Altitude Sickness on the Everest Base Camp Trek (2026)

Acute Mountain Sickness is the common form of altitude illness: headache as the defining symptom, alongside nausea, fatigue, or dizziness, typically appearing above 3,000 m within six to twelve hours of arrival at a new elevation. The 2018 Lake Louise consensus requires headache for a diagnosis, and disturbed sleep alone is a poor signal, since periodic breathing at altitude disrupts most healthy trekkers' sleep regardless of whether AMS is present. Most cases resolve within one to two days at the same altitude, provided ascent stops until symptoms clear. If symptoms get worse at the same altitude, descend.

High Altitude Cerebral Oedema, or HACE, is fluid accumulation on the brain and the dangerous extension of AMS. It shows up as confusion, a severe headache unrelieved by medication, and ataxia, a loss of coordination visible as an inability to walk a straight line heel-to-toe. HACE can progress from first symptom to unconsciousness within hours, and a confused person may not notice their own confusion, so ask a guide or companion to watch you too.

High Altitude Pulmonary Oedema, or HAPE, is fluid in the lungs and a separate condition rather than a later stage of AMS. It is marked by breathlessness at rest rather than just on exertion, a persistent cough, and a gurgling or crackling sound when breathing, and it can occur with little or no headache. Both HACE and HAPE are medical emergencies requiring immediate descent of at least 500 to 1,000 m, regardless of the itinerary schedule or proximity to Base Camp.

For how acclimatisation fits into the rest of the trip, from route to permits to timing, see the complete Everest Base Camp Trek guide.

Above 3,000 m, the Wilderness Medical Society advises raising your sleeping elevation by no more than 500 m per night, with a rest day every 3 to 4 days. Many trekkers treat 300 to 500 m as a more conservative target. Ascent rate is the biggest lever a trekker has over their own AMS risk, bigger than fitness level or trekking experience.

No standard EBC itinerary meets that limit on every night. On this site's own elevations, the Classic 14-Day gains 830 m from Phakding to Namche Bazaar, 550 m from Tengboche to Dingboche, and 530 m from Dingboche to Lobuche, and shorter schedules that walk Namche to Dingboche in a single day gain 970 m. Rest days at Namche Bazaar (3,440 m) and Dingboche (4,410 m) are what offset the biggest jumps, so both are non-negotiable on any standard-length itinerary, and a schedule that compresses the climb carries more risk even with both kept.

Acetazolamide, sold under the brand name Diamox, is a carbonic anhydrase inhibitor that speeds up the body’s natural acclimatisation process by encouraging faster, deeper breathing. Many trekkers start a prophylactic dose the night before ascending above 3,000 m, though a same-day start still helps, and continue for two to three days after arrival at the highest point of a given ascent stage.

The Wilderness Medical Society’s 2024 clinical practice guidelines, the current reference standard for altitude illness prevention, recommend 125 mg every 12 hours for a standard, well-paced ascent. They say 250 mg every 12 hours can be considered for high-risk ascents up to 5,000 m, such as a shorter itinerary with less built-in acclimatisation time, and the CDC also recommends that dose for people above 100 kg. The same guidelines note that dosing for ascent above 5,000 m, which every EBC itinerary eventually involves at Gorak Shep or Kala Patthar, isn't clearly established in the research, which is one more reason Diamox supplements pacing rather than replacing it. For treating AMS rather than preventing it, the WMS and CDC dose is 250 mg every 12 hours. The child prevention dose is 1.25 mg/kg every 12 hours, to a maximum of 125 mg. Agree any dose with a doctor before departure.

Common side effects include tingling in the fingers and toes, increased urination, and a temporary change in how carbonated drinks taste. They usually pass, but they surprise first-time users who haven't been warned. Trekkers with a sulfa allergy should discuss alternatives with a doctor before departure, since acetazolamide is a sulfonamide-derived medication.

AMS susceptibility varies significantly between individuals and correlates weakly with fitness. A strong runner with no prior altitude exposure can develop severe AMS while a less fit trekker with a slow, well-acclimatised pace stays symptom-free, because the underlying mechanism is about how quickly an individual’s body adapts to reduced oxygen, not cardiovascular fitness. Researchers studying high-altitude populations have linked some of this variation to inherited differences in how the body senses and responds to low oxygen, but no test currently exists that reliably predicts an individual trekker's AMS risk before departure. See the physical training guide for what fitness actually does prepare you for on this trek.

Age alone isn’t a strong predictor either way, though trekkers over 50 with underlying cardiovascular or respiratory conditions should get a medical clearance before booking. A prior history of AMS on a previous high-altitude trip is one of the stronger predictors of future susceptibility, and trekkers with that history should discuss a Diamox prophylaxis plan with their doctor well before departure rather than deciding on the trail.

Dehydration and alcohol can both make symptoms worse and harder to read, so keep drinking water and skip alcohol while you are gaining altitude. Rate of ascent remains the factor a trekker can actually control day to day. Genetics, prior altitude exposure, and individual physiology all influence susceptibility, but none of them are adjustable on a two-week trek. Pacing is.

Helicopter evacuation is available from villages as high as Gorak Shep (5,164 m), and municipal limits on commercial flights above Lukla exempt rescue flights. Commonly quoted costs run USD 3,000 to 6,000 depending on aircraft type, weather, and rescue company, but we found no current published source for that range, so ask your insurer what limit applies. Weather can ground aircraft for days: Lukla closed to air traffic on 25 September and again on 30-31 October 2026. Never delay a descent you can make on foot while waiting for a flight. Confirm your policy covers evacuation to at least 6,000 m before departure, since the route's highest point, Kala Patthar at 5,644 m (some sources list 5,545 m for a different point on the ridge), sits above many generic travel-insurance altitude ceilings.

Guides on Swotah itineraries carry a pulse oximeter for daily oxygen saturation checks and are trained to initiate an evacuation call well before a trekker reaches the severe end of the Lake Louise Score. Call your insurer's assistance line before the flight whenever you can: after Nepal's 2026 rescue-fraud prosecutions, some insurers have reportedly tightened verification of claims. See the travel insurance guide for exactly what a policy needs to cover and how the evacuation claims process works in practice.