Fitness does not protect you from altitude sickness. Age does not predict it, and neither does previous experience — people who have been to 5,000 m without trouble can be hit on their next trip. The only reliable protections are a sensible ascent rate, enough fluid, and the discipline to act on early symptoms instead of walking through them.
Acute mountain sickness (AMS) is common above 3,000 m and usually mild. Its two dangerous forms, high-altitude pulmonary oedema (HAPE) and high-altitude cerebral oedema (HACE), are rare but can be fatal within hours. Every guide we send out is trained to recognise all three, but you should be able to recognise them too — you will notice your own symptoms long before anyone else does.
This article is general information for trip planning, not medical advice. Talk to a travel health clinic or a doctor who knows high-altitude medicine before your trek, especially if you have a heart or lung condition, are pregnant, or take regular medication.
What Altitude Does to You
Air at 5,364 m — Everest Base Camp — holds a little over half the oxygen available at sea level. Your body responds by breathing faster and deeper, raising your heart rate, producing more red blood cells, and changing your blood chemistry. Those adaptations take days, not hours. Altitude sickness is what happens when you climb faster than your physiology can follow.
The useful number to remember is sleeping altitude. You can walk up to a high viewpoint and come back down the same day with no ill effect — that is how acclimatisation days work. What your body reacts to is where you spend the night.
| Altitude | Oxygen available vs sea level | What trekkers notice |
|---|---|---|
| 2,500 m | About 74% | Mild breathlessness on climbs, slightly broken sleep |
| 3,500 m | About 64% | Noticeably slower pace, poorer sleep, reduced appetite |
| 4,500 m | About 57% | Headache common, every task feels like effort |
| 5,400 m | About 51% | Short breath at rest, very poor sleep, no appetite |
The Acclimatisation Rules That Matter
Every well-designed Nepal itinerary is built around a small set of rules. If a schedule you are offered breaks them, that is a reason to question the schedule.
- Above 3,000 m, raise your sleeping altitude by no more than 300–500 m per night. Our Everest and Annapurna itineraries are built on this, which is why they look slower than the fastest trips on the market.
- Take a rest day every 3–4 days, or every 1,000 m of gain. Namche Bazaar at 3,440 m and Dingboche at 4,410 m exist in the schedule for exactly this reason.
- Climb high, sleep low. On a rest day, walk several hundred metres above your lodge and come back down. It is more useful than sitting still.
- Drink three to four litres a day. Dry air and fast breathing dehydrate you much more than the temperature suggests.
- Walk slowly enough to hold a conversation. If you cannot, you are going too fast for the altitude, regardless of your fitness.
- Avoid alcohol and sleeping pills above 3,000 m. Both suppress breathing at night, which is when acclimatisation happens.
Our Everest Base Camp itinerary, for instance, holds two full acclimatisation days — one at Namche and one at Dingboche — and climbs to a viewpoint on each. That is the difference between a trip that succeeds and one that turns around at Lobuche.
Recognising the Symptoms
AMS usually begins 6 to 12 hours after arriving at a new altitude. A headache is the cardinal symptom; the rest varies.
| Severity | Symptoms | Action |
|---|---|---|
| Mild AMS | Headache, nausea, tiredness, poor sleep, reduced appetite, mild dizziness | Stop ascending. Rest, hydrate, simple painkillers. If it clears, continue next day. |
| Moderate AMS | Headache not relieved by painkillers, vomiting, weakness, breathlessness at rest | Descend at least 500 m the same day. Do not go higher. |
| HACE (brain) | Confusion, loss of coordination, stumbling, irrational behaviour, severe headache, drowsiness | Immediate descent, oxygen, dexamethasone, evacuate. Life-threatening. |
| HAPE (lungs) | Breathlessness at rest, wet cough, pink or frothy sputum, crackling in the chest, blue lips | Immediate descent, oxygen, nifedipine. Life-threatening. |
The single most useful field test for HACE is the heel-to-toe walk: ask the person to walk a straight line placing each heel against the toe of the other foot. A trekker who cannot do this when they could yesterday needs to go down now, at night if necessary.
Descent is the treatment for all of it. Drugs and oxygen buy time; losing altitude is what actually fixes the problem. A drop of 500–1,000 m usually brings dramatic improvement.
Acetazolamide and Other Medication
Acetazolamide (Diamox) speeds acclimatisation by mildly acidifying the blood, which drives deeper breathing. A common preventive dose is 125 mg twice daily, started a day before going above 3,000 m; your own doctor should set the dose and confirm it suits you. Expect tingling in the fingers and toes, more frequent urination, and flat-tasting fizzy drinks. It is a sulfonamide, so it matters if you have a sulfa allergy.
Dexamethasone and nifedipine are treatment drugs for HACE and HAPE, not preventives. Our guides carry them on high-altitude trips, along with a pulse oximeter and, on climbing trips and high passes, bottled oxygen. Do not self-medicate with either and continue climbing — the drug masks the symptom while the underlying condition worsens.
Ibuprofen or paracetamol is fine for an altitude headache. Do not use sleeping tablets, and go easy on antihistamines for the same reason.
Evacuation and Insurance
Above the road head there is no ambulance. Serious altitude illness means walking or riding down if the patient can move, and a helicopter if they cannot. Helicopter operators in Nepal dispatch against a guarantee of payment, which in practice means an insurance policy that explicitly covers helicopter evacuation at the maximum altitude of your route.
Many standard travel policies cap cover at 3,000 m, which excludes almost every trek in this catalogue. Check the altitude clause, not the marketing copy — our insurance guide walks through exactly what to look for and the altitude each region demands.
Which Treks Carry the Most Altitude Risk
Risk tracks both the maximum altitude and how fast the route gets there.
- Highest risk: routes crossing 5,000 m passes — the Everest Three Passes, Thorong La on the Annapurna Circuit, Larke La on the Manaslu Circuit — and any trekking-peak ascent.
- Moderate risk: Everest Base Camp, Gokyo, Tilicho Lake, Makalu Base Camp — high, but with gradual profiles and rest days built in.
- Low risk: Annapurna Base Camp at 4,130 m, Langtang Valley, Mardi Himal — high enough for mild AMS, low enough that descent is always quick.
- Minimal risk: Poon Hill, Pikey Peak, and the Kathmandu Valley rim, all below 4,100 m with short approaches.
One route deserves a special warning: a helicopter tour that lands at Kala Patthar or Gorak Shep takes you from 1,400 m to above 5,300 m in under an hour. Operators keep landings short for exactly that reason. If you have a heart or lung condition, discuss a heli tour with a doctor before booking.
