Nepal Altitude Sickness: Symptoms, Prevention & Treatment (2026)

Altitude sickness affects roughly one in three trekkers in Nepal, and every year it sends unprepared hikers down in helicopters — or worse. The good news: acute mountain sickness (AMS) is almost entirely preventable if you ascend slowly, recognise early symptoms, and know when to act. This guide covers everything you need to stay safe on Nepal's trails, from Everest Base Camp to the Annapurna Circuit.

What Is Altitude Sickness and Why Does It Happen?

When you ascend rapidly, the partial pressure of oxygen in the air drops. Your body has not had time to produce extra red blood cells or adapt its breathing pattern, so your tissues become mildly hypoxic. The result is AMS — a spectrum of symptoms that ranges from a dull headache to the life-threatening emergencies of High Altitude Cerebral Oedema (HACE) and High Altitude Pulmonary Oedema (HAPE).

In Nepal, the risk is particularly pronounced because popular treks gain altitude very quickly. Flying into Lukla (2,860 m) from Kathmandu (1,400 m) is a jump your body notices immediately. The Thorong La pass on the Annapurna Circuit tops out at 5,416 m, and the trail to Everest Base Camp reaches 5,364 m — altitudes where even fit, experienced mountaineers take careful acclimatisation days.

Recognising Altitude Sickness: The Three Stages

Stage 1 — Acute Mountain Sickness (AMS)

AMS is the mildest and most common form. Symptoms typically appear within 6–12 hours of arriving at a new altitude and include:

• Headache — the cardinal symptom; usually frontal and throbbing
• Nausea or vomiting
• Fatigue and weakness out of proportion to effort
• Dizziness and light-headedness
• Poor sleep and vivid dreams
• Loss of appetite

Doctors use the Lake Louise Score to quantify AMS. A score of 3 or above (with headache as mandatory) indicates AMS. If you have mild AMS — headache and nausea but no other symptoms — the correct action is to stop ascending, rest at the same altitude, drink 3–4 litres of water, and take ibuprofen (400 mg) or paracetamol for the headache. Do not go higher until every symptom has resolved completely for 24 hours.

Stage 2 — High Altitude Cerebral Oedema (HACE)

HACE is the brain swelling with fluid. It is a medical emergency. Signs that AMS has progressed to HACE include:

• Severe, worsening headache that does not respond to ibuprofen
• Ataxia — have the person walk heel-to-toe in a straight line; staggering is a red flag
• Confusion, disorientation, or unusual behaviour
• Extreme lethargy — difficulty staying awake
• Hallucinations in severe cases

At the first sign of HACE, descend immediately — at least 500–1,000 m — and give dexamethasone 8 mg loading dose, then 4 mg every six hours. A Gamow bag (portable hyperbaric chamber) can buy time if descent is impossible in the dark, but it is not a substitute for descending. Most teahouses on the Everest trail above Namche carry one. Call for a helicopter rescue.

Stage 3 — High Altitude Pulmonary Oedema (HAPE)

HAPE is fluid accumulating in the lungs. It is the leading cause of altitude-related death, and it can develop without preceding AMS. Warning signs:

• Breathlessness at rest — not just on exertion
• A persistent cough, often productive with pink or frothy sputum
• Crackling sounds in the chest (a guide or medic can hear these with a stethoscope)
• SpO2 (blood oxygen saturation) dropping well below other trekkers at the same altitude — a pulse oximeter is invaluable here
• Cyanosis: blue tinge to lips or fingertips in severe cases

HAPE treatment: immediate descent, nifedipine 30 mg slow-release (30 mg repeat after 12 hours if needed), supplemental oxygen, and a Gamow bag if available. HAPE can kill within hours — do not wait for morning.

Acclimatisation: The Only Reliable Prevention

No drug replaces proper acclimatisation. The principle is straightforward: ascend gradually, build in rest days, and follow the maxim "climb high, sleep low."

On the Everest Base Camp trek, the standard safe schedule includes:

• Two nights in Namche Bazaar (3,440 m) — the first acclimatisation stop
• A day hike from Namche up to Everest View Hotel (3,880 m) or Khumjung (3,790 m), returning to sleep in Namche
• A second acclimatisation night in Dingboche (4,410 m) or Pheriche (4,240 m) before pushing higher
• Rest day in Lobuche (4,940 m) area before Base Camp

On the Annapurna Circuit, the critical acclimatisation point is Manang (3,519 m) — always take a full rest day here before heading toward Thorong La. A day hike to Ice Lake (4,600 m) or Gangapurna Lake is the recommended acclimatisation excursion.

For the Manaslu Circuit, the rules are even stricter because the trail is more remote and rescue is slower. Lho (3,180 m) and Samagaon (3,530 m) are mandatory rest stops before crossing the 5,160 m Larkya La pass.

Diamox: Dosage, Timing and Side Effects

Acetazolamide (brand name Diamox) is a carbonic anhydrase inhibitor that speeds up your breathing rate, increasing blood oxygen saturation at altitude. It is the only drug with robust evidence for both prevention and treatment of AMS.

Standard prophylactic dosage: 125 mg (half a standard 250 mg tablet) twice daily, starting 24 hours before ascending above 2,500 m and continuing until you have been at your highest altitude for 48 hours. Some physicians prescribe 250 mg twice daily for faster ascents; the lower dose has fewer side effects with similar efficacy.

Treatment of established AMS: 250 mg twice daily alongside rest and descent if symptoms worsen.

