Altitude Sickness: Symptoms, Prevention & Treatment for Trekkers
Altitude sickness is the most common medical problem faced by trekkers at elevations above 2,500 meters (8,200 feet). From the Everest Base Camp trail in Nepal to Kilimanjaro in Tanzania, the thin air at high altitude causes real physiological stress that no amount of fitness can override. Understanding how to prevent altitude sickness — and recognizing when it becomes dangerous — can mean the difference between a summit and an evacuation.
This guide covers the three forms of altitude illness, evidence-based acclimatization strategies, when and how to use Diamox (acetazolamide), and what to pack for high-altitude adventures. Whether you are planning a Himalayan trek, a visit to La Paz, or a climb of Kilimanjaro, these principles apply.
Altitude sickness key facts
Trekkers affected above 3,500m
25-50%
Altitude where AMS can begin
2,500m
HACE/HAPE incidence above 4,000m
0.5-1%
Recommended ascent rate
300m/day
What Is Altitude Sickness? The Three Forms
Altitude sickness — technically called high-altitude illness — occurs when the body cannot adapt quickly enough to the reduced oxygen and lower air pressure at elevation. At 3,500 meters, each breath delivers roughly 60% of the oxygen available at sea level. The body compensates by increasing breathing rate, heart rate, and producing more red blood cells — but this adaptation takes time.
There are three recognized forms of altitude illness, ranging from uncomfortable to life-threatening:
Three forms of altitude illness — know the difference
AMS — Acute Mountain Sickness
Most common form. Uncomfortable but not directly life-threatening. Usually resolves with rest and descent.
- Headache (the cardinal symptom, present in nearly all cases)
- Nausea, loss of appetite, sometimes vomiting
- Fatigue, dizziness, lightheadedness
- Difficulty sleeping (common above 3,000m)
- Onset: 6-24 hours after arrival at altitude
- Affects 25-50% of people above 3,500m
HACE — High Altitude Cerebral Edema
Life-threatening brain swelling. Requires immediate descent. Fatal without treatment.
- Confusion, disorientation, irrational behavior
- Severe headache unresponsive to painkillers
- Ataxia (inability to walk a straight line — the key diagnostic sign)
- Drowsiness progressing to unconsciousness
- Onset: usually evolves from untreated severe AMS over 1-3 days
- Incidence: 0.5-1% of trekkers above 4,000m; fatality rate 25% if untreated
HAPE — High Altitude Pulmonary Edema
Life-threatening fluid accumulation in lungs. Most common cause of death from altitude illness.
- Breathlessness at rest (not just during exertion)
- Persistent dry cough, progressing to frothy or pink sputum
- Extreme fatigue and weakness disproportionate to exertion
- Chest tightness or congestion
- Crackling sounds when breathing (audible without stethoscope in severe cases)
- Onset: typically 2-4 days after arrival above 3,000m; can occur without AMS
- Incidence: 0.2-2% above 4,000m; fatality rate up to 50% without descent
HACE and HAPE require immediate descent
HACE and HAPE are medical emergencies. The single most important treatment for both is immediate descent — go down at least 300-1,000 meters as quickly as possible. Do not wait until morning. Do not continue ascending. If descent is impossible due to weather or terrain, use a portable hyperbaric chamber (Gamow bag) if available and administer supplemental oxygen.
High-Altitude Destinations: Know Your Risk
These popular travel destinations all involve significant altitude exposure. Risk varies based on how quickly you ascend and how high you sleep:
Popular high-altitude destinations
Everest Base Camp, Nepal
5,364m
Kilimanjaro Summit, Tanzania
5,895m
Machu Picchu trail (high point)
4,130m
La Paz, Bolivia (city center)
3,640m
Destination-specific risks
Destination altitude risks
- ○Everest Base Camp (Nepal): 12-14 day trek allows gradual acclimatization; AMS rate ~50% despite slow ascent
- ○Kilimanjaro (Tanzania): Rapid ascent over 5-7 days is the main danger; AMS rate 50-75%; HAPE/HACE significant risk
- ○Cusco and Machu Picchu (Peru): Cusco sits at 3,400m — many tourists fly in from sea level and feel AMS within hours
- ○La Paz (Bolivia): At 3,640m, the world's highest capital. Visitors arriving by air from sea level are at immediate risk
- ○Quito (Ecuador): At 2,850m — near the AMS threshold; most visitors adjust within 1-2 days
- ○Lhasa, Tibet (China): At 3,650m — similar to La Paz. Chinese authorities require permits that include altitude advisory
Kilimanjaro: longer routes are safer
Kilimanjaro has the highest rate of altitude illness among popular treks because most itineraries ascend too fast. Choose a 7-8 day route (Lemosho, Northern Circuit) rather than a 5-day route (Marangu). Longer itineraries have significantly lower AMS, HACE, and HAPE rates.
