Altitude sickness on Indian treks

What altitude sickness is

As you go higher, each breath carries less oxygen. Over a few days your body adjusts. This is called acclimatisation. If you climb faster than your body can adjust, you can get altitude illness.

There are three forms. Acute mountain sickness (AMS) is common and usually mild. High-altitude cerebral oedema (HACE) and high-altitude pulmonary oedema (HAPE) are rare but can kill.

Fitness does not protect you. Strong, fit trekkers get AMS too, often because they climb faster. What protects you is a slow ascent.

Symptoms: AMS, HAPE and HACE

AMS (acute mountain sickness)

The main symptom is a headache, usually with at least one of: nausea, dizziness, tiredness or loss of appetite. Some people vomit.
It usually starts 2–12 hours after you reach a new height.
If you stop climbing, it usually settles within 12–48 hours.
If you feel ill after three days at the same height without climbing higher, it is probably not AMS. See a doctor.

HACE (fluid on the brain)

The CDC describes HACE as “end-stage” AMS. Watch for:

confusion or strange behaviour
unsteady walking, as if drunk (ataxia)
unusual drowsiness

Coma can follow within 24 hours. Anyone who cannot walk in a straight line at altitude must be taken down.

HAPE (fluid in the lungs)

Early signs: chest congestion, cough, more breathlessness on effort than your companions, and a sudden drop in your ability to keep up.
Later: breathless even while resting, and frothy or blood-stained spit.

The CDC warns that HAPE can kill faster than HACE. Do not wait for it to get worse.

Acclimatise: how fast to climb

Both the Wilderness Medical Society (WMS) and the CDC say gradual ascent is the main way to prevent altitude illness. What counts is the height where you sleep, not the highest point you reach during the day.

Above 3,000 m, sleep no more than about 500 m higher than the night before.
Add a rest day for every 1,000 m of sleeping height you gain.
Do not go from low altitude straight to a sleeping height of 2,750 m or more in one day. Flying into Leh, or driving from the plains to a high roadhead in a single day, breaks this rule. Plan rest days on arrival.
The CDC says spending two or three nights at 2,450–2,750 m before going higher strongly protects against AMS.
For the first 48 hours at a new height, keep to light activity and avoid alcohol.

The WMS rates your risk by how you climb:

How you climb above 3,000 m	Risk
Sleep up to 500 m higher each night, with a rest day for every 1,000 m	Low
Sleep more than 500 m higher each night, but with a rest day for every 1,000 m	Moderate
Sleep more than 500 m higher each night, with no rest days	High

If a trek itinerary gains more than this, ask the organiser why, or add a night.

When to descend

These rules come from the CDC and the WMS. Learn them before you leave home.

Never go up to sleep higher if you have any symptoms of altitude illness, however mild.
With mild AMS, stop and rest at the same height. Go down if you get worse despite rest or treatment.
Descending about 300 m or more usually improves AMS.
With HACE or HAPE, go down urgently. The WMS says to keep going down until symptoms ease, usually 300–1,000 m. For HAPE, go down at least 1,000 m and let the sick person do as little work as possible. Carry them, or put them on a mule.
If descent is truly impossible (bad weather, night), oxygen or a portable hyperbaric bag can buy time. Go down as soon as you can.
Never leave a sick trekker alone, and never send them down alone.

Tell your trek leader early. Hiding a headache to keep up with the group is how mild AMS turns into an emergency.

Medicines: talk to a doctor first

Medicines can help, but they need a prescription and do not replace a slow ascent or going down.

Acetazolamide (sold in India as Diamox) is the CDC’s preferred medicine to prevent AMS. It also speeds up acclimatisation. The dose, and whether it is safe for you, is for a doctor to decide. It is a sulfonamide, so tell your doctor about any drug allergies.
The WMS says acetazolamide should not be used to treat HAPE.
Dexamethasone prevents and treats AMS and HACE. The CDC says it is generally kept for treatment, alongside descent. Do not climb higher after taking it for symptoms.
Do not take someone else’s tablets on the trail, and do not start a medicine because a group chat said so.

See a doctor well before a high trek, not the night before you leave. Take your planned itinerary with its sleeping heights, and ask what to carry and when to use it.

Children and older trekkers

Children. The CDC says children are as likely as adults to get altitude illness, so keep a close eye on how they eat, sleep and walk. The CDC and WMS say dexamethasone should not be used to prevent AMS in children. The WMS does recommend acetazolamide for prevention in children, with the dose worked out from body weight by a doctor. Keep to the same slow ascent rules.

Older trekkers. Age alone is not a bar. The CDC notes that people over 50 have a slightly lower risk of AMS. Health conditions matter more:

People with heart disease, lung disease or low oxygen levels should see a doctor who knows altitude medicine before going, even if their condition is under control.
People with diabetes should check blood sugar closely, because altitude illness can trigger dangerous complications.
Severe sleep apnoea usually means avoiding high altitude unless you use a CPAP machine with oxygen.
Pregnant women are advised not to sleep above 3,050 m.

If you are unsure, choose a lower trek this season and build up.