What is Altitude Sickness?
Altitude sickness — clinically known as Acute Mountain Sickness (AMS) — occurs when you ascend to high altitude faster than your body can adapt. At higher elevations, atmospheric pressure drops and there's less oxygen per breath. Your body needs time to compensate by producing more red blood cells and increasing breathing rate.
When you ascend too quickly, this acclimatisation process can't keep up. The result: oxygen delivery to tissues falls short, triggering a cascade of symptoms ranging from a mild headache all the way to life-threatening fluid build-up in the lungs or brain.
AMS typically begins above 2,500 metres (8,200 ft), though some individuals are affected as low as 2,000 m. In the Himalayas, most treks cross 3,500–5,000 m — making this a topic every trekker must understand.
The Golden Rule
Altitude sickness has nothing to do with fitness. World-class athletes get it. Sedentary first-timers don't. It's purely about how your individual physiology responds to lower oxygen — and that's not predictable. Treat every ascent with respect.
Who is at Risk?
Studies suggest that roughly 40–50% of trekkers experience some form of AMS above 3,500 m. Risk factors include:
- Previous history of AMS — the strongest predictor. If you've had it before, you're more likely to get it again.
- Fast ascent rate — the most controllable risk factor and the most common cause.
- Starting altitude — flying directly to Leh (3,500 m) is much higher risk than driving up gradually from Manali.
- Age under 50 — counterintuitively, younger people are slightly more susceptible, possibly due to being more aggressive in their ascent pace.
- Dehydration and alcohol — both impair acclimatisation significantly.
- Certain medications — sleeping pills, diuretics, and some antihistamines can worsen symptoms.
Fitness Does NOT Protect You
This is the most dangerous misconception in high-altitude trekking. Being extremely fit may actually increase risk — fit trekkers push harder and ascend faster. Physical conditioning is important, but it is not a shield against AMS.
Severity Levels — AMS, HACE & HAPO
There are three distinct conditions on a spectrum of severity. Knowing which is which can be the difference between a bad day and a medical emergency:
Acute Mountain Sickness (Mild)
Headache + one or more: fatigue, loss of appetite, dizziness, nausea. Common above 2,500 m. Uncomfortable but not dangerous. Stop ascending, rest, hydrate — usually resolves in 24–48 hours.
Moderate Altitude Sickness
Severe headache not relieved by ibuprofen, vomiting, increasing fatigue, difficulty walking straight. Requires immediate descent of 300–500 m minimum. Do not sleep at this altitude.
High Altitude Cerebral Edema
Fluid build-up in the brain. Symptoms: confusion, altered consciousness, loss of coordination (can't walk heel-to-toe), extreme lethargy. Life-threatening. Requires immediate descent and emergency medical care.
High Altitude Pulmonary Edema
Fluid in the lungs — the #1 killer at altitude. Symptoms: extreme breathlessness at rest, persistent dry cough (sometimes with pink frothy sputum), blue lips/fingernails, gurgling sounds in chest. Emergency descent NOW. Carries oxygen if available.
Recognizing Symptoms Early
The Lake Louise Score is the standard clinical tool used in the field. Check these symptoms every morning and evening on any trek above 2,500 m:
- Headache — present + any of the following = AMS:
- GI symptoms: nausea, vomiting, loss of appetite
- Fatigue that is disproportionate to exertion
- Dizziness or light-headedness when standing
- Insomnia — notably different from your normal sleep quality
- Shortness of breath at rest — this is a red flag requiring action
- Dry, persistent cough — another red flag
Use the thumb test: If someone is showing symptoms, ask them to stand, close their eyes, and walk heel-to-toe in a straight line for 10 steps. Inability to do this cleanly suggests HACE is developing — descend immediately.
How to Prevent AMS — 10 Proven Strategies
Treatment — What to Do if You're Affected
For Mild AMS
- Stop ascending immediately. Do not go higher until all symptoms have completely resolved — typically 24–48 hours.
- Rest and hydrate. Drink water consistently. Avoid caffeine and alcohol entirely.
- Ibuprofen 600mg every 8 hours helps significantly with AMS headache and has some evidence of reducing overall AMS severity.
- Monitor closely. Check SpO₂ every 2 hours if available. Have someone stay with the affected person overnight.
- If symptoms improve within 24 hours: cautious resumption of ascent is permissible.
- If symptoms worsen at any point or don't improve after 24 hours: descend.
For Moderate to Severe AMS / HACE / HAPE
Immediate Descent — No Exceptions
Descent is the definitive treatment. Even 300 m of descent can produce dramatic improvement. Do not wait until morning. Do not give the person another night to "see if they improve." Descend now, with assistance if needed. Supplemental oxygen and a Gamow bag are supportive measures, not substitutes for descent.
Medications — Diamox and Alternatives
Acetazolamide (Diamox)
How it works: Diamox stimulates faster breathing by making the blood slightly more acidic, which speeds up acclimatisation. It's the only medication with strong evidence for both prevention and treatment of AMS.
- Prevention dose: 125–250 mg twice daily, starting 1–2 days before ascent.
- Treatment dose: 250 mg twice daily.
- Side effects: Tingling in fingers and toes (very common, harmless), increased urination, occasional nausea. Carbonated drinks taste flat.
- Allergy note: Diamox is a sulfa drug. If you have a sulfa allergy, do NOT take it without specialist advice.
Dexamethasone (Dex)
A corticosteroid used for treatment of moderate-to-severe AMS and HACE. Not recommended for prevention (masks symptoms without aiding acclimatisation). Typically only carried by guides and expedition teams. Dose: 8 mg initial, then 4 mg every 6 hours. This is an emergency medication — descent must still occur.
Nifedipine
Used specifically for HAPE treatment. A calcium channel blocker that reduces pulmonary artery pressure, easing the fluid build-up in the lungs. Carried by experienced expedition teams. Cannot replace descent.
Get a Prescription Before Your Trek
None of these medications should be taken without prior consultation with a doctor who understands high-altitude medicine. Bring your medication kit assembled before departure — pharmacy access in remote trek areas is zero.
When to Descend Immediately
Memorise this list. These are non-negotiable descent triggers — never wait for morning:
- Inability to walk heel-to-toe in a straight line (ataxia)
- Any altered mental state — confusion, irrational behaviour, disorientation
- Breathlessness at complete rest
- Persistent cough with frothy or pink sputum
- SpO₂ below 75% at rest and not improving
- Loss of consciousness at any point
- Any AMS symptoms that are rapidly worsening
The Patient Cannot Make This Decision
A person with HACE or severe AMS often loses the cognitive ability to recognize how serious their situation is — or actively resists descending. It is the guide's and group's responsibility to insist on descent even against the patient's will. This rule has saved lives on MilesNPeople treks.
Arjun holds a Wilderness First Responder certification and has guided over 80 high-altitude expeditions across Uttarakhand, Ladakh, and Himachal Pradesh. He developed MilesNPeople's safety protocols and leads our guide training programme. He writes extensively on high-altitude medicine and responsible trekking.
