Altitude Sickness on the Everest Base Camp Trek (2026)

Acute Mountain Sickness is the mild, common form of altitude illness: headache, nausea, fatigue, and disturbed sleep, typically appearing above 3,000 m within six to twelve hours of arrival at a new elevation. Most cases resolve within one to two days at the same altitude, provided ascent stops until symptoms clear.

High Altitude Cerebral Oedema, or HACE, is fluid accumulation on the brain and the more dangerous of AMS’s two possible complications. 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.

High Altitude Pulmonary Oedema, or HAPE, is fluid in the lungs rather than the brain, marked by breathlessness at rest rather than just on exertion, a persistent cough, and a gurgling or crackling sound when breathing. 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 golden rule is simple: gain no more than 300 to 500 m of net sleeping elevation per day, with a full rest day built in for every 900 to 1,000 m gained. Every responsible EBC itinerary follows this pacing, and it’s the single biggest lever a trekker has over their own AMS risk, bigger than fitness level or trekking experience.

Two rest days are non-negotiable on any standard-length itinerary: one at Namche Bazaar (3,440 m) and one at Dingboche (4,410 m). Skipping either is the single most common cause of AMS on this route, more common than any individual physiological risk factor covered below.

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 one day before reaching 3,000 m and continue for two to three days after arrival at the highest point of a given ascent stage.

A typical prophylactic dose is 125 mg twice daily, though dosing varies by individual and should always be confirmed 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. None of these are dangerous, but they surprise first-time users who haven’t been warned.

Diamox is not a substitute for the 300-500 m golden rule above. It reduces AMS risk at a given ascent rate; it doesn’t make a reckless ascent rate safe. 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. 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.

Rate of ascent remains the one factor a trekker can actually control. Genetics, prior altitude exposure, and individual physiology all influence susceptibility, but none of them are adjustable on a two-week trek. Pacing is.

For serious cases, helicopter evacuation from villages as high as Gorak Shep (5,164 m) is well-established infrastructure in the Khumbu, typically costing USD 3,000 to 6,000 depending on aircraft type, weather, and rescue company. 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, 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. See the travel insurance guide for exactly what a policy needs to cover and how the evacuation claims process works in practice.