How to avoid altitude sickness on a trek (what actually works)

Short answer: go up slowly. Almost everything else is detail. Above about 3,000m, raise the height you sleep at by no more than roughly 300–500m a night, take a rest day every few days, walk at an easy pace, drink plenty, skip alcohol high up — and if symptoms come on, stop climbing until they pass. The one rule that overrides all the others: if you get worse, go down. Fitness does not protect you, and no pill or gadget replaces a sensible itinerary.

Altitude sickness ends more big treks than sore knees, bad weather and blisters combined. It is also the one risk that catches strong, fit people off guard — because the thing that prevents it has nothing to do with how hard you can push. The good news is that the prevention is simple and well understood. Here is the honest version, without the scaremongering or the magic cures.

What is altitude sickness, really?

As you climb, the air thins and every breath delivers less oxygen. Your body adapts — making more red blood cells, breathing deeper — but that adaptation takes time, and if you climb faster than your body can keep up, you feel ill. That's acute mountain sickness (AMS), the mild and common form. Most people are fine up to around 2,500m; symptoms become common above roughly 3,000m, and the risk rises the higher and faster you go. Treks like Kilimanjaro (5,895m) and Everest Base Camp (5,364m) spend days well into that zone, which is exactly why their itineraries are built around acclimatising.

There are three things to know by name. The table is the honest one-look summary; the rule underneath it never changes.

AMS is uncomfortable but manageable if you respect it. HACE and HAPE are rare but genuinely dangerous, and both can develop from ignored AMS. You do not need to memorise the medicine — you need to recognise that things are getting worse and act on it.

What actually prevents it?

One factor dominates everything else: how fast you gain sleeping altitude. Day walks that go high and come back down to sleep low are far kinder than a steady climb that beds you higher each night. The widely used guideline above 3,000m is to raise your sleeping height by no more than about 300–500m per night, and to take a rest (or "climb high, sleep low") day every few days or every 1,000m of ascent. A well-designed trek itinerary already does this for you — which is the single best reason not to cut days to save money.

Everything else is supporting cast, but it helps:

• Walk slowly. "Pole pole" — slowly, slowly — is the Kilimanjaro mantra for a reason. An easy pace lets your body adapt as you climb.
• Drink plenty of water. You dehydrate fast at altitude in dry, cold air; dehydration mimics and worsens AMS.
• Eat, even when you don't fancy it. Appetite drops high up, but your body needs the fuel.
• Skip alcohol and sleeping pills high up. Both blunt your breathing overnight, exactly when you need it most.
• Don't fly straight to a high start and walk off. On Andean treks, arrive in Cusco (3,400m) two or three days early to acclimatise before you start.

Does being fit help?

Not against altitude. This is the most important — and most ignored — truth on this page: fitness does not prevent altitude sickness. Strong young trekkers get it as often as anyone, and a good trip last year is no guarantee for the next one; some people are simply more susceptible. Training still matters hugely, because it lets you walk the long days comfortably and gives you reserve when you're tired — but it earns you nothing in the acclimatisation stakes. Train for the distance and the back-to-back days; let the itinerary handle the altitude.

What about Diamox and oxygen gadgets?

Acetazolamide (Diamox) is a prescription medicine that can genuinely help some people acclimatise, and it's widely used on high treks. But it is not a free pass: it has side effects (tingling fingers, frequent urination, a metallic taste are common), it doesn't let you skip going slowly, and whether you should take it — and at what dose — is a decision for a doctor or travel clinic who knows your health, ideally a few weeks before you travel. Don't self-prescribe off a forum.

The clip-on "oxygen meters" (pulse oximeters) some trekkers carry can be reassuring, but a number on a screen is no substitute for how you actually feel; readings at altitude are noisy and easy to misread. Bottled oxygen and portable pressure bags exist on the big peaks as emergency tools, not as a way to push through symptoms. None of these change the basic rule.

The one rule that overrides everything: go down

If your symptoms are getting worse despite resting — and especially if you see any sign of confusion, an unsteady stagger, or breathlessness at rest — descend, and descend now. Going down even a few hundred metres is the single most effective treatment there is, and it works fast. No summit, no permit, no sunk cost is worth gambling on. The trekkers who get into real trouble are almost always the ones who "didn't want to ruin it for the group" and kept climbing. Tell your guide early and honestly how you feel; a good operator would far rather turn one person around than carry them down.

A simple plan you can actually follow

• Choose a slow itinerary — on Kilimanjaro, the extra day on a longer route is the cheapest insurance you'll ever buy.
• Acclimatise before high treks and don't start the day you arrive.
• Walk slow, drink, eat, no booze up high.
• Know the warning signs and treat AMS as a stop sign, not a challenge.
• Talk to a travel clinic a few weeks out about whether preventive medication is right for you.
• If you get worse, go down. Always.

None of this is exotic. It's just discipline — and the willingness to put getting home safely ahead of a photo at the top.

Our honest take

You can't train your way out of altitude, and you can't buy your way out of it either. What works is boring and reliable: pick a route with enough days, go up gently, watch yourself honestly, and treat "down" as a tool rather than a failure. Do that and most people sail through. Ignore it because you feel fit, and the mountain doesn't care how many squats you've done.

This is general information from an independent trekking guide, not medical advice. Altitude illness can be serious — speak to a doctor or a travel clinic about your own health and any medication before a high-altitude trek.

Next steps: put it into practice on a specific trek — see how Kilimanjaro's routes trade days for success rates, what Everest Base Camp's altitude really feels like, and how to train so the walking, at least, is easy.

Related guides

• Kilimanjaro routes compared: which one to actually climb
• Everest Base Camp: what the trek is really like
• How to train for a multi-day trek (a real plan)