Altitude sickness kills trekking dreams in Nepal every season. Trekkers who arrive fit, motivated, and well-prepared still get turned around at 4,000 meters - not because of poor fitness, but because they underestimated what thin air does to the human body.
The good news is that altitude sickness is largely preventable. Understanding how it works, recognizing it early, and responding correctly dramatically reduces both your risk and the severity of symptoms. Our guides at Navigate Globe have spent years on the trails around Everest and Annapurna, and we have seen what works and what does not.
This guide gives you everything you need to know about altitude sickness before you set foot on a Nepal trek. Read it. Take it seriously. Your time in the mountains depends on it.
What Is Altitude Sickness?
Altitude sickness is the body's response to reduced oxygen levels at elevation. As you ascend, atmospheric pressure drops and each breath delivers fewer oxygen molecules to your lungs. Above 2,500 meters, most people begin to notice the change. Above 3,500 meters, it becomes a genuine physiological challenge.
The umbrella term for altitude-related illness covers three distinct conditions. Understanding each one helps you gauge how serious a situation is.
Acute Mountain Sickness (AMS)
AMS is the most common form - a mild to moderate response to rapid altitude gain. It presents similarly to a hangover: headache, nausea, fatigue, and poor sleep. AMS affects an estimated 25-40% of trekkers attempting the Everest Base Camp route. It is uncomfortable but manageable, and most people recover with rest, hydration, and no further ascent.
High Altitude Cerebral Edema (HACE)
HACE is severe AMS that has progressed to brain swelling. The brain accumulates fluid, causing symptoms far beyond a headache - confusion, loss of coordination, altered consciousness, and in extreme cases, coma. HACE is a medical emergency. A trekker with HACE must descend immediately, regardless of time of day or weather.
High Altitude Pulmonary Edema (HAPE)
HAPE is fluid accumulation in the lungs. It is the leading cause of altitude-related death and can progress from first symptoms to life-threatening condition within hours. HAPE can develop independently or alongside HACE. It requires immediate descent and, where available, emergency oxygen or a Gamow bag.
The UIAA Mountain Medicine Centre notes that HAPE is often misread as exhaustion or a chest cold in its early stages - which is precisely why knowing the symptoms in detail matters.
Altitude Sickness Symptoms: Mild AMS to Life-Threatening HACE and HAPE
The Lake Louise AMS Score is the standard tool used by medical professionals and guides to assess altitude sickness severity. It assigns points based on headache, gastrointestinal symptoms, fatigue, dizziness, and sleep quality. A score of 3 or more in the presence of a headache indicates AMS.
For practical trail use, here is how to read the symptoms across the three categories:
Mild to Moderate AMS Symptoms
- Headache (the hallmark symptom of AMS)
- Nausea or loss of appetite
- Fatigue and general weakness
- Dizziness or light-headedness
- Disturbed sleep - vivid dreams, waking frequently, difficulty breathing at night
- Mild shortness of breath at rest
These symptoms typically appear 6-12 hours after arriving at a new elevation. They often peak on the second night at altitude and ease by the third day, if no further ascent is made.
Warning: Do not ascend further if you have any AMS symptoms. "Walking it off" to a higher camp is one of the most dangerous things a trekker can do.
Severe AMS and HACE Symptoms
- Worsening, persistent headache that does not respond to ibuprofen or paracetamol
- Loss of coordination - the "drunk walk" test (inability to walk heel-to-toe in a straight line)
- Confusion, disorientation, or unusual behavior
- Extreme fatigue - inability to stand or walk without assistance
- Vomiting
If any of these are present, descend immediately. Do not wait until morning.
HAPE Symptoms
- Dry cough that progresses to producing pink or frothy sputum
- Breathlessness at rest, not just during exertion
- Crackling or gurgling sounds in the lungs when breathing
- Blue tinge to lips or fingernails (cyanosis)
- Rapid heart rate and feeling of tightness or pressure in the chest
- Severe weakness and inability to exert effort
HAPE can come on quickly with minimal warning. Any trekker showing these signs needs to descend at once, even in the middle of the night.
