Altitude sickness in Nepal: symptoms, prevention, and what to do

What AMS is, and why it happens

Air pressure drops as you gain elevation, which means each breath delivers less oxygen even though the air’s oxygen percentage stays the same — this is a physiological response to reduced air pressure, not simply a matter of the air being “thinner” in a way that scales with fitness. Acute mountain sickness is the body’s common, usually short-lived reaction to ascending faster than it can adjust: headache, nausea, fatigue, dizziness, and disturbed sleep, typically appearing within 6 to 12 hours of reaching a higher sleeping altitude above roughly 2,500 metres (Wilderness Medical Society). AMS on its own is common and usually resolves with rest and no further ascent — the risk is in ignoring it and continuing upward.

Prevention: the ascent-rate rule and “climb high, sleep low”

The single most effective prevention tool is pacing, not fitness or medication. The Wilderness Medical Society’s current clinical guideline recommends: avoid ascending directly to a sleeping altitude of 2,750 metres or higher in a single day from a lower starting point; once above 3,000 metres, do not increase your sleeping altitude by more than 500 metres per night; and add an extra acclimatisation night for roughly every 1,000 metres of sleeping-altitude gain (WMS 2024 guideline). The altitude you walk up to during the day matters less than the altitude you sleep at — the basis of the “climb high, sleep low” principle used across established Nepal trekking itineraries, where a rest day usually means a day hike to a higher point followed by a return to the same or a lower lodge for the night, not a day of complete inactivity.

Two supporting habits: stay well hydrated, since dehydration can mimic or worsen AMS symptoms and makes them harder to read accurately; and treat the ascent-rate guideline as a target to plan a route around, not a rule to break because a schedule is tight. If a trek’s itinerary compresses acclimatisation days to save time, that is a real trade-off, not a minor scheduling detail.

Fitness, age, and experience do not reliably protect you. Multiple independent medical and trekking-specific sources agree that susceptibility to altitude illness does not correlate reliably with fitness level, age, or prior high-altitude experience — ascent rate and individual physiology are the stronger predictors. Read alongside Nepal trekking for beginners on choosing a realistic route: a slower, better-paced itinerary is a more effective safeguard than training harder for a fast one.

Recognising AMS, HAPE, and HACE

AMS is the common starting point. HAPE (fluid in the lungs) and HACE (swelling in the brain) are its two dangerous progressions — both life-threatening, both treated the same way: immediate descent.

Sources: NCBI StatPearls, HAPE and NCBI StatPearls, HACE, cross-referenced against the WMS guideline and independent clinical summaries. HACE most often follows AMS that was not heeded, though it can occasionally appear without a clear AMS warning first — another reason not to treat a mild headache at altitude as background noise.

What to actually do, stage by stage

• Mild AMS (headache, nausea, fatigue): stop ascending to a higher sleeping altitude, hydrate, treat symptoms (rest, over-the-counter pain relief as appropriate), and reassess the next morning before deciding whether to continue.
• Worsening or moderate AMS: descend — even a few hundred metres of elevation loss commonly brings rapid improvement. Do not continue ascending while symptomatic.
• Any sign of HAPE or HACE (breathlessness at rest, a worsening cough, confusion, loss of coordination): descend immediately, as far and as fast as can be done safely, and get to the nearest aid post, clinic, or evacuation point. These are medical emergencies, not conditions to wait out at altitude.

Descent is the primary treatment for all three conditions (WMS guideline); supplemental oxygen, if available, is a supportive measure alongside descent for HAPE and HACE, not a substitute for it.

Diamox (acetazolamide): what it is, and what it is not

Acetazolamide (brand name Diamox) is commonly cited in clinical references as a preventive medication for AMS, with 125 mg taken twice daily as the most commonly cited prophylactic dose, usually started 24–48 hours before ascent and continued for at least 48 hours at the highest altitude reached. A published clinical trial (the RADICAL trial) found a lower 62.5 mg twice-daily dose non-inferior to the standard dose for AMS prevention, but a separate later trial did not find the lower dose equally effective and reported an increased risk of AMS at that dose — the lower-dose research is genuinely mixed, not a settled alternative, so this page notes it exists without recommending either dose.

Reported common side effects include increased urination, tingling in the fingers, toes, or around the mouth, and an altered taste in carbonated drinks; it is a mild diuretic, so hydration matters more while taking it. It is generally avoided with impaired kidney function. On the sulfa-allergy question specifically: acetazolamide is a non-antibiotic sulfonamide, and the literature reports a low but non-zero cross-reactivity risk for people with a prior antibiotic-sulfa allergy — genuinely a “discuss with a doctor” question rather than a settled “safe” or “unsafe” answer. None of this is a substitute for your own doctor’s assessment; see Nepal packing list for why any altitude medication belongs in a pre-departure conversation, not a packing checklist decision.

Existing safety infrastructure on the ground

The Himalayan Rescue Association Nepal (HRA) runs seasonally staffed aid posts at Pheriche (Khumbu, on the Everest Base Camp route, roughly 4,250m) and Manang (Annapurna Circuit, roughly 3,550m), opening for the spring and autumn trekking seasons and focused specifically on assessing and treating AMS, HAPE, and HACE. HRA confirmed both posts open and staffed for the Spring 2026 season — a reasonable sign of continued operation, not a guarantee for whichever season you are trekking in; confirm current-season status with your guide or agency before relying on it. For what your travel insurance needs to cover, how helicopter rescue and claims actually work, and more on the HRA aid posts, see Nepal travel insurance — this page deliberately does not restate that page’s payment-model and claims detail.