Altitude sickness: what it feels like, and the three rules that matter

The thing most likely to end your trek is not fitness or weather. It is going up faster than your body will allow — and it does not care how strong you are.

By Prakash Tamang · 8 min read · Updated 1 August 2026

Acute mountain sickness is the most common reason a trek in Nepal ends early, and it is the one risk that is almost entirely within your control. It is also poorly understood by most people who arrive here — including some who are very fit, which turns out to be the group most likely to get into trouble.

Fitness does not protect you. Age does not predict it. Being strong mainly means you are capable of walking uphill faster than your body can adapt, which is exactly the mistake.

This page is what our guides brief, and it is not a substitute for medical advice. If you have a heart or lung condition, are pregnant, or take regular medication, talk to a doctor who knows altitude medicine before you book.

What it feels like

Mild AMS is a headache, usually starting in the evening after you arrive somewhere higher, plus some combination of nausea, poor appetite, fatigue out of proportion to the day, dizziness and a genuinely bad night's sleep.

Trekkers routinely explain all of that away. The headache is dehydration, the tiredness is the long day, the bad sleep is the plywood walls. Sometimes that is true. The safe assumption above 3,000 m is that a new headache is altitude until proven otherwise. The rule of thumb our guides use: if you feel unwell at altitude, it is altitude. Prove it is something else before you act as though it is.

The three rules

One: do not go higher if you have symptoms. Not 'a bit higher', not 'just to the next lodge'. Stay where you are until they have gone.

Two: if symptoms get worse while you are resting, go down. Not tomorrow morning — then. Descent of even three or four hundred metres reverses mild AMS remarkably reliably, and it is the only treatment that always works.

Three: never leave someone with symptoms alone, and never let them descend alone. Judgement is one of the first things altitude takes, and a person who seems merely tired can deteriorate quickly.

The serious forms

High altitude cerebral oedema is AMS that has progressed to affect the brain. The signature sign is loss of coordination — someone who cannot walk a straight line heel-to-toe — along with confusion, unusual behaviour or drowsiness. This is a medical emergency and the response is immediate descent, at night if necessary.

High altitude pulmonary oedema affects the lungs. Breathlessness at rest that does not settle, a persistent cough, a crackling sound when breathing, extreme fatigue, sometimes pink or frothy sputum. Also an emergency, also treated by descent. Both are uncommon on standard teahouse routes and both are survivable when recognised early. What kills people is delay — waiting until morning, or continuing up because a flight is booked.

How to ascend so it does not happen

Above about 3,000 m, sleep no more than roughly 300 to 500 m higher than the night before, and build in a rest day every 1,000 m or so of gain. Our itineraries are built around this, and it is most of the reason a trek takes the number of days it does.

Climb high, sleep low. An acclimatisation day is not a lie-in — you walk several hundred metres up and come back down to sleep. The Manang day on the Circuit and the Namche day on Everest exist for exactly this and skipping them is the single most common cause of trouble further up.

Drink considerably more than you feel like — three to four litres a day is normal. Eat even without appetite; your body needs the calories to adapt. Avoid alcohol and sleeping tablets at altitude.

Walk slowly. Absurdly slowly, at a pace that feels wrong for the first twenty minutes. Every experienced guide in Nepal walks more slowly than their clients want to at the start of a climb.

Diamox, honestly

Acetazolamide, sold as Diamox, helps the body acclimatise and is widely used. It is a real drug with real effects, not a vitamin, and it is a prescription decision for a doctor who knows your history — particularly if you have a sulfa allergy or kidney problems.

It is a diuretic, so it makes you urinate more and you must drink more to compensate. Tingling in the fingers and toes is common and harmless. Some people report that fizzy drinks taste strange, which is an odd side effect.

What it does not do is make it safe to ascend faster. It reduces the chance of AMS on a sensible profile. It does not license a bad one, and it can mask early symptoms — which is exactly why the three rules still apply while you are taking it.

What we do about it

Our guides carry a pulse oximeter and check the group each evening from around 3,000 m. It takes thirty seconds and turns a vague worry into a number to watch across days.

Itineraries are built with the ascent profile first and the day count second. When someone asks us to compress a trek, this is the thing we are protecting, and it is why we sometimes say no.

And we turn people around. Several times a season, and it is never framed as failure. A trek you came down from is a trek you can do again.

Walk it with us

Private departures on your dates, guided by the people who wrote this page.

Annapurna Base Camp7 days · 4,130 mRs 100,400 · See dates →Annapurna Circuit & Thorong La12 days · 5,416 mRs 167,400 · See dates →Everest Base Camp14 days · 5,545 mRs 187,500 · See dates →Tilicho Lake Trek13 days · 5,416 mRs 180,800 · See dates →

Questions we get asked

At what altitude does it start?

Symptoms are uncommon below about 2,500 m and can appear any time above roughly 3,000 m. Most trekking AMS shows up between 3,500 and 5,000 m.

Does being fit help?

Not against altitude. Fit people are at slightly higher risk in practice because they can climb faster than they should. Susceptibility varies between individuals and is not predicted by fitness, age or sex.

If I got AMS once, will I get it again?

Previous AMS is the best predictor of future AMS, so tell us if it has happened before and we will build a slower profile. Plenty of people who struggled once are fine on a gentler ascent.

Should I take Diamox as a precaution?

That is a conversation with a doctor, not with a trekking company. Many people do; it is not a substitute for a sensible ascent rate and it can mask early symptoms.

What happens if I have to come down?

Your guide descends with you and we sort out transport, accommodation and getting you comfortable. Insurance covers evacuation if it comes to that — which is why it must include helicopter cover.