Altitude sickness can hit any trekker above 2,500 metres and becomes dangerous above about 3,500 metres, the band where Nepal's high passes like Thorong La (5,416 metres) sit. The rules for avoiding it are simple. Climb slowly. Above 3,000 metres sleep no more than about 500 metres higher each night, drink plenty, and go down the moment serious symptoms appear. This guide covers the three forms of altitude illness, how they show up on different Nepal treks, and when a helicopter becomes the only safe option.
Doctors call it acute mountain sickness, or AMS. It's a set of symptoms caused by the lower air pressure at height, appearing when your body can't adjust fast enough to the thinner air. It runs from a mild headache to two conditions that can kill, high altitude cerebral oedema (HACE) and high altitude pulmonary oedema (HAPE), and it hits a fit 25-year-old as readily as anyone, because fitness doesn't predict who gets it.
AMS: the early warning above 2,500 m
AMS usually starts above 2,500 metres with a headache plus at least one of nausea, tiredness, dizziness or poor sleep. It feels like a hangover and typically arrives six to twelve hours after you reach a new height. That's why the height you sleep at matters more than the height you touch during the day. On the Annapurna Circuit most people first feel it around Manang at 3,540 metres.
Treat it as a stop sign. With mild AMS you stay at the same height, rest and drink until the headache clears, and only then carry on. Climbing higher with symptoms is how AMS turns into HACE.
Mountain doctors score AMS with the Lake Louise system, adding points for headache, stomach upset, tiredness and dizziness. Three or more, including a headache, confirms it. You don't need the formal score to act, though it explains why your guide asks the same questions every evening. Track how you feel night by night, not how hard the day was. That habit catches AMS early.
HACE and HAPE: when AMS turns fatal
HACE is swelling of the brain, usually following AMS that wasn't dealt with. It shows as confusion, poor balance and being unable to walk a straight line. Someone with HACE may look drunk, struggle to stand or become oddly sleepy, and it can kill within hours. Treatment is immediate descent, oxygen, and dexamethasone given by a guide trained to use it.
HAPE is fluid on the lungs. Look for breathlessness at rest, a wet cough, a tight chest and bluish lips. It can appear without any AMS first, and it's the most common cause of altitude deaths, so anyone gasping for breath while sitting still must go down at once. Both conditions are why guided groups on the Everest Base Camp trek, which reaches 5,364 metres, carry a pulse oximeter and watch for falling oxygen saturation.
Where the risk sits: altitude profile by trek
Every trek reaches its danger zone on a different day. Knowing your own route's profile matters more than memorising symptoms. The Annapurna Base Camp trek has one of the kinder profiles in Nepal, because the trail drops and climbs again several times before the final push, giving your body repeated time at moderate height.
| Stage | Altitude | Risk level |
|---|---|---|
| Nayapul to Ghorepani | 1,070 m – 2,874 m | Low. Physically demanding, not altitude-limited. |
| Poon Hill | 3,210 m | Low. A dawn side-trip, not a sleeping altitude. |
| Chhomrong | 2,160 m | Low. The trail descends here before climbing again. |
| Deurali | 3,230 m | Moderate. First night above 3,000 m on most itineraries; mild AMS is common. |
| Annapurna Base Camp | 4,130 m | Significant. Guides monitor SpO2 here; symptomatic trekkers descend to Machhapuchhre Base Camp (3,700 m) immediately. |
Other routes work the same way. Manang at 3,540 metres is where the Annapurna Circuit's real acclimatising happens, two nights there before the push toward Thorong La at 5,416 metres. On the Everest side, Namche Bazaar (3,440 metres) and Dingboche (4,410 metres) do that job before Kala Patthar.
Acclimatisation rules that prevent illness
The 500-metre rule is your best defence. Above 3,000 metres, raise your sleeping height by no more than about 500 metres a night, and take a rest day for every 1,000 metres gained. The Annapurna Circuit keeps two nights at Manang for exactly this, as our Manang acclimatisation guide explains, with a hike up to Ice Lake at about 4,600 metres and back down to sleep.
Climb high and sleep low. Drink three to four litres of water a day to make up for the dry air. Skip alcohol and sleeping pills, both of which slow your breathing. Walk at a pace where you can still talk, and keep eating even when you aren't hungry, because your body needs the calories to adapt.
These rules matter most before a high pass. People who rush the Manang days are the ones who turn back on pass day, so treat the rest stops as part of the route, not padding.
