Altitude sickness ends more Nepal treks than bad weather, poor fitness, and blown knees combined. It doesn’t discriminate by age, experience, or physical shape — I’ve watched fit ultramarathon runners turn grey-faced at Namche Bazaar (3,440m) while a sixty-year-old retired schoolteacher walked on to Everest Base Camp without so much as a headache. The difference, almost always, comes down to how well you understand what’s happening to your body and whether you’re willing to act on it.
This guide covers the three forms of altitude illness you might encounter in Nepal, how to prevent them, how to recognise when things are getting serious, and what to do about it. Read it before you go. The people who get into real trouble at altitude are almost always the people who assumed it wouldn’t happen to them.
Why Altitude Makes You Sick
The atmosphere at sea level contains roughly 21% oxygen, and that proportion doesn’t change with altitude. What changes is air pressure — the higher you go, the thinner the air, and the fewer oxygen molecules you pull into your lungs with each breath. At Kathmandu (1,400m), you’re getting about 85% of the oxygen you’d get at sea level. At Namche Bazaar (3,440m), it drops to around 67%. At Thorong La pass on the Annapurna Circuit (5,416m), you’re working with roughly 50% of sea-level oxygen. At Everest Base Camp (5,364m), similar.
Your body can adapt to lower oxygen levels — this is acclimatization. Red blood cell production increases, breathing rate rises, circulation adjusts. But these changes take time, typically 24–72 hours at each significant altitude step. Go too high too fast and your body simply can’t keep up. The result is altitude sickness, in one of its three forms.
The Three Forms of Altitude Illness
AMS: Acute Mountain Sickness
AMS is the most common form and the one most trekkers will encounter at some point. The diagnostic standard used by most wilderness medicine organizations is the Lake Louise Score: a headache at altitude plus at least one of the following — fatigue or weakness, dizziness or lightheadedness, gastrointestinal symptoms (nausea, vomiting, loss of appetite), or difficulty sleeping. Onset is typically 6–12 hours after arriving at a new altitude, which is why you often feel fine at lunch and terrible by 9 PM.
Mild AMS is uncomfortable but not dangerous if you stop ascending. The appropriate response is: rest at the same altitude, hydrate, take ibuprofen or paracetamol for the headache, and do not go higher until all symptoms have resolved. Descending even 300–500m can bring noticeable relief within an hour or two.
The danger with AMS is misreading it. People conflate headache with dehydration, mistake nausea for bad food, and push on because they’ve paid for a flight to Lukla and don’t want to waste it. AMS that isn’t respected becomes something else.
HACE: High Altitude Cerebral Edema
HACE is AMS that has progressed to the point where fluid is accumulating around the brain. It is a medical emergency. The cardinal signs are a severe, worsening headache that doesn’t respond to ibuprofen, confusion or altered mental state, and ataxia — the inability to walk in a straight line. The heel-to-toe walking test is the standard field check: if someone can’t walk a straight line placing heel directly in front of toe, suspect HACE.
Other symptoms include extreme fatigue, drowsiness, double vision, and in severe cases, seizures or loss of consciousness. HACE typically develops above 3,500m and is rare below that. On the EBC route, it’s most likely to appear between Dingboche (4,410m) and Gorak Shep (5,164m). On the Annapurna Circuit, the Thorong La approach from High Camp (4,925m) is the high-risk section.
Treatment is immediate descent — do not wait for morning, do not wait for a helicopter if descent on foot is possible. Dexamethasone (8mg initial dose, then 4mg every six hours) can buy time but is not a substitute for descent. If a Gamow bag is available, use it while preparing to descend.
HAPE: High Altitude Pulmonary Edema
HAPE is statistically the deadliest form of altitude illness and is responsible for the majority of altitude-related deaths in Nepal. Fluid accumulates in the lungs rather than the brain. Early symptoms include breathlessness during exertion that is disproportionate to the effort, a dry cough, and reduced exercise tolerance compared to the day before. As it progresses: breathlessness at rest, a wet cough that may produce pink or frothy sputum, crackling or gurgling sounds audible in the lungs, bluish lips or fingertips (cyanosis), and rapid heart rate.
HAPE can develop more quickly than HACE and can be fatal within hours if not treated. It typically appears on the second night at a new altitude, often in someone who seemed to be coping fine the previous day. The first symptom — breathlessness during activity that seems excessive — is easily dismissed as fitness or exertion. Don’t dismiss it above 3,500m.
Treatment: immediate descent of at least 500–1,000m. Supplemental oxygen if available (most lodges above 4,000m on the EBC route have oxygen cylinders). Nifedipine (30mg slow-release) can help as a bridging treatment. Again, descent takes priority over everything else.