Acclimatization: The Golden Rules
Acclimatization is the body's process of adapting to reduced oxygen at altitude. It cannot be rushed — no amount of fitness, training, or willpower substitutes for time. The Wilderness Medical Society (2019) provides clear guidelines:
The golden rules of acclimatization
- 1
Ascend gradually above 3,000m
Above 3,000 meters, increase your sleeping altitude by no more than 300-500 meters per day. This is the single most important rule for preventing altitude illness.
- 2
Build in rest days
For every 1,000 meters of altitude gained, spend an extra night at the same elevation. On rest days, you can hike higher during the day but return to sleep at your current altitude.
- 3
Climb high, sleep low
During the day, trek to a higher altitude, then descend to a lower elevation to sleep. This accelerates acclimatization by exposing your body to lower oxygen during exertion but allowing recovery at a more comfortable altitude.
- 4
Stay hydrated
Drink 3-4 liters of water per day at altitude. Dehydration worsens AMS symptoms and mimics altitude illness. Avoid alcohol for the first 48 hours at each new altitude.
- 5
Eat carbohydrate-rich meals
Carbohydrates require less oxygen to metabolize than fats or proteins. A high-carb diet (60-70% of calories) during acclimatization supports your body's oxygen needs.
- 6
Listen to your body — never ignore symptoms
If you develop AMS symptoms, do NOT ascend further until symptoms resolve. If symptoms worsen despite rest, descend immediately. Never leave a symptomatic person alone.
Fitness is not protection
Fitness does NOT protect against altitude sickness. Elite athletes get AMS at the same rate as sedentary individuals. In fact, very fit trekkers may be at higher risk because they tend to ascend faster and push through early symptoms. The only proven protection is gradual ascent and acclimatization time.
Diamox (Acetazolamide): Prevention and Treatment
Acetazolamide (brand name Diamox) is the most studied and recommended medication for preventing and treating altitude sickness. It works by causing mild metabolic acidosis, which stimulates breathing and improves oxygen uptake — essentially accelerating the acclimatization process.
Diamox for prevention (prophylaxis)
Diamox prophylaxis protocol
- ○Dose: 125mg twice daily (morning and evening)
- ○Start: 1 day before ascent above 3,000m
- ○Continue: Until 2 days after reaching maximum altitude, or throughout the trek
- ○Effectiveness: Reduces AMS incidence by ~50%
- ○Prescription required in most countries — arrange before travel
- ○A trial dose at home is recommended to check for side effects before the trek
Diamox for treatment
Diamox treatment protocol
- ○Dose: 250mg twice daily for established AMS
- ○Combine with: rest (no further ascent), hydration, and paracetamol/ibuprofen for headache
- ○If no improvement in 24 hours: descend at least 300-500m
- ○Diamox does NOT treat HACE or HAPE — these require immediate descent
Side effects of Diamox
Common side effects
- ○Tingling in fingers, toes, and lips (very common, harmless — called paresthesia)
- ○Increased urination (by design — take last dose before 6 PM)
- ○Altered taste — carbonated drinks taste flat (bicarbonate effect)
- ○Mild nausea in some people
- ○Photosensitivity — wear sunscreen and sunglasses
- ○Contraindicated: sulfa allergy, severe kidney/liver disease, pregnancy
Sulfa allergy? Ask about alternatives
If you have a sulfa allergy, Diamox is contraindicated. Alternatives include dexamethasone (for AMS/HACE prevention, prescription only) and nifedipine (for HAPE prevention). Discuss options with your doctor at least 4-6 weeks before travel.
Other Medications for Altitude Illness
Dexamethasone
A corticosteroid used for HACE treatment and AMS prevention when Diamox cannot be used. Dose for AMS prevention: 2mg every 6 hours or 4mg every 12 hours. For HACE treatment: 8mg initial dose, then 4mg every 6 hours. Reduces brain swelling rapidly but does NOT aid acclimatization — symptoms return if stopped at altitude.
Nifedipine
A calcium channel blocker used for HAPE prevention and treatment. Dose: 30mg extended-release every 12 hours. Reduces pulmonary artery pressure, which is elevated at altitude. Primarily used for people with a history of HAPE. Not for routine prophylaxis.
Ibuprofen
Studies show ibuprofen (600mg three times daily) may be effective for AMS prevention, similar to Diamox. It is available over the counter and may be an option for those who cannot take Diamox. However, Diamox remains the first-line recommendation.
