You can be young, fit, and well-trained and still get altitude sickness. Above the treeline, thinner air forces your body to retool itself — breathing quickens and red blood cell production ramps up, and that adaptation takes time. Almost every serious altitude problem on treks is preventable with a sensible ascent schedule and the discipline to stop or turn back when something feels wrong. This guide covers the essentials — it is general travel information, not medical advice.
What altitude sickness actually is
Medical sources describe three related illnesses, all triggered by low oxygen at height:
Acute mountain sickness (AMS) is the common one, typically above around 2,500 metres — often 6–10 hours after reaching a new sleeping altitude. A headache plus nausea, fatigue, dizziness, or poor sleep. It feels like a bad hangover, which is exactly why trekkers underestimate it.
High-altitude pulmonary edema (HAPE) is fluid leaking into the lungs, generally in people who ascended too fast above 2,500 metres. Early signs are breathlessness out of proportion to effort and unusual fatigue; warning signs are breathlessness at rest, a persistent cough, and frothy, pink-tinged or blood-flecked sputum. Untreated HAPE can be rapidly fatal.
High-altitude cerebral edema (HACE) is brain swelling, generally seen above 3,000 metres. Red flags are loss of coordination, confusion, altered behaviour, or abnormal drowsiness. Like HAPE, it can kill if not recognized and treated promptly.
How acclimatization works
The most useful fact in altitude medicine is this: your body adapts to the altitude where you sleep, not the height you reach during the day. The CDC’s Yellow Book and the Wilderness Medical Society guidelines both emphasize this. It is why the classic “climb high, sleep low” technique works: hike higher during the day, then descend to sleep lower.
The day-by-day rules that prevent most problems
The ascent rules are consistent across medical sources — build your itinerary around them:
Cap sleeping-altitude gains at 300–500 metres per day above 3,000 metres. The number that matters is how much higher tonight’s camp is than last night’s.
Take a rest day every 3–4 days (or an extra acclimatization day for every 1,000 metres gained). A rest day means no upward movement of your sleeping altitude; an easy acclimatization hike during the day is fine.
Avoid sleeping high on day one. Sleep below about 2,750 metres on the first night. Flying or driving straight to high altitude and exerting yourself the same day is the classic recipe for trouble.
Know the golden rule: never ascend while you have symptoms. The CDC’s guidance puts it plainly — never go up to sleep at a higher altitude with any symptoms of altitude illness, however minor they seem.
Warning signs you must not ignore
Stop ascending and rest if you develop any AMS symptoms. If they do not improve with rest, or they worsen despite treatment at the same altitude, descend — medical guidance is explicit that descent is the definitive treatment, and ascent should never continue while someone remains symptomatic.
Treat these as emergencies triggering immediate descent: breathlessness at rest, a persistent cough, or pink/frothy sputum (possible HAPE — even a few hundred metres of descent can be lifesaving); stumbling, confusion, or abnormal drowsiness (possible HACE — descend immediately and seek medical help); any symptoms that worsen despite a rest day.
On a guided trek, have your guide check symptoms every morning and evening, and agree in advance that turning around is always acceptable. The summit will be there next year.
Hydration, alcohol, and pacing
Dehydration and altitude symptoms amplify each other: dry mountain air and faster breathing increase fluid loss, and dehydration causes headaches and fatigue that mimic AMS. Drink noticeably more than usual, keep your urine pale, and eat well — you burn far more energy at altitude. Keep alcohol intake minimal or skip it during ascent days: it dehydrates, disrupts sleep, and can mask symptoms. Walk slower than you think you need to, especially in the first days at altitude; light daytime activity aids acclimatization, but overexertion on arrival is a known trigger of illness.
Medicines and oxygen — talk to your doctor
A sensible schedule is always the best prevention, but medication has a recognized supporting role. Acetazolamide (Diamox) is the drug with the strongest evidence for preventing AMS; guidelines generally point it at people at moderate to high risk — those with a history of altitude illness or forced rapid ascents — and it is typically started the day before ascent. It speeds acclimatization rather than masking symptoms, and side effects like tingling fingers are common. Discuss suitability with your doctor before the trip, not at the trailhead.
Dexamethasone is reserved for emergency treatment, not routine prevention, and guidance cautions against casual use. Supplemental oxygen and portable hyperbaric chambers are valuable temporizing measures that can stabilize someone with HAPE or HACE while a descent is organized — but they are no substitute for going down.
Before any high-altitude trek, talk to your doctor: bring up acetazolamide, know your risk factors (a previous bad experience at altitude is the most important one), and carry the medication even if your itinerary is conservative.
In short: ascend slowly, cap sleeping-altitude gains, rest regularly, never go up with symptoms, and descend without hesitation if things worsen. Altitude sickness punishes impatience — give the mountains the days they demand.
Arjun Mehta is the pen name behind Mountains Bliss — practical mountain-travel guides with routes, costs, seasons and safety notes, researched from real trekker discussions and cross-checked before publishing. No sponsored posts, ever.
More about Mountains Bliss →