The Gamow Bag
A Gamow bag is a portable hyperbaric chamber — essentially a large airtight bag that you inflate with a foot pump to increase the pressure inside, effectively simulating a descent of 1,500–2,500m without physically moving. The person with altitude illness is placed inside and the bag is pressurized. Sessions of one to two hours can produce dramatic improvement in symptoms.
Most well-equipped lodges above 4,000m on the Everest and Annapurna routes have Gamow bags, as do many trekking agencies that operate in high-altitude areas. They are genuinely useful for buying time when immediate descent isn’t possible — for example, if a HACE patient is too disoriented to walk safely and you need to stabilize them before a night descent. But the bag treats symptoms, not causes. The moment the patient exits the bag, their body is back at the same altitude. Descent is still mandatory.
Diamox (Acetazolamide): What It Does and How to Use It
Acetazolamide, sold under the brand name Diamox, is the most widely used medication for altitude sickness prevention. It works by acidifying the blood, which stimulates faster and deeper breathing, accelerating the acclimatization process. It does not mask symptoms — if you develop AMS on Diamox, you still feel it, which is an important safety distinction.
The standard prophylactic dose is 125mg twice daily, starting one to two days before your major ascent. Some doctors prescribe 250mg twice daily; the lower dose has similar efficacy with fewer side effects. Common side effects include increased urination (Diamox is a diuretic — stay hydrated), tingling in the fingers and toes (very common, harmless), and a metallic taste that makes carbonated drinks unpleasant. Rare but possible: allergic reaction in people with sulfa drug allergies. If you’re allergic to sulfonamide antibiotics, discuss Diamox with a doctor before the trip.
Diamox is available in Kathmandu pharmacies in Thamel without a prescription, typically for NPR 150–300 for a strip of 10 tablets. That said, it’s worth discussing with a doctor before your trip — not because it’s dangerous, but because getting a baseline assessment and confirming you have no contraindications is sensible preparation for high-altitude trekking.
Should everyone take Diamox? Not necessarily. People who acclimatize well, follow a conservative ascent schedule, and have trekked at altitude before without issues often skip it. The medication is most useful for those with a history of AMS, those on an aggressive itinerary with limited acclimatization days, or those who have had a bad experience at altitude before. It’s also worth having in your kit regardless, as a treatment option if symptoms develop.
Acclimatization: The Schedule That Actually Works
The foundational rule of high-altitude trekking is this: above 3,000m, your sleeping altitude should not increase by more than 300–500m per night. “Climb high, sleep low” is the governing principle — you can go higher during the day to stimulate acclimatization, but return to a lower elevation to sleep.
EBC Route Acclimatization Schedule
The standard itinerary for Everest Base Camp is structured around two built-in acclimatization days. Here’s how the altitude progression looks in practice:
- Day 1: Fly Kathmandu (1,400m) to Lukla (2,860m), trek to Phakding (2,610m) — a deliberate descent to start
- Day 2: Trek Phakding to Namche Bazaar (3,440m)
- Day 3: REST DAY in Namche — acclimatization hike up to Everest View Hotel (3,880m), sleep back in Namche (3,440m)
- Day 4: Namche to Tengboche (3,860m)
- Day 5: Tengboche to Dingboche (4,410m)
- Day 6: REST DAY in Dingboche — hike to Nangkartshang peak (5,083m), sleep back at Dingboche (4,410m)
- Day 7: Dingboche to Lobuche (4,940m)
- Day 8: Lobuche to Gorak Shep (5,164m), continue to Everest Base Camp (5,364m), return to Gorak Shep to sleep
Itineraries that cut this to 10–11 days by skipping acclimatization days dramatically increase the risk of serious altitude illness. The two rest days are not optional extras — they’re the reason most people make it to base camp.
Annapurna Circuit Acclimatization
The Annapurna Circuit’s altitude spike happens at Thorong La (5,416m), but the approach allows good acclimatization if you follow the route properly:
- Chame sits at 2,710m — most trekkers feel fine here
- Upper Pisang (3,310m) or Ngawal (3,660m) — first significant altitude, slow down here
- Manang (3,519m) — mandatory acclimatization stop, minimum one full rest day; most guides recommend two if you have any symptoms
- High Camp / Thorong Phedi (4,450–4,925m) — sleep here before the Thorong La crossing
- Thorong La (5,416m) — cross early morning, before 10 AM ideally, to avoid afternoon weather
Manang is where most Thorong La failures originate. People arrive feeling good, skip the rest day, push to High Camp, and hit the wall at 5,000m. The headache that seemed manageable at 4,000m becomes incapacitating at altitude. One rest day in Manang — with a day hike to Ice Lake at 4,620m — is the single most effective thing you can do to improve your chances of crossing Thorong La.
