Altitude sickness affects most trekkers above 2,500 m and turns dangerous above 3,500 m, the zone where Nepal's high passes like Thorong La at 5,416 m sit. The prevention rules are simple: climb slowly, sleep no more than 500 m higher each night above 3,000 m, hydrate, and descend the moment severe symptoms appear. This guide covers the three forms of altitude illness, how they show up differently depending on which Nepal trek you're on, and when a helicopter evacuation becomes the only safe option.
Altitude sickness, clinically called acute mountain sickness or AMS, is a set of symptoms caused by reduced oxygen pressure at elevation that appears when the body cannot adjust quickly enough to thinner air. It ranges from a mild headache to two life-threatening conditions, high-altitude cerebral edema (HACE) and high-altitude pulmonary edema (HAPE), and it can strike a fit 25-year-old as readily as an older trekker because fitness does not predict susceptibility.
AMS: the early warning above 2,500 m
AMS usually begins above 2,500 m with a headache plus at least one of nausea, fatigue, dizziness, or broken sleep. The symptoms feel like a hangover and typically appear six to twelve hours after reaching a new altitude, which is why the night you sleep higher matters more than the height you touch during the day. On the Annapurna Circuit most trekkers first notice AMS around Manang at 3,540 m.
AMS is the body asking you to stop climbing, not to push through. The correct response to mild AMS is to halt at the same altitude, rest, hydrate, and only continue once the headache clears, because climbing higher with active symptoms is how AMS escalates into HACE.
The Lake Louise scoring system, used by mountain doctors worldwide, rates AMS by adding points for headache, gastrointestinal upset, fatigue, and dizziness, and a score of three or more with a headache confirms the diagnosis. Trekkers do not need the formal score to act, but the framework explains why a guide asks the same questions each evening at altitude. Tracking how you feel night by night, rather than how hard the day's walk was, is the habit that catches AMS early.
HACE and HAPE: when AMS turns fatal
HACE, high-altitude cerebral edema, is fluid swelling in the brain that follows untreated AMS and shows as confusion, loss of balance, and an inability to walk a straight line. A trekker with HACE may seem drunk, struggle to stand, or fall asleep abnormally, and the condition can become fatal within hours. The only treatment is immediate descent, supplemental oxygen, and the drug dexamethasone administered by a guide trained in its use.
HAPE, high-altitude pulmonary edema, is fluid in the lungs and presents as breathlessness at rest, a wet cough, chest tightness, and a blue tinge to the lips. HAPE can develop independently of AMS and kills faster than HACE, so any trekker gasping for breath while sitting still must descend at once. Both conditions are the reason guided groups on the Everest Base Camp trek at 5,364 m carry a pulse oximeter and watch for falling oxygen saturation.
Where the risk actually sits: altitude profile by trek
Every Nepal trek reaches its risk zone on a different day, and knowing your own route's profile matters more than memorising general symptoms. The Annapurna Base Camp trek has one of the better-designed acclimatisation profiles of any route in Nepal, largely because the trail descends and re-climbs several times before the final push, giving the body repeated exposure at moderate altitude.
| 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. |
The same logic applies on other routes. Manang at 3,540 m is where the Annapurna Circuit's real acclimatisation work happens, two nights before the push toward Thorong La at 5,416 m. On the Everest side, Namche Bazaar (3,440 m) and Dingboche (4,410 m) play the same role ahead of Kala Patthar.
Acclimatisation rules that prevent illness
The 500 m rule is the single most effective defence: above 3,000 m, raise your sleeping altitude by no more than 500 m per night and take a rest day every 1,000 m of net gain. The Annapurna Circuit builds in two nights at Manang for exactly this reason, a pattern explained in our Manang acclimatisation guide, where trekkers hike high to Ice Lake at 4,600 m and return to sleep low.
- Climb high, sleep low: gain altitude during the day, then drop to sleep.
- Drink 3 to 4 litres of water daily to offset dry mountain air.
- Avoid alcohol and sleeping pills, which suppress breathing.
- Walk at a pace that lets you hold a conversation.
- Eat enough; appetite drops at altitude but calories drive acclimatisation.
These rules matter most before a high pass. Trekkers who rush the Manang acclimatisation days are the ones who turn back on pass day, so the rest stops are not optional padding.
Diamox and medical support
Diamox, the brand name for acetazolamide, speeds acclimatisation by mildly acidifying the blood and stimulating breathing, and the standard preventive dose is 125 mg twice a day starting one day before going above 3,000 m. It is a prevention aid, not a cure, so a trekker on Diamox who develops HACE or HAPE must still descend immediately. Tingling fingers and a metallic taste are common harmless side effects, but anyone allergic to sulfa drugs should consult 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
Descent of 500 to 1,000 m is the universal cure for serious altitude illness, and it should never be delayed to nightfall or to the next scheduled stage. Any sign of HACE or HAPE means descending immediately, even in darkness with a headlamp, because these conditions worsen by the hour. Guided groups make this call early because a TAAN-certified guide is watching for the symptoms that a tired trekker often ignores.
