Altitude sickness ends more Nepal treks than bad weather, poor fitness, or bad luck combined. The mountain does not care how hard you trained or how much your gear cost. Once your body falls behind on acclimatization, the only cure is descent.
The encouraging reality is that with the right schedule, the right information, and the discipline to follow both, most healthy people can trek to Everest Base Camp, cross Thorong La on the Annapurna Circuit, and reach Gorak Shep at 5,164 meters without serious altitude complications. Altitude acclimatization in Nepal is not a mystery. It is a system, and this guide explains how to use it.
Understanding What Altitude Does to Your Body
The air at sea level contains roughly 21 percent oxygen by volume. That percentage stays constant as you climb, but the atmospheric pressure drops. At 3,500 meters, each breath delivers about 65 percent of the oxygen it would at sea level. At 5,000 meters, that figure drops below 55 percent.
Your body compensates through a cascade of physiological responses: increased breathing rate, higher heart rate, rising red blood cell production, and gradual chemical changes in the blood. These adaptations take time. They cannot be rushed by willpower, caffeine, or fitness. The only variable you can control is pace.
Acute Mountain Sickness (AMS) occurs when ascent outpaces the body's ability to adapt. Fluid accumulates in tissues where it does not belong. In mild cases, this produces headache, nausea, and fatigue. In serious cases, it causes High Altitude Pulmonary Edema (HAPE, fluid in the lungs) or High Altitude Cerebral Edema (HACE, fluid in the brain). Both are life-threatening. Both are largely preventable with proper acclimatization.
The Core Rules of Altitude Acclimatization
Every altitude medicine professional, every experienced trekking guide, and every peer-reviewed study on the subject points to the same foundational principles.
Ascend slowly. Above 3,000 meters, gain no more than 300 to 500 meters of sleeping altitude per day. This is the sleeping altitude that matters, not how high you climb during the day.
Climb high, sleep low. Day hikes that go higher than your sleeping altitude and then return accelerate acclimatization without increasing risk. This is the principle behind rest days in Namche Bazaar (3,440m), where trekkers hike to 3,800 or 4,000 meters and return to sleep at the same elevation.
Rest days are mandatory, not optional. The standard EBC itinerary includes two nights in Namche and one extra night in Dingboche. These are not for sightseers who want more time. They are medically necessary for the vast majority of trekkers.
Never ascend with symptoms. A mild headache at altitude might be dehydration. It might also be the first sign of AMS. Ascending with any unresolved symptoms is how mild AMS becomes HACE. If you feel off, stay put. If you worsen, descend.
Hydration matters. At altitude, you lose more water through respiration than at sea level. Aim for 3 to 4 liters of fluid daily. Avoid alcohol in the first three days at any new elevation. Alcohol suppresses the breathing response your body relies on to adapt.
AMS Symptoms: What to Watch For and When to Act
The Lake Louise Score is the standard tool altitude medicine practitioners use to assess AMS severity. You do not need to memorize the scoring system, but you do need to recognize the symptoms.
Mild AMS:
- Headache (the primary indicator)
- Fatigue and general weakness
- Dizziness or lightheadedness
- Loss of appetite
- Nausea
Mild AMS is common and not dangerous, provided you stop ascending and let it resolve. Most mild cases clear within 12 to 24 hours at the same elevation.
Moderate AMS:
- Severe headache that does not respond to ibuprofen
- Persistent vomiting
- Increasing fatigue and weakness
- Difficulty with coordination (ataxia)
Moderate AMS requires descent of at least 300 to 500 meters. Do not sleep at the same elevation and hope for improvement.
Severe AMS / HAPE / HACE:
- Breathlessness at rest
- Coughing, particularly a productive cough or pink frothy sputum (HAPE)
- Confusion, disorientation, inability to walk in a straight line (HACE)
- Loss of consciousness
Severe AMS is a medical emergency. Descend immediately, by any means available. In Nepal's high mountains, this often means helicopter evacuation. Supplemental oxygen and Gamow bags (portable hyperbaric chambers) can stabilize a patient while evacuation is arranged, but they are not substitutes for descent.
If in doubt, descend. The Himalayan Rescue Association uses this principle as their first commandment: when uncertain whether symptoms represent AMS, treat them as AMS.
Acclimatization Schedule: Everest Base Camp Trek
The Everest Base Camp trek reaches 5,364 meters and keeps trekkers above 4,000 meters for five or more consecutive nights. A rushed EBC itinerary is the primary cause of altitude evacuations in the Khumbu.