Which Nepal Treks Carry the Highest Altitude Risk?
Not all Nepal treks involve the same altitude exposure. The risk scales sharply above 3,500 meters and increases significantly above 4,500 meters.
Everest Base Camp Trek - Moderate to High Risk
The Everest Base Camp trek reaches 5,364 meters. Trekkers spend multiple nights above 4,000 meters, with sleeping elevations at Dingboche (4,410 m), Lobuche (4,910 m), and Gorak Shep (5,164 m). The extended time at high altitude and the gradual approach make EBC manageable with proper acclimatization, but it remains the trek where AMS most commonly causes evacuations in Nepal.
AMS affects 25-40% of EBC trekkers. Most cases are mild and resolved with rest. Helicopter evacuations for HACE and HAPE occur every season.
Annapurna Circuit - High Risk at Thorong La
The Annapurna Circuit crosses Thorong La Pass at 5,416 m - slightly higher than EBC. The risk here is the rapid ascent on summit day. Trekkers start from Thorong Phedi or High Camp at 3 AM and climb more than 1,600 meters to the pass before descending steeply to Muktinath. HAPE cases are not uncommon at the pass itself and in the teahouses just below.
Gokyo Ri and Three Passes Trek - High Risk
Gokyo Ri (5,357 m), Renjo La (5,360 m), Cho La (5,420 m), and Kongma La (5,535 m) all push trekkers into very high altitude. The Three Passes route involves more cumulative time above 5,000 meters than the standard EBC itinerary, placing it among the higher-risk options in the Khumbu.
Lower-Risk Treks
Poon Hill (3,210 m), Mardi Himal (4,500 m up to base camp), and Langtang Valley (up to 4,984 m at Kyanjin Ri) carry lower risk but still require awareness. Annapurna Base Camp at 4,130 m sees mild AMS cases regularly, particularly when trekkers ascend too quickly from Pokhara.
For full context on the demands of the EBC route, read our detailed Everest Base Camp trek difficulty guide.
How to Prevent Altitude Sickness on Nepal Treks
Prevention is far more effective than treatment. Every acclimatization tip here comes from guides who have walked these trails for decades and watched what separates trekkers who reach the top from those who turn back.
Ascend Slowly - the Non-Negotiable Rule
Above 3,000 meters, never increase your sleeping elevation by more than 300-500 meters per day. This is not a guideline - it is the physiological threshold at which most bodies can adapt without developing AMS.
The golden rule of high-altitude trekking is: climb high, sleep low. On acclimatization days, hike to a higher point during the day and return to a lower sleeping elevation. This triggers red blood cell production without overwhelming your system overnight.
Rushing the ascent is the single most common cause of AMS on Nepal treks. Cut days from your itinerary and you cut your chances of reaching the summit.
Plan Acclimatization Days Into Your Itinerary
On EBC, acclimatization days at Namche Bazaar (3,440 m) and Dingboche (4,410 m) are not optional extras - they are core parts of a safe schedule. On these days, hike up 300-400 meters and sleep lower. Your body does the adaptation work while you sleep.
If you feel unwell on an acclimatization day, take an additional rest day. Do not push forward on a fixed schedule when your body is telling you otherwise.
Stay Hydrated
Dehydration significantly worsens altitude symptoms. Drink 3-4 liters of water per day while trekking at altitude. Urine color is your guide - aim for pale yellow. Dark urine means you are behind on fluids.
Hot herbal teas (ginger, lemon) available at every teahouse are excellent. Avoid relying on sugary drinks or caffeine for hydration.
Avoid Alcohol and Sedatives Above 3,000 Meters
Alcohol impairs acclimatization and sleep quality at altitude. It dilates blood vessels and reduces oxygen saturation, amplifying every symptom of AMS. Even one beer at 3,500 meters can turn a manageable headache into a rough night.
Sleep medications and sedatives - including antihistamines - suppress the brain's drive to breathe, which is already reduced at altitude. Avoid them entirely above 3,000 meters.