What to Pack for High-Altitude Travel
Beyond your regular trekking gear, these altitude-specific items could save your life. For a complete packing guide, see our travel health kit guide.
Altitude-specific packing list
- ○Diamox (acetazolamide) — prescription; bring enough for your entire trek plus extra
- ○Pulse oximeter — small, lightweight; monitors blood oxygen saturation (SpO2)
- ○Paracetamol and ibuprofen — for altitude headaches
- ○Oral rehydration salts (ORS) — for maintaining hydration
- ○Sunscreen SPF 50+ and lip balm with SPF — UV radiation increases 10-12% per 1,000m
- ○High-quality sunglasses (category 4) — snow blindness risk above 4,000m
- ○Insulated water bottle — prevents water from freezing at high camps
- ○Quick-energy snacks — glucose tablets, energy bars, dried fruit, chocolate
- ○Headlamp with fresh batteries — essential for early morning summit attempts
- ○Portable hyperbaric bag (Gamow bag) — for organized expeditions above 4,500m
- ○Emergency evacuation insurance — helicopter rescue coverage for Nepal, Peru, Tanzania
Carry a pulse oximeter
A pulse oximeter is one of the most valuable tools for monitoring altitude acclimatization. Normal SpO2 at sea level is 95-100%. At 3,500m, 85-90% is typical. Below 80% with symptoms is a red flag requiring descent. Cost is minimal (under 20 dollars) and weight is negligible.
When to Descend: Recognizing an Emergency
Knowing when to turn back is the most important altitude safety skill. Follow these absolute rules:
Decision ladder — when to descend
- 1
Mild AMS: stop ascending
If you develop headache plus one or more AMS symptoms (nausea, fatigue, dizziness, poor sleep), do not go higher. Rest at current altitude, hydrate, take Diamox/paracetamol. Most cases resolve within 12-48 hours.
- 2
Worsening AMS: descend 300-500m
If symptoms persist or worsen after 24 hours of rest, or if you develop severe headache, persistent vomiting, or marked fatigue — descend immediately to the last altitude where you felt well.
- 3
Ataxia: descend immediately (HACE)
Test for ataxia: walk heel-to-toe in a straight line. If you or your trekking partner cannot do this, this is HACE until proven otherwise. Descend immediately, day or night. Administer dexamethasone if available.
- 4
Breathless at rest: descend immediately (HAPE)
If you are breathless while sitting still, or develop a wet cough with frothy/pink sputum, this is HAPE. Descend at least 1,000m immediately. Administer nifedipine and supplemental oxygen if available.
Frequently Asked Questions
Can I train to prevent altitude sickness?
Physical fitness does not prevent altitude sickness. AMS affects athletes and non-athletes equally. However, good cardiovascular fitness helps with the physical demands of trekking at altitude (carrying a pack uphill while oxygen is reduced). Train for endurance, but do not expect fitness to replace acclimatization.
Does sleeping with extra pillows help?
Sleeping with your head slightly elevated (15-30 degrees) may reduce the periodic breathing pattern common at altitude, which disrupts sleep. This is not a substitute for acclimatization but can improve sleep quality.
Is altitude sickness worse the second time?
Not necessarily. Some people have consistent susceptibility, while others experience it on one trip but not another. Previous successful acclimatization does not guarantee future immunity. Apply the same precautions every time.
Should I take Diamox even if I feel fine?
Prophylactic Diamox is recommended for rapid ascent profiles (flying into high-altitude cities like La Paz, Cusco, or Lhasa) and for people with a history of AMS. For gradual treks with proper acclimatization days, it is optional but still reduces risk by approximately 50%.
Can children get altitude sickness?
Yes. Children are at least as susceptible as adults and may not be able to articulate symptoms. The Wilderness Medical Society advises against taking infants below 6 months above 2,500m. For older children, follow the same ascent guidelines as adults and monitor closely for irritability, loss of appetite, and unusual fatigue.
Related Reading
- Peru & Bolivia Travel Health: Altitude, Vaccines & Essential Preparation
- Travel Health Kit: What to Pack
- Pre-Travel Health Checklist: The Complete Guide
Important Disclaimer
Medical disclaimer
This article is for educational purposes only and does not constitute medical advice. Altitude illness can be life-threatening. Before traveling to high-altitude destinations, consult a travel medicine specialist or your healthcare provider for personalized advice on Diamox, acclimatization planning, and fitness-to-travel assessment. In an emergency at altitude, descend immediately and seek medical help.
Sources: Wilderness Medical Society Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness (2019), CDC Yellow Book 2026, BMJ Altitude Illness Review (2024). Last updated: April 2026.