The standard acclimatization schedule:
| Day | Stage | Sleeping Altitude | Notes |
|---|---|---|---|
| Day 1 | Fly Lukla, trek to Phakding | 2,610m | Short first day. Deliberate gentle start. |
| Day 2 | Phakding to Namche Bazaar | 3,440m | Steep climb through forest. Many trekkers feel first altitude effects. |
| Day 3 | Rest day in Namche Bazaar | 3,440m | Hike to Everest View Hotel (3,880m) or national park visitor center. Return to Namche to sleep. |
| Day 4 | Namche to Tengboche | 3,867m | Gentle elevation gain. Second acclimatization night. |
| Day 5 | Tengboche to Dingboche | 4,410m | Significant altitude gain. Monitor for symptoms. |
| Day 6 | Rest day in Dingboche | 4,410m | Day hike to Nangkartshang (5,083m). Critical acclimatization stop. |
| Day 7 | Dingboche to Lobuche | 4,940m | Above 4,500m for the first time. Altitude effects common. |
| Day 8 | Lobuche to Gorak Shep, EBC | 5,164m | Trek to base camp (5,364m) and return to Gorak Shep to sleep. |
| Day 9 | Gorak Shep to Kala Patthar | 5,545m | Summit attempt at dawn. Descend to Pheriche or Dingboche. |
Do not compress this schedule. The Namche acclimatization day and the Dingboche rest day are the two non-negotiable anchors. Skipping either significantly raises the probability of a forced evacuation.
For a broader picture of the region and its villages, see our Khumbu region guide.
Acclimatization Schedule: Annapurna Circuit
The Annapurna Circuit reaches 5,416 meters at Thorong La Pass but handles altitude very differently from EBC. The circuit starts at low elevation (around 800 to 900 meters) and gains height gradually over two full weeks before crossing the pass. This gradual gain is one reason the circuit's altitude profile is generally better tolerated than EBC.
Key acclimatization stages on the circuit:
Manang (3,519m): This is the standard rest day location on the Annapurna Circuit. Spend two nights in Manang. Day hike to Ice Lake (4,600m) or Gangapurna viewpoint to maximize acclimatization benefit. The Himalayan Rescue Association runs an altitude medicine clinic in Manang during trekking season with free informational talks every afternoon. Attendance is strongly recommended.
High Camp (4,850m): Some trekkers spend a night at High Camp before attempting Thorong La. This reduces the elevation gain on pass day (from 1,900 meters round trip from Manang to 650 meters from High Camp) and spreads the altitude stress more comfortably. It is the recommended approach for trekkers who felt any discomfort through the Manang valley.
Thorong La crossing: Cross early. Aim to start before 6am. Weather builds on the pass through the afternoon, and descending the far side to Muktinath (3,760m) before wind and snow arrive is both easier and safer.
Acclimatization Schedule: Annapurna Base Camp
The Annapurna Base Camp (ABC) trek reaches 4,130 meters, which is well within manageable range for most healthy trekkers without the same level of concern as EBC or the Annapurna Circuit pass. That said, altitude sickness can still occur at 4,000 meters, particularly in individuals who are more sensitive or who ascend too quickly from Pokhara.
The standard ABC itinerary allows adequate acclimatization through its natural pace. The key stages are Chhomrong (2,170m), Himalaya Hotel (2,900m), and Deurali (3,230m) before the final push to base camp. No formal rest days are built into standard ABC itineraries, but slowing your pace and spending an extra night at Chhomrong if you feel any symptoms is a sound strategy.
Other High-Altitude Treks: Key Elevation Markers
Different treks in Nepal cross different altitude thresholds. Knowing where each route peaks helps you plan your acclimatization schedule.
| Trek | Maximum Elevation | Key Risk Elevation |
|---|---|---|
| Everest Base Camp | 5,545m (Kala Patthar) | Above 4,000m for 5+ nights |
| Three Passes Trek | 5,535m (Kongma La) | Multiple crossings above 5,000m |
| Annapurna Circuit | 5,416m (Thorong La) | Single-day high-altitude crossing |
| Annapurna Base Camp | 4,130m | Moderate risk |
| Langtang Valley | 4,773m (Tserko Ri) | Rest day at Kyanjin Gompa important |
| Gokyo Lakes | 5,357m (Gokyo Ri) | 3-4 nights above 4,700m |
| Kanchenjunga Base Camp | 5,143m | Remote with limited evacuation options |
Diamox: What It Does and Who Should Use It
Acetazolamide, sold under the brand name Diamox, is the most commonly used pharmaceutical prophylactic against AMS. It works by stimulating faster breathing, which accelerates acclimatization. It does not eliminate altitude risk; it reduces it.
Diamox is commonly prescribed at 125mg twice daily, starting 24 hours before ascending above 3,000 meters. Some altitude medicine specialists recommend a test dose before the trek to identify potential allergic reactions, as Diamox is a sulfonamide derivative and some individuals have sulfa drug allergies.
Common side effects include tingling in the hands, feet, and face, increased urination (which is a sign it is working), and altered taste of carbonated drinks. These are normal and generally tolerable.