Eat Well and Move Slowly
Dal bhat and other carbohydrate-rich foods are ideal at altitude because carbohydrates require less oxygen to metabolize than fats or proteins. High-carb meals support energy without adding metabolic stress.
Walk slowly - the "bistaarai bistaarai" principle. Your pace at altitude should allow easy conversation. If you cannot speak without pausing to breathe, you are moving too fast.
For more on acclimatization protocols and safety resources, see our altitude sickness resource page.
Diamox for Altitude Sickness: Does It Work?
Diamox (acetazolamide) is the most widely used medication for altitude sickness prevention and treatment. It has a strong evidence base and is used by high-altitude mountaineers, trekkers, and rescue teams worldwide.
How Diamox Works
Diamox is a carbonic anhydrase inhibitor. At altitude, it works by acidifying the blood slightly, which tricks the brain's respiratory center into increasing the breathing rate. More breaths per minute means more oxygen per minute - effectively mimicking a faster acclimatization process.
It does not eliminate altitude sickness entirely, but it significantly reduces the severity and onset of AMS symptoms in most people.
Dosage
The standard preventive dose is 125 mg twice daily (morning and evening), starting 24 hours before ascending above 2,500 meters. Some guides recommend 250 mg twice daily for higher-risk itineraries or for people with a history of AMS.
For treatment of existing AMS symptoms, the same dosage is used. Diamox does not replace descent - if symptoms are severe, descend first.
Side Effects
- Increased urination - expect to wake once or twice in the night. Stay hydrated
- Tingling in fingers, toes, and lips - very common and harmless, caused by electrolyte shifts
- Carbonated drinks taste flat - a minor inconvenience
- Sensitivity to sunlight - use sunscreen and sunglasses
Diamox is a sulfa-class drug. People with sulfa allergies should not take it. Pregnant women and people with kidney conditions should consult a doctor before use.
Who Should Use Diamox
Diamox is not compulsory for every trekker. Consider it if you:
- Have had AMS on a previous trek
- Are on an aggressive itinerary with limited acclimatization time
- Know you are prone to AMS at moderate altitude
- Have any concerns about your acclimatization capacity
Always consult your doctor before starting Diamox. It requires a prescription in most countries. Discuss your itinerary, your medical history, and the correct dosage for your specific situation.
The World Health Organization includes acetazolamide in its guidance for altitude illness prevention as a supported intervention with a clear evidence base.
Treatment: What to Do If You Get AMS on the Trail
Most trekkers experience some form of altitude discomfort. Knowing how to respond is as important as knowing how to prevent it.
Mild AMS: Rest and Reassess
If you develop a mild headache and some nausea at your sleeping elevation, stop ascending. Rest at your current altitude. Drink fluids, eat lightly, and take ibuprofen (400-600 mg) or paracetamol (1,000 mg) for the headache.
Do not ascend higher the following day if symptoms persist. Two consecutive nights at the same elevation often resolve mild AMS entirely.
Moderate AMS: No More Ascent, Possible Descent
If your Lake Louise score is 3 or above, or if mild symptoms are not improving after 24-48 hours of rest, do not go higher. Discuss with your guide whether a descent of 300-500 meters is needed to stabilize your condition.
Supplemental oxygen, if available, provides temporary relief but does not replace descent or treatment.
HACE or HAPE: Descend Immediately
There is no waiting, no consulting the weather, and no debating with the schedule. If HACE or HAPE is suspected:
- Descend immediately - even 300-500 meters makes a significant physiological difference
- Administer oxygen if available - target SpO2 above 90%
- Use a Gamow bag if descent is impossible and one is available
A Gamow bag is a portable hyperbaric chamber. Inflating it around the patient increases the effective air pressure, simulating a lower altitude. It buys critical time when immediate descent is blocked by weather or terrain. Many larger teahouses and rescue checkpoints along popular routes keep one on site.
- Contact your guide and your travel insurance to arrange helicopter evacuation immediately
Emergency Evacuation in Nepal
Helicopter rescue is the primary emergency transport in the Himalaya. Nepal's mountain rescue infrastructure is well-developed, with helicopters operating out of Kathmandu and Lukla. Response times from activation to evacuation typically range from 1-4 hours depending on weather.
