Why altitude sickness happens, the early signs to watch for, and the simple habits that keep most trekkers well above 4,000 m.
Table of contents
Acute mountain sickness (AMS) is the most common reason trekkers turn back in Nepal, and it has nothing to do with fitness. Marathon runners get it; retirees walking slowly often don't. What matters is how quickly you gain height and how well you listen to your body.
Why it happens
At 5,000 m each breath carries roughly half the oxygen it does at sea level. Your body adapts by breathing faster, producing more red blood cells and shifting fluid balance, but these changes take days. Climb faster than they can happen and you feel it.
The early signs
- Headache that doesn't go away with water and rest
- Loss of appetite or nausea
- Unusual tiredness and poor sleep
- Dizziness or light-headedness
Mild symptoms are common and usually settle with a rest day. Tell your guide about any of them — a good guide would rather hear about a headache early than discover it late.
Habits that work
- Climb high, sleep low. Acclimatisation days are not rest days; you hike higher and come back down to sleep.
- Above 3,000 m, raise your sleeping altitude by about 300–500 m a night, with a rest day every three to four days.
- Drink three to four litres a day. Garlic soup and ginger-lemon tea count.
- Walk slower than feels necessary. If you can't hold a conversation, slow down.
- Skip alcohol and sleeping pills until you are heading down.
When to descend
Breathlessness at rest, confusion, a wet cough or difficulty walking in a straight line are signs of HAPE or HACE, which are emergencies. The treatment is immediate descent. Our guides carry a pulse oximeter and first-aid kit, log every trekker's oxygen saturation each evening, and coordinate helicopter evacuation through your insurer when needed.
What about Diamox?
Acetazolamide (Diamox) can speed acclimatisation and is widely used on Himalayan treks. It is a prescription medicine with side effects, so talk to your doctor before you travel about whether it suits you.