How to Acclimatize and Avoid Altitude Sickness on a Trek: A Practical Guide

Altitude sickness does not care how fit you are. Marathon runners get it on their first Himalayan teahouse trek; seventy-year-olds cruise through 5,000-metre passes without a headache. It is the great leveller of mountain travel — and the one trekking hazard that is almost entirely preventable with planning. Whether your trek tops out at 3,000 or 6,000 metres, the same principles apply: ascend slowly, hydrate relentlessly, and know the difference between a bad headache and a medical emergency. This is the practical playbook.

What altitude sickness actually is

Above roughly 2,700–2,750 metres (about 9,000 feet), the air holds too little oxygen for an unadapted body, and acute mountain sickness (AMS) can set in. The classic symptoms — headache, nausea, dizziness, poor sleep, loss of appetite — are common and usually mild. The danger lies in what untreated AMS can become: high-altitude pulmonary edema (HAPE) and high-altitude cerebral edema (HACE), both potentially fatal and both signalled by worsening symptoms such as breathlessness at rest, loss of coordination, or confusion. Fitness, age and previous experience offer no reliable immunity; the only proven protection is a slow, well-designed ascent.

The golden rules of acclimatization

First, keep your sleeping altitude gains small. Above about 2,500–2,750 metres, aim to increase the altitude you sleep at by no more than 300–500 metres per day, and build in a rest day after every significant gain (after every 900 metres or so, or every three to four days of climbing, depending on whose rulebook you follow). Second, practice “climb high, sleep low”: you can safely hike several hundred metres above your camp during the day, as long as you descend to sleep no more than about 300 metres above the previous night’s altitude. Third, take it genuinely easy for the first two to three days at altitude — this is not the time for summit pushes or Strava records. Your body is building millions of extra red blood cells; that work happens mostly while you sleep, so protect your sleep.

Hydration, food and the alcohol question

Drink two to three litres of water a day even when you do not feel thirsty — altitude is a diuretic, dry mountain air accelerates fluid loss, and dehydration mimics AMS so closely that a headache may simply mean you are dry. Eat carbohydrates generously, even when your appetite vanishes (altitude reliably kills it): at low oxygen your body burns through glycogen faster, and carb-rich foods cost less oxygen to digest. Keep high-energy snacks on hand for the trail. Alcohol, meanwhile, depresses your breathing response to low oxygen and dehydrates you — skip it for the first couple of nights at altitude, or at minimum keep it to a single drink.

Medication: talk to your doctor first

Acetazolamide (commonly sold as Diamox) is the standard preventive medication: it speeds up acclimatization and is widely used by trekkers and expeditions. Ibuprofen helps with altitude headaches. But neither is a substitute for a slow itinerary — medication can mask the early warning signs that would otherwise tell you to slow down. If you have a history of severe altitude illness, heart or lung conditions, or are considering medication for children, get proper medical advice before the trip and carry a written plan for what to do if symptoms appear.

Recognising trouble and when to descend

Mild symptoms — a headache that responds to rest, water and painkillers — usually resolve within a day at the same altitude; do not ascend until they are gone. The hard rules: if symptoms persist beyond 24 hours, descend at least 300 metres (1,000 feet) and recover there. If symptoms worsen, or if you notice breathlessness at rest, a wet cough, loss of coordination or confusion in yourself or a companion, descend immediately and keep descending — HAPE and HACE improve dramatically with lower altitude, and hesitation is the real danger. Tell your companions the moment you feel unwell, however mild it seems; hiding symptoms on a mountain is one of the most dangerous things you can do.

Putting it into practice on common treks

Build these principles into your itinerary from the start. On a teahouse trek like Everest Base Camp, that means the standard acclimatisation days at Namche and Dingboche, not sprinting the trail. On a volcano climb like Kinabalu or Teide, where the summit day gains altitude brutally fast, it means sleeping at the mountain base the night before, moving slowly, and accepting that the rope section at 4,000 metres will feel harder than it looks. Wherever you walk, watch your teammates as closely as yourself — agree before setting out that anyone can call a stop, no questions asked. The mountain will still be there tomorrow; the only summit that matters is getting everyone home healthy.