Every year, hundreds of trekkers on Nepal's trails ignore subtle warning signs that their body is struggling with altitude — and a handful pay for that mistake with their lives. Acute Mountain Sickness (AMS) is not a sign of weakness or poor fitness. It is a physiological response to reduced oxygen, and it can affect a 25-year-old marathon runner just as easily as a 60-year-old first-time trekker.
The good news: AMS is almost always preventable, and when caught early, it is entirely manageable. The key is knowing what to look for and having the confidence to act on what you find.
What Is Altitude Sickness?
When you ascend to altitude, the air pressure drops. Your lungs draw in the same volume of air but receive fewer oxygen molecules per breath. Your body compensates — your breathing rate increases, your heart beats faster, your blood thickens slightly — but this adaptation takes days. If you ascend faster than your body can adapt, fluid can accumulate in the lungs or brain, with potentially fatal consequences.
AMS typically begins above 2,500 metres (8,200 ft). On the Everest Base Camp trek, you cross 3,440m at Namche Bazaar on day three. On the Annapurna Circuit, you hit 4,130m at Thorong Phedi. Almost every major Nepal trek will take you into AMS territory.
The 5 Warning Signs You Must Know
1. Persistent Headache
The hallmark symptom of AMS is a headache that does not go away with rest or mild pain relief (paracetamol/ibuprofen). It typically feels like a throbbing pressure behind the eyes or across the forehead. Many trekkers dismiss this as dehydration — and while dehydration does cause headaches at altitude, the difference is that an AMS headache persists even after you drink 500ml of water and rest for an hour.
What to do: Stop ascending. Rest at your current altitude for 24 hours. Take ibuprofen or paracetamol. Drink 3–4 litres of water through the day. If the headache worsens or does not improve after 24 hours, descend immediately.
2. Nausea and Loss of Appetite
Your digestive system is one of the first casualties of altitude. At 4,000m+, many trekkers find that food — even foods they normally love — becomes deeply unappealing. Mild nausea is common and not always alarming on its own. But combined with a headache or fatigue, it is a clear AMS signal.
Vomiting at altitude is more serious. It rapidly accelerates dehydration and makes it impossible to take oral medication or maintain energy. If you are vomiting and above 3,500m, descend the same day — do not wait to see if it passes.
Practical note: Force yourself to eat even when you have no appetite. A few biscuits or half a bowl of soup provides enough energy to keep moving. Dal Bhat — Nepal's traditional lentil-rice dish — is ideal as it is calorie-dense, easy to digest, and available at every teahouse on every trail.
3. Fatigue and Weakness Out of Proportion to Effort
You will be tired on any mountain trek. Sore legs, heavy breathing, the desire to sit down — all normal. AMS fatigue is qualitatively different: a bone-deep exhaustion that does not ease with rest, combined with general weakness that makes simple tasks feel difficult.
A useful field test: if you feel distinctly worse after sleeping than you did before — especially if a night at altitude has left you more tired, not less — that is a significant warning sign. Healthy acclimatization works the other way: a rest night at the same altitude should leave you feeling measurably better the next morning.
4. Dizziness and Loss of Coordination
Mild lightheadedness when standing quickly is common at altitude. The AMS version is more concerning: a persistent unsteadiness, difficulty walking in a straight line, or a feeling of being "drunk" on a trail where you have done nothing to earn that sensation.
Coordination problems are a red flag for High Altitude Cerebral Edema (HACE) — the dangerous progression of AMS where fluid accumulates in the brain. The Tandem Gait Test is used by guides and medical teams on Nepal's trails: walk heel-to-toe in a straight line for six steps. If you cannot do this cleanly, HACE must be suspected and immediate descent is required, along with emergency oxygen and Dexamethasone if available.
5. Sleep Disturbances and Periodic Breathing
At altitude, the body's breathing regulation is disrupted during sleep. Many trekkers experience Cheyne-Stokes respiration — a cycle of deep breathing, then a pause of 10–15 seconds where breathing stops entirely, then a sudden gasp. It is alarming to witness in a tent-mate and deeply disturbing if you wake up in the middle of one of the apnea (no-breathing) phases.
Disrupted sleep is both a symptom of AMS and a cause of further deterioration — poor sleep at altitude means poor recovery. If you consistently wake at 3am feeling anxious, short of breath, or with a pounding heart, note it as part of the picture alongside your other symptoms.
