Altitude Sickness in Nepal — Symptoms, Diamox, and When to Turn Back

Above 2,500 meters, your body starts working harder to extract oxygen from thinner air. Above 3,500 meters, it can't keep up — and that's where altitude sickness shows up for about a third of trekkers, regardless of fitness, age, or trekking history. Fit marathoners get AMS. Casual hikers don't. There's no reliable predictor except your personal track record at altitude.

Here's what to recognize, what to do, and the single decision that matters.

What AMS actually feels like

Stage 1 — mild AMS (3,000–4,000m, often appears 6–12 hours after arrival at new altitude):

• Headache that doesn't respond to ibuprofen the way it should
• Mild nausea or loss of appetite
• Trouble sleeping, weird dreams, periodic breathing
• Mild fatigue beyond what the day's walking justifies
• Dizziness when you stand up fast

Most trekkers feel some of this above 3,500m. It's not an emergency. It's a signal to slow down, drink water, and not gain more altitude that day.

Stage 2 — moderate AMS (any altitude where symptoms get worse despite resting):

• Headache that won't respond to anything
• Vomiting
• Shortness of breath at rest (not just on the trail)
• Loss of coordination — tripping, slurred speech, can't walk a straight line
• Mental confusion or unusual irritability

This is the line where you stop ascending and start descending. Not tomorrow. That day.

Stage 3 — HACE / HAPE (rare but life-threatening):

• HACE (cerebral edema): can't walk heel-to-toe, unable to function, hallucinations
• HAPE (pulmonary edema): gurgling breath, pink/frothy sputum, severe shortness of breath, blue lips

Both kill within hours if you stay at altitude. Descent is the only treatment that works. Oxygen and Diamox buy time; they don't fix it.

How Diamox actually works

Diamox (acetazolamide) doesn't prevent AMS — it accelerates your body's natural acclimatization by making your blood slightly more acidic, which makes you breathe faster, which delivers more oxygen. Side effects include tingling in fingers and toes, and making carbonated drinks taste flat. Both are harmless.

The standard prophylactic dose is 125mg twice a day, starting one day before reaching 3,000m and continuing until you start descending. A 125mg dose is half of the standard 250mg pill — yes, you split them.

Diamox is sold over the counter in every pharmacy in Thamel for about NPR 700 ($5) for a strip of 10. Bring your own from home if you prefer brand-name. It is not a substitute for descent if you have moderate AMS — it can mask symptoms that get worse silently.

The acclimatization rules

The two non-negotiable principles:

1. Above 3,000m, sleep no more than 500m higher than the previous night.
2. Take an acclimatization day every 1,000m gained.

Standard EBC itineraries already build these in: Namche (3,440m) two nights, Dingboche (4,410m) two nights. Don't compress them to save days. The trekkers who skip the second Namche night are over-represented in evacuation statistics by a factor of four.

The "climb high, sleep low" principle is real: a day-walk to a higher viewpoint (e.g. Hotel Everest View from Namche) accelerates acclimatization even though you sleep back down low.

When to turn back

The clearest signal is moderate AMS that doesn't improve after 24 hours at the same altitude. Not gone — improved. If your headache is the same on the morning of day 3 in Dingboche as it was on day 2, you descend.

The hardest case: you feel mostly fine but your body's giving subtle warnings. Slower pace than yesterday. Less appetite. Sleeping badly. These aren't reasons to descend immediately, but they're reasons to add an unplanned acclimatization day before pushing higher.

The trekker who turns around at 4,800m because they don't feel right is the trekker who comes back the next year and finishes. The trekker who pushes through is the trekker on the helicopter — sometimes.

The Nepali phrase that matters

Malai lek lagyo — मलाई लेक लाग्यो — "I have altitude sickness." Literally "altitude has caught me." Every guide, porter, lodge owner, and HRA medic above Namche knows this phrase. It removes all ambiguity from the situation faster than any English explanation.

The medical scenario script covers the full set: telling someone you need descent, asking for the aid post, refusing a premature helicopter evacuation when descent is the right call.

The helicopter-evac warning

Mild AMS does not require helicopter evacuation. The textbook treatment is descent of 500m and a rest day. Most cases resolve overnight.

Some unscrupulous guides and lodge owners push helicopter evacuation on trekkers with mild symptoms because insurance pays out USD 3,000–6,000 and a kickback goes to the person who arranged the call. The defence is buying time — see the scam-defence script for the exact phrases.

If you can walk down on your own legs, walk down. Real HACE/HAPE evacuations save lives. Manufactured ones drain insurance pools and feed fraud.

The HRA aid posts

Two posts staffed by Western volunteer doctors during peak seasons:

• Manang (3,540m) — Annapurna Circuit, before Thorong La
• Pheriche (4,371m) — EBC trail, before the high passes

Both offer free altitude consultations and a daily talk on AMS. If a guide or lodge owner is pressuring evacuation, the HRA medic is an independent second opinion. Use them.

Pre-trek checklist

• Diamox 125mg × 2 daily, starting 24h before 3,000m
• Day-pack ibuprofen, anti-nausea, ORS sachets
• Eight trail phrases including the AMS one
• Travel insurance with helicopter evacuation cover
• The mental commitment to descend if it's the right call

Altitude sickness isn't a character test. It's an oxygen test, and your body decides — not your willpower.