Diamox is not mandatory for every trekker. Plenty of people complete the EBC and Annapurna Circuit without it. But for trekkers with a history of altitude sensitivity, those on compressed itineraries, or those with pre-existing medical conditions that limit oxygen uptake (mild anemia, sleep apnea), Diamox reduces risk meaningfully.
Consult a physician before your trek, particularly if you have kidney disease, are pregnant, or take other medications. Carry a letter from your doctor if you are bringing Diamox to Nepal; it is a prescription drug in most countries.
Ibuprofen (400-600mg) is effective for altitude headache and is widely used by trekkers. It does not treat AMS, but it manages the headache symptom. Never use ibuprofen to mask symptoms and continue ascending.
Medical Resources in Nepal's Mountains
Nepal has better high-altitude medical infrastructure than most people expect, though coverage is uneven.
Himalayan Rescue Association (HRA): The HRA operates clinics at Pheriche (4,371m) on the EBC route and in Manang (3,519m) on the Annapurna Circuit during peak trekking seasons (March-May, October-November). These clinics are staffed by volunteer physicians, mostly from Western countries, with expertise in high-altitude medicine. Walk-in consultations are free. Presentations on altitude sickness are held every afternoon during season. Visiting the clinic is one of the smartest things you can do during your acclimatization rest day. More information at himalayarescue.org.
Helicopter evacuation: Private helicopter evacuation is available across most of Nepal's trekking regions. Response times from Kathmandu to the Khumbu are typically 45 minutes to 90 minutes depending on weather. Costs range from $3,000 to $8,000 USD depending on location and provider. Travel insurance with helicopter evacuation coverage is not optional for high-altitude trekking. It is a prerequisite.
Gamow bags: These portable hyperbaric chambers simulate descent by increasing atmospheric pressure around the patient. Many teahouses on the EBC route and at higher points on the Annapurna Circuit keep Gamow bags. Ask at your lodge if you are in an area where one might be needed. A Gamow bag buys time for evacuation; it does not replace descent or definitive medical care.
Government hospitals: Namche Bazaar has a small hospital with basic facilities. Lukla and Kathmandu have airports that facilitate medical transport. Below the high mountains, most district hospitals have limited capacity for altitude emergencies. On remote routes like Dolpo, Nar Phu Valley, or Kanchenjunga, medical resources are extremely limited. Plan accordingly.
What Teahouses Cannot Fix
Nepal's teahouse system is excellent at providing warmth, food, and shelter. But teahouse owners and staff are not medical professionals. They have seen altitude sickness many times and often recognize the signs, but their response options are limited.
Do not rely on your teahouse host to make medical decisions for you. Know your symptoms, know when to act, and know that the decision to descend is yours to make. Trekkers who wait to feel dramatically worse before acting are the ones who end up in evacuation scenarios. Act at the first serious signs, not the last.
Practical Checklist Before You Depart
Before arriving in Nepal for a high-altitude trek, tick these boxes.
Before you leave home:
- Get a medical checkup. Undiagnosed anemia, cardiac issues, or pulmonary conditions significantly increase altitude risk.
- Discuss Diamox with your physician. Get a prescription if appropriate.
- Purchase comprehensive travel insurance that explicitly covers helicopter evacuation.
- Research the Nepal Tourism Board for current trek and permit conditions.
Packing for altitude:
- Diamox and ibuprofen, plus any personal prescription medications.
- A pulse oximeter. These small, inexpensive devices measure blood oxygen saturation and provide objective data when you are trying to decide whether symptoms are serious. A reading below 85 percent at rest at altitude warrants attention.
- A sleeping bag rated for temperatures at your maximum sleeping altitude.
- A headlamp for nighttime bathroom trips to high-altitude outdoor facilities.
On the trail:
- Log your symptoms daily. Morning headache, appetite, sleep quality. This pattern tells you how your body is responding.
- Communicate with your guide. Licensed Nepali guides have training in altitude first aid. They have seen AMS presentations many times. Listen to them.
- Do not compare yourself to other trekkers. Altitude response is largely genetic and unpredictable. The fit ultramarathoner in your group might suffer worse than the retired schoolteacher. This is not a performance competition.
The Mountain Decides the Pace
The Himalayas do not reward speed. Every experienced trekking guide in Nepal will tell you this, and the medical literature supports it completely. The trekkers who reach Everest Base Camp, cross Thorong La, and return with stories rather than evacuation invoices are the ones who treated their acclimatization schedule as seriously as they treated their training.
Build your itinerary conservatively. Add buffer days for weather delays and bad acclimatization days. Choose a guide who prioritizes your health over a fast turnaround. Know when to stop.
If you want to plan a high-altitude trek with an itinerary built around safe acclimatization from the start, get in touch with our team. Our guides have led thousands of trekkers through Nepal's mountains and understand how to pace an itinerary that balances the goal of reaching altitude with the reality of what the human body needs to get there safely.