When to Descend Immediately
The golden rule of altitude medicine: if in doubt, descend. Not in the morning. Not after one more night to see if it improves. Now.
Descend immediately if any of the following are present:
- Confusion, disorientation, or unusual behavior at altitude
- Inability to walk in a straight line (positive ataxia test)
- Breathlessness at rest — not just on the trail, but sitting still or lying down
- A wet or gurgling cough with any of the above
- Loss of consciousness, even briefly
- Severe headache that does not respond to ibuprofen after one hour
- Extreme fatigue combined with any cognitive changes
A descent of even 500m can be enough to stabilize someone with early HACE or HAPE. The goal isn’t to get them to Kathmandu immediately — the goal is to get them lower, quickly, and reassess. At Dingboche (4,410m), descending to Tengboche (3,860m) is often sufficient to bring someone back from the edge. From Lobuche (4,940m), dropping to Pheriche (4,243m) where there’s a Himalayan Rescue Association clinic provides both altitude relief and medical support.
Helicopter Rescue: Costs and Insurance
Nepal has a well-developed helicopter rescue infrastructure, particularly in the Khumbu and Annapurna regions. Multiple operators — Simrik Air, Fishtail Air, Karnali Helicopters among others — can reach most high-altitude areas within 45 minutes to 2 hours of a rescue request.
But rescue comes at a cost. A helicopter evacuation from the EBC area runs approximately $3,500–5,000 USD. From remote locations on the Annapurna Circuit or Manaslu, expect $4,000–8,000 USD or more depending on distance, weather delays, and whether the rescue requires multiple flight legs. These costs are typically not covered by standard travel insurance — you need a policy that specifically includes high-altitude evacuation and helicopter rescue.
World Nomads, IMG Global, and Tata AIG (popular with Indian trekkers) are commonly used policies that cover helicopter rescue in Nepal. Before you buy, confirm the policy covers trekking above a specific altitude — some policies cut off coverage above 4,000m or 5,000m, which is exactly where you need it. Print your insurance details and keep them with your passport. Your guide, your lodge owner, and the rescue coordination services at the Himalayan Rescue Association (HRA) clinics in Pheriche and Manang all need your policy number before a helicopter can be dispatched on credit.
The HRA clinics are worth knowing about. Pheriche (4,243m) on the EBC route and Manang (3,519m) on the Annapurna Circuit both have staffed clinics during the main trekking seasons (March–May and September–November). They run daily altitude lectures that are free, genuinely informative, and worth attending even if you feel completely fine. The staff see altitude illness every day of the season and can give you a quick assessment if you’re unsure whether what you’re feeling is normal acclimatization or something more serious.
Prevention Checklist Before You Trek
- Choose a conservative itinerary. A 14-day EBC trek is safer than a 10-day one. The extra days cost money but they exist for a reason.
- Hydrate consistently. 3–4 liters of water per day at altitude. Dehydration worsens altitude symptoms and is easy to fall behind on when you’re walking all day.
- Avoid alcohol and sleeping pills in the first few nights at each new altitude. Both suppress breathing, which reduces your oxygen intake when your body is already struggling.
- Know the symptoms. Brief your trekking partners, not just yourself. People with serious HACE are often the last to recognize something is wrong — confusion is a symptom.
- Discuss Diamox with your doctor before leaving home. Even if you don’t end up taking it, having it in your kit gives you a treatment option.
- Buy evacuation insurance that specifically covers high-altitude helicopter rescue. Check the altitude ceiling on the policy.
A Note from Nepal Trail Guide
Altitude sickness is not a character flaw and it’s not a reason to feel embarrassed. Some of the strongest, most experienced high-altitude trekkers descend from Lobuche or Thorong La because their body simply isn’t acclimatizing that day, and they make the right call. The mountain stays put. You can come back.
What we’ve tried to do here is give you the specific information — the altitudes, the symptoms, the medication doses, the costs — that makes the difference between recognizing a problem early and being evacuated in a much worse state. The Himalayan Rescue Association’s field clinics exist because altitude illness is common and treatable when caught in time. Go read their materials too; they’re the people who see these cases every single season and know exactly what happens when trekkers push on when they shouldn’t.
Trek carefully, acclimatize properly, and don’t let a tight flight schedule or a paid itinerary become the reason you make a decision that puts you in a helicopter instead of on the trail.