straight into a high-altitude city, or aggressive trekking schedules). Pre-trip acclimatisation and a well-paced plan matter most for these travellers.

The bottom line

Altitude sickness is common, unrelated to fitness, and mostly preventable by one habit: ascend slowly, and never climb higher with worsening symptoms. Learn to tell ordinary AMS from the emergency signs of HACE and HAPE, consider acetazolamide with your doctor if your schedule is rushed, and remember that descent is the treatment that always works. Plan the pace and you can enjoy the high places safely.

Check your high-altitude destinations on the interactive travel vaccine map, and see our guides to Peru & Machu Picchu, Mexico and South America.

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This guide is general information based on CDC and Wilderness Medical Society guidance and is not a substitute for personalised medical advice. Acetazolamide and other altitude medications are prescription drugs — consult a travel-health professional before you travel. If you or a companion show signs of HACE or HAPE, descend and seek medical help immediately.