Helicopter evacuation from the high Annapurna or Everest trails costs USD 2,500 to USD 5,000, which is why comprehensive travel insurance that covers trekking to 6,000 m and helicopter rescue is non-negotiable. Confirm your policy includes high-altitude evacuation before you fly, and carry the policy number on your TIMS card so rescuers can authorise the flight. Difficulty planning around altitude also shapes route choice, a topic in our Annapurna Circuit difficulty guide.
Why weather forecasting matters as much as acclimatisation
Altitude illness is not the only high-mountain risk, and Nepal's worst trekking tragedy was a weather event rather than a medical one. On 14 October 2014, the remnants of Cyclone Hudhud dumped close to 1.8 metres of snow across the Annapurna region within twelve hours, triggering avalanches around Thorong La and the Manang and Mustang districts on the Annapurna Circuit. At least 43 people died, including trekkers caught on the pass with no warning of the incoming storm. It remains Nepal's deadliest trekking disaster.
In response, Nepal's government and TAAN (Trekking Agents Association of Nepal) overhauled weather warning systems and emergency communication along major routes. Guided groups today carry satellite communicators and check real-time forecasts before committing to a pass crossing or a push to base camp, and a guide's authority to delay a departure for weather is exactly the kind of decision that prevented a repeat of 2014.
Who is most at risk
Susceptibility to altitude sickness varies by individual physiology, not by age or fitness, so a marathon runner can suffer AMS while an unfit trekker walks through unaffected. The strongest predictor is your own history: a trekker who has had AMS before is more likely to get it again, and rapid ascent profiles raise the risk for everyone. Pre-existing heart or lung conditions and recent respiratory infections also lower your tolerance for thin air.
Because fitness offers no protection, the discipline of slow ascent matters for every trekker regardless of training. The most common cause of serious illness on Nepal's trails is not weakness but speed, with trekkers on tight schedules skipping the acclimatisation days that the body needs. Flying directly to Lukla at 2,860 m for the Everest route, rather than walking up gradually, is one reason that trek demands extra caution from day one.
The role of a guide in altitude safety
A TAAN-certified guide carries the equipment and training that turn early symptoms into a manageable problem rather than an emergency. The guide monitors each trekker's oxygen saturation with a pulse oximeter, knows the descent routes from every high camp, and holds the dexamethasone and nifedipine that treat HACE and HAPE during an evacuation. Crucially, the guide makes the objective call to descend when a tired trekker wants to push on.
That independent judgement is the single biggest safety advantage of a guided trek, because the trekker experiencing HACE is often the last to recognise their own confusion. Our guides also pace the group to the slowest member and enforce the acclimatisation rest days that a self-guided trekker might skip to save time. The combination of monitoring, medication, and decision-making is why guided groups record far fewer serious altitude incidents than independent walkers did before the 2023 rule requiring a licensed guide on most Nepal treks.
Diet, sleep, and the small daily habits
Hydration and calories drive acclimatisation more than any supplement, so drinking 3 to 4 litres of water a day and eating full meals even with a suppressed appetite gives the body the raw material to adjust. Garlic soup and ginger tea are tea-house staples that trekkers swear by, and while the evidence is informal, the warm fluids genuinely help hydration. Avoiding alcohol and sleeping pills matters because both depress the breathing rate that the body needs to raise at altitude.
Sleep quality drops at elevation, and broken, dream-heavy sleep with periodic pauses in breathing is normal above 3,500 m rather than a danger sign on its own. The habit that protects you is honest daily self-assessment: noting your headache, appetite, and sleep each morning so a worsening trend is caught before it becomes serious. These small routines, repeated every day on a route like the Annapurna Circuit or the climb to Annapurna Base Camp, are what keep a trekker moving safely toward the high point.
Trek safely with structured acclimatisation
Annapurna Trekking builds structured acclimatisation into every high-altitude itinerary, with rest days at Manang on the circuit and at Namche and Dingboche on the Everest route, and our TAAN-certified guides carry a pulse oximeter and a first-aid kit with emergency oxygen. We keep groups to a maximum of twelve so a guide can monitor every trekker daily. To plan a safe ascent of the Annapurna Circuit, Annapurna Base Camp, or Everest Base Camp, message our team on WhatsApp at +977 984 159 5962 or use our contact page.
