Acute Mountain Sickness affects anywhere from roughly a quarter to over half of trekkers who ascend too quickly on the Everest Base Camp route, depending on the study and the checkpoint measured, but it is largely preventable with a properly paced itinerary and basic self-monitoring.
Acclimatisation is the physiological process by which the body adapts to reduced oxygen at altitude, primarily through a faster breathing rate and, over several days, a rise in red blood cell production driven by the hormone erythropoietin. The Everest Base Camp Trek's mandatory rest days exist specifically to give this process time to work before ascending further.
The 300-500 m rule
Above 3,000 m, the trekking medicine consensus is to gain no more than 300-500 m of net sleeping elevation per day, and to build in a rest day for every 900-1,000 m gained. This is why every responsible EBC itinerary includes acclimatisation days at Namche (3,440 m) and Dingboche (4,410 m). Skipping either to save a day is the single most common cause of AMS on this route.
Climb high, sleep low
On rest days, the best acclimatisation strategy is to hike higher during the day, to the Everest View Hotel (3,880 m) from Namche, or Nangkartshang Peak (5,083 m) from Dingboche, then descend back to the lower sleeping elevation. This exposes the body to reduced oxygen without requiring it to sleep there yet.
Diamox: what it does and how to use it
Acetazolamide, sold under the brand name Diamox, is the standard prophylactic medication for AMS prevention on this route. It works by making the kidneys excrete bicarbonate, which acidifies the blood slightly and triggers faster, deeper breathing, the same adaptation the body would otherwise take days to build naturally. The commonly recommended dose is 125 mg twice daily, started one to two days before ascent and continued for two days after reaching the highest sleeping altitude of the trek. A Journal of Travel Medicine trial found roughly a 48% relative risk reduction in AMS among trekkers taking prophylactic Diamox compared to placebo. Common side effects include tingling in the fingers and toes and a metallic taste in carbonated drinks, both harmless, along with increased urination, which makes the hydration target below even more important. Diamox is a prescription medication in most countries, so this conversation belongs with a doctor before departure, not a decision made on the trail.
Hydration, diet, and self-monitoring
Drink at least 3-4 litres of water daily; dehydration mimics and worsens AMS symptoms. Avoid alcohol above 3,000 m, and eat enough carbohydrate-heavy food even if appetite drops, since appetite loss is itself a mild altitude symptom. A pulse oximeter, a small clip-on device measuring blood oxygen saturation, is a useful trail companion, though it supplements symptom tracking rather than replacing it: readings vary enough between individuals at the same altitude that one number rarely predicts AMS risk on its own, and a downward trend over several days matters more than any single reading. Track symptoms using the Lake Louise Score: a total of 3 to 5, with headache present, signals mild-to-moderate AMS and a stop in ascent; a score of 6 or higher, or any sign of ataxia or confusion, means descend immediately regardless of the itinerary schedule.
When acclimatisation isn't working: HACE and HAPE
AMS, HACE, and HAPE sit on the same spectrum of altitude illness, and recognising which one is happening changes the correct response. AMS causes headache along with nausea, fatigue, or dizziness, and is the only one of the three considered mild to moderate. High Altitude Cerebral Edema (HACE) is brain swelling that typically appears three to five days after a significant ascent, marked by ataxia (a loss of coordination visible in something as simple as walking a straight line), confusion, and drowsiness that can progress toward coma without immediate descent. High Altitude Pulmonary Edema (HAPE) is fluid in the lungs, often appearing without any preceding AMS symptoms, marked by breathlessness at rest, a persistent cough, and in advanced cases blood-tinged sputum. Both HACE and HAPE are medical emergencies requiring immediate descent regardless of distance already covered, a call no guide should hesitate to make.
Frequently Asked Questions
What is the Diamox dosage for Everest Base Camp?
The commonly recommended prophylactic dose is 125 mg twice daily, started one to two days before ascent and continued for two days after reaching your highest sleeping altitude. Consult a doctor before your trip, since it's a prescription medication with some contraindications.
How much does Diamox reduce AMS risk?
A Journal of Travel Medicine study found roughly a 48% relative risk reduction in AMS among trekkers taking prophylactic Diamox compared to placebo, though it doesn't eliminate risk entirely and proper acclimatisation pacing still matters.
What's the difference between AMS, HACE, and HAPE?
AMS is headache plus nausea, fatigue, or dizziness, and is mild to moderate. HACE is brain swelling causing loss of coordination and confusion. HAPE is fluid in the lungs causing breathlessness and cough. Both HACE and HAPE require immediate descent.
Can I still get AMS even with acclimatisation days?
Yes. Acclimatisation days reduce risk substantially but don't eliminate it entirely, since individual susceptibility to altitude varies for reasons not fully understood, even at similar fitness levels. Self-monitoring on rest days matters just as much as the schedule itself.
Is a pulse oximeter worth carrying on the trek?
It's a useful supplement to symptom tracking, though blood oxygen readings vary enough between individuals at the same altitude that a single reading rarely predicts AMS on its own. A downward trend over several days is more meaningful than any single number.