Diamox and medical support
Diamox, or acetazolamide, speeds acclimatisation by nudging your breathing rate up. The usual preventive dose is 125 milligrams twice a day, starting the day before you go above 3,000 metres. It helps you adjust and cures nothing. Someone on Diamox who develops HACE or HAPE still has to go down at once. Tingling fingers and a metallic taste in fizzy drinks are common and harmless, and anyone allergic to sulfa drugs should talk to a doctor before the trip.
| Condition | Key symptom | Action |
|---|---|---|
| AMS | Headache, nausea | Stop, rest, hydrate |
| HACE | Confusion, poor balance | Descend now, oxygen, dexamethasone |
| HAPE | Breathless at rest | Descend now, oxygen, nifedipine |
When to descend and call a helicopter
Going down 500 to 1,000 metres is the cure for serious altitude illness. Never wait for nightfall or the next scheduled stage. Any sign of HACE or HAPE means going down now, by head-torch if necessary, because both get worse by the hour. Guided groups make the call early. A TAAN-certified guide is watching for the signs a tired trekker tends to ignore.
A helicopter from the high Annapurna or Everest trails costs roughly USD 3,000 to 6,500 depending on where it picks you up, so your travel insurance must cover trekking to 6,000 metres and helicopter rescue. Check the policy before you fly, and put the policy number on your TIMS form so rescuers can approve the flight. Altitude also shapes which route to choose, which our Annapurna Circuit difficulty guide covers.
Why weather forecasting matters as much as acclimatisation
Altitude isn't the only danger up high. Nepal's worst trekking disaster was a storm. On 14 October 2014 the remnants of Cyclone Hudhud dropped up to 1.8 metres of snow across the Annapurna region in about twelve hours, setting off avalanches around Thorong La and in Manang and Mustang. At least 43 people died, many of them trekkers caught near the pass with no warning of what was coming.
Since then forecasts and communications on the main routes have improved, and guides check the latest forecast before committing a group to a pass or a push to base camp. The late September 2026 snowfall that turned 150 trekkers back from Thorong La, all safely, showed how much that habit matters. A guide's authority to hold a group for a day is the decision that keeps 2014 from repeating.
Who is most at risk
Your body decides, not your age or fitness. A marathon runner can get AMS while an unfit friend walks through untouched. The best predictor is your own history. If you've had AMS before, you're more likely to get it again, and a fast ascent raises the risk for everyone. Heart or lung conditions and a recent chest infection also lower your tolerance.
Fitness gives no protection, so everyone needs the slow ascent. Speed causes more serious illness on Nepal's trails than weakness does, mostly among people on tight schedules who skip rest days. Flying straight to Lukla at 2,860 metres for the Everest trek, with no gradual walk-in, is one reason that route needs extra care from day one.
The role of a guide in altitude safety
A TAAN-certified guide has the kit and training to turn early symptoms into a manageable problem. The guide checks each trekker's oxygen saturation with a pulse oximeter, knows the way down from every high camp, and carries the dexamethasone and nifedipine used for HACE and HAPE during an evacuation. Most important of all, the guide makes the call to go down when a tired trekker wants to keep going.
That outside judgement is the biggest safety benefit of a guided trek. Someone developing HACE is often the last to notice their own confusion. Our guides also set the pace by the slowest walker and keep the rest days a trekker on their own might skip. Monitoring, medicine and an independent decision-maker together are a large part of why Nepal now requires a licensed guide on most routes.
Diet, sleep, and the small daily habits
Water and food do more for acclimatisation than any supplement. Drink three to four litres a day and eat full meals even when your appetite has gone. Garlic soup and ginger tea are teahouse favourites. The evidence for them is thin, but hot drinks help you hydrate. Skip alcohol and sleeping pills, which slow the breathing your body needs to speed up.
Above 3,500 metres, sleep gets broken and full of vivid dreams, sometimes with short pauses in breathing. That's normal on its own. What protects you is an honest check each morning. Headache? Appetite? Sleep? Note them, so a worsening trend gets caught early. Small routines like that, repeated every day on the Annapurna Circuit or the climb to Annapurna Base Camp, keep you moving safely toward the top.
Trek safely with structured acclimatisation
Every high itinerary we run has acclimatisation days built in, at Manang on the Circuit and at Namche and Dingboche on the Everest route. Our TAAN-certified guides carry a pulse oximeter and a first-aid kit with emergency oxygen, and groups are capped at twelve so a guide can check everyone daily. To plan a safe ascent of the Annapurna Circuit, Annapurna Base Camp or Everest Base Camp, message us on WhatsApp at +977 984 159 5962 or use our contact page.
Frequently asked questions
At what altitude does altitude sickness start?
Acute mountain sickness usually starts above 2,500 metres, and the risk rises with every 1,000 metres you gain. Most Nepal treks cross that line, so acclimatisation matters on almost every route.
What is the best way to prevent altitude sickness?
Climb slowly. Above 3,000 metres raise your sleeping height by no more than about 500 metres a day, take the rest days and drink plenty. A slow ascent works far better than any medicine.
Should you take Diamox for trekking in Nepal?
Acetazolamide (Diamox) can help if you've had altitude sickness before or your route climbs fast, usually 125 mg twice a day. It helps you acclimatise and never replaces a slow ascent. Talk to your doctor first.
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Founder & Lead Guide, Annapurna Trekking
AJ founded Annapurna Trekking in 2016 and has personally guided treks across Nepal ever since. More about our team →