The cost of a helicopter evacuation from the Khumbu region is $3,000-$7,000 or more. This is why travel insurance with helicopter evacuation cover is non-negotiable for every Nepal trek.
Confirm before you leave home that your policy covers:
- High-altitude trekking to at least 6,000 meters
- Emergency helicopter evacuation
- Medical treatment and hospitalization in Nepal
Contact us to discuss permits, itineraries, and safety planning before your trek. Our team at Navigate Globe coordinates with established rescue networks and knows the exact protocols for each route. Get in touch here.
Acclimatization Itinerary Tips: EBC and Annapurna Circuit
The best way to prevent altitude sickness is to follow a schedule that gives your body time to adapt. Here are the key acclimatization checkpoints for the two most popular high-altitude treks in Nepal.
Everest Base Camp - Recommended Acclimatization Schedule
A 14-day EBC itinerary should include these rest or acclimatization days:
| Day | Location | Sleeping Elevation | Action |
|---|---|---|---|
| 2-3 | Namche Bazaar | 3,440 m | Full acclimatization day - hike to Everest View Hotel (3,880 m) and return |
| 6 | Dingboche | 4,410 m | Acclimatization day - hike to Nagarjun Hill (5,100 m) and return |
| 8 | Lobuche | 4,910 m | Short day walk only. Do not push to Gorak Shep same day |
| 9-10 | Gorak Shep | 5,164 m | EBC summit day, then descend to Pheriche for sleep |
The critical rule on EBC is to sleep low after your summit day. Trekkers who sleep at Gorak Shep (5,164 m) after reaching Base Camp (5,364 m) report significantly more AMS symptoms than those who descend to Pheriche (4,240 m) the same evening.
Annapurna Circuit - Managing the Thorong La Crossing
The Thorong La crossing (5,416 m) requires a pre-dawn start because afternoon cloud and wind make the pass dangerous. Acclimatization strategy:
| Checkpoint | Elevation | Note |
|---|---|---|
| Manang | 3,519 m | Minimum 2 nights. Day hike to Ice Lake (4,600 m) is essential |
| Yak Kharka | 4,018 m | One night - acclimatization step |
| Thorong Phedi / High Camp | 4,450 m / 4,850 m | Start Thorong La crossing at 3-4 AM |
| Thorong La Pass | 5,416 m | Do not linger. Descend immediately to Muktinath (3,800 m) |
Trekkers who skip the Manang acclimatization days and go straight to the pass attempt face a disproportionately high rate of HAPE and HACE. Manang's two nights are not wasted time - they are what makes the crossing safe.
General Acclimatization Principles for All Treks
- If you feel unwell, do not move up. The mountain does not have a deadline
- A lost day at altitude is far better than a helicopter evacuation
- Never underestimate your second night at a new elevation. AMS often peaks then
- Drink water before bed, not just during the day
- Use the Nepal trekking permits system to understand the formal requirements that apply to your route
Trek Smart, Summit Safe
Altitude sickness in Nepal is not a rare complication or a freak event. It is a predictable physiological response to a predictable environmental challenge. The trekkers who avoid it are not stronger or fitter than those who get it - they are simply better informed and better prepared.
Follow the acclimatization schedule. Drink your water. Watch for symptoms. Listen to your guide. Descend without hesitation if the warning signs appear.
At Navigate Globe, every itinerary we build includes proper acclimatization days, licensed guides trained in altitude response, and pre-trek briefings on exactly the information in this article. Our guides have walked the Khumbu Valley and the Annapurna Circuit hundreds of times. They know the signs, they know the protocols, and they will not let you push past your limits.
If you are planning an Everest Base Camp trek or any high-altitude route in Nepal, start your planning with safety at the center of it. Browse our altitude sickness resources or contact our team to discuss the right itinerary for your experience level and goals.
The Himalayas are extraordinary. Go prepared, go informed, and go with people who know the mountain.