When AMS Becomes an Emergency: HACE and HAPE
AMS is the mild-to-moderate form of altitude illness. If the underlying cause — ascending too fast — is not corrected, it can progress to two life-threatening conditions.
HACE — High Altitude Cerebral Edema
Fluid accumulates in the brain. Symptoms: severe headache that does not respond to any medication, profound confusion or disorientation, inability to walk straight, loss of consciousness. This is a medical emergency. The treatment is immediate descent of at least 300–500 metres, plus Dexamethasone 8mg IM/IV immediately. A Gamow bag (portable hyperbaric chamber) buys time for evacuation if descent is delayed. Helicopter rescue should be called without hesitation.
HAPE — High Altitude Pulmonary Edema
Fluid accumulates in the lungs. Symptoms: persistent dry cough, pink or frothy sputum, extreme breathlessness even at rest, blue tinge to lips (cyanosis), inability to lie flat. HAPE is more common than HACE and kills more trekkers. The treatment is immediate descent — do not wait until morning, do not wait for weather to improve. Nifedipine 30mg extended-release (or Tadalafil 10mg) and supplemental oxygen if available. Descend, descend, descend.
Prevention: The Rules That Actually Work
- Ascend slowly: The standard guideline above 3,000m is to gain no more than 300–500 metres of sleeping altitude per day.
- Acclimatisation days: On the EBC route, Namche Bazaar (3,440m) has a mandatory acclimatisation day for good reason. Use it — hike to 3,800m and sleep lower. Same rule applies at Dingboche and Lobuche.
- Hydrate aggressively: Aim for 3–4 litres per day. Urine should be pale yellow. Dark urine at altitude is a warning sign.
- Avoid alcohol and sleeping tablets: Both suppress respiration and worsen Cheyne-Stokes breathing. Save the drinks for the celebration in Kathmandu.
- Consider Diamox (Acetazolamide): 125mg twice daily, starting 24 hours before ascent, is effective prophylaxis. Consult a travel medicine doctor before your trip. Side effects include tingling in fingers and increased urination.
- Know your escape routes: Before each day's walk, know the nearest lower-altitude village where you could spend the night if symptoms develop.
The Golden Rule
Never ascend with symptoms. Never. Altitude sickness at 4,000m that you push through becomes HACE or HAPE at 5,000m. The mountain will always be there next year.
Frequently Asked Questions
Can fit, young trekkers still get altitude sickness?
Yes. AMS has no correlation with fitness level, age, or prior trekking experience. Fitness helps you walk faster — which can paradoxically increase risk if it leads to ascending too quickly. AMS susceptibility is largely genetic and unpredictable.
How long does acclimatisation take?
Your body needs roughly 2–4 days to acclimatise to each significant altitude gain (1,000m+). Full adaptation to very high altitudes (above 5,000m) takes weeks, which is why mountaineers spend months above base camp before summit attempts.
Is Diamox necessary for every trekker?
No. Many trekkers acclimatise successfully on a well-paced itinerary without medication. Diamox is recommended for those with a history of AMS, those on compressed itineraries, or those trekking to extreme altitudes. Discuss with a travel medicine doctor at least 4 weeks before departure.
What if I get symptoms on a day hike and return to lower altitude each night?
The "climb high, sleep low" principle is highly effective. If you are sleeping at 3,440m but hiking to 3,800m during the day, you gain acclimatisation benefit from the high altitude while recovering at the lower sleeping altitude. This is built into most reputable itineraries.
Can altitude sickness kill you on day one of symptoms?
AMS itself rarely kills directly. The danger is ignoring mild AMS and continuing to ascend, which can lead to HACE or HAPE developing over 12–48 hours. People die from altitude illness because they underestimated early symptoms — not because the illness struck without warning.
Where is the nearest medical facility on the EBC route?
The Himalayan Rescue Association operates a medical clinic at Pheriche (4,280m) staffed by altitude medicine specialists during trekking season (March–May, September–November). There is also a HRA clinic at Manang on the Annapurna Circuit. Both offer free altitude sickness consultations.
Trekking in Nepal is one of the most rewarding experiences on earth. With the right knowledge and a willingness to slow down and listen to your body, altitude sickness is something you will read about rather than experience. Book your Everest Base Camp trek with Yeti Routes and our guides will brief you fully on altitude protocols before you take your first step on the trail.