Common side effects — these are expected, not allergic reactions:

• Tingling in fingers, toes and lips (paraesthesia) — very common, usually harmless
• Increased urination — drink extra water to compensate
• Carbonated drinks taste flat
• Mild nausea

Do not take Diamox if you are allergic to sulphonamide antibiotics, are pregnant, or have kidney or liver disease. Consult your doctor before your trip. In Kathmandu and Pokhara, Diamox is available over the counter at pharmacies for approximately USD 0.50–1.00 per tablet — but confirm the dosage with a doctor beforehand.

Hydration, Nutrition and Lifestyle on the Trail

Dehydration worsens the symptoms of AMS. At altitude, your respiratory rate increases and you lose water rapidly through breathing — often without feeling thirsty. Aim for 3–4 litres of fluid per day above 3,500 m. Signs your urine should be pale straw-coloured; dark urine means drink more.

Alcohol and sleeping pills are vasodilators and respiratory depressants respectively — both impair your body's ability to acclimatise and should be avoided above 3,000 m, especially in the first 48 hours at any new altitude. This is a hard rule that many trekkers ignore to their detriment.

Eat carbohydrate-rich meals (dal bhat is ideal). Your appetite will likely drop at altitude; force yourself to eat. Rest completely on acclimatisation days — this is not the day for a strenuous side hike.

Using a Pulse Oximeter

A pulse oximeter clips to your fingertip and reads your blood oxygen saturation (SpO2) and pulse rate. It is one of the most useful tools you can carry in Nepal's high country, available for USD 8–15 in Namche Bazaar or Lukla.

Typical SpO2 values at altitude:

• Kathmandu (1,400 m): 96–99%
• Namche Bazaar (3,440 m): 88–95%
• Dingboche (4,410 m): 82–90%
• Everest Base Camp (5,364 m): 75–85%

If your reading is significantly lower than others at the same altitude, or if it drops sharply overnight, that is a warning sign. A reading below 70% at any altitude, combined with symptoms, warrants immediate descent and emergency assistance.

Emergency Protocols: When to Descend and How

The rule is simple: never ascend with symptoms of AMS; always descend with symptoms of HACE or HAPE. Descending even 300–500 m can produce rapid improvement. Do not wait until morning. Do not let a trekker with HACE or HAPE sleep at the same altitude "to see how they feel."

In the Khumbu region, helicopter evacuation can be arranged through your guide, the nearest teahouse, or via the TIMS emergency number. The Himalayan Rescue Association (HRA) maintains an aid post in Pheriche (4,240 m) staffed by volunteer doctors during the main trekking seasons (March–May and October–November). An HRA consultation costs USD 30–40 and is worth every cent if you are unsure about symptoms.

Helicopter rescue from the Khumbu costs approximately USD 1,500–4,000 depending on pickup point and insurance arrangements. Travel insurance with helicopter evacuation cover is mandatory for any Nepal trek above 3,500 m — verify your policy covers altitudes of 5,500 m+ if you are attempting EBC or similar. Before you leave for Nepal, make sure your insurance company's emergency number is saved in your phone.

Who Is Most at Risk?

AMS does not discriminate by fitness level, age, or gender. Ironically, very fit athletes sometimes ascend too fast because they feel strong, which increases their risk. Previous altitude experience does reduce risk but does not eliminate it — your susceptibility can vary trip to trip. People with pre-existing heart or lung conditions should consult a specialist before trekking above 4,000 m. Check our full Nepal trekking tours to see which treks include built-in acclimatisation days.

Altitude Sickness by Route: What to Expect

For permit requirements and route logistics by region, see our Nepal trekking permits guide. For visa information before you travel, see Nepal visa requirements. If you want a fully guided experience with an experienced local team who manage acclimatisation schedules, explore our Nepal trekking packages.

Frequently Asked Questions

At what altitude does altitude sickness start in Nepal?

AMS can begin as low as 2,500 m (8,200 ft), though most trekkers first notice symptoms between 3,000–3,500 m. Namche Bazaar at 3,440 m is where the majority of AMS cases on the Everest trail are first reported. Above 4,000 m, virtually every trekker experiences some physiological effect from reduced oxygen.

How long does acclimatisation take at Namche Bazaar?

Two full nights in Namche Bazaar is the minimum recommended. On the second day, hike up to 3,800–3,900 m (the Everest View Hotel ridge or Khumjung village) and return to sleep at Namche. This "climb high, sleep low" day activates your body's acclimatisation response significantly faster than a pure rest day would.

Can I buy Diamox in Nepal without a prescription?

Yes. Acetazolamide (Diamox) is available over the counter at pharmacies in Kathmandu, Pokhara, and Namche Bazaar without a prescription, for roughly USD 0.50–1.00 per 250 mg tablet. However, you should still consult a doctor before your trek to confirm the correct dose for your body weight and to rule out sulpha allergies or contraindications.

Is it safe to trek in Nepal if I had altitude sickness on a previous trip?

Previous AMS does increase your statistical risk of future AMS, but it does not mean you cannot trek in Nepal. The key adjustments are a more conservative ascent profile, prophylactic Diamox (discuss with your doctor), and a guide who is authorised and experienced to recognise early symptoms. Many repeat trekkers manage altitude well with these precautions in place.

What is the difference between HACE and HAPE?

HACE (High Altitude Cerebral Oedema) is fluid on the brain — its hallmark signs are coordination loss (ataxia), confusion, and extreme drowsiness. HAPE (High Altitude Pulmonary Oedema) is fluid in the lungs — its signs are breathlessness at rest, a persistent wet cough, and rapidly dropping SpO2. Both are emergencies requiring immediate descent; HAPE is statistically the more common killer of the two. Either condition can develop within hours and must be treated before the person sleeps another night at the same altitude.

Featured image: Freshfield, Douglas William, 1845-1934 via Wikimedia Commons (No restrictions).