Pre-acclimatisation: what actually works, ranked by evidence

Real nights at altitude in the two weeks before you go. The best evidence. A study by Muza, Beidleman and Fulco found that six days at 2,200 m substantially reduced sickness and improved work capacity on a rapid ascent to 4,300 m, and that five or more days above 3,000 m within the previous two months significantly reduced sickness on a rapid ascent to 4,500 m. UK Climbing's altitude physiologist notes that people who had been above 4,000 m within two weeks summited about an hour faster. For British walkers this means an Alpine hut weekend, a couple of nights at a Pyrenean refuge, or the Toubkal refuge itself in the fortnight before a bigger trip. Nothing in the UK is high enough: Ben Nevis is 1,345 m, so there is no pre-acclimatising at home in Britain.

A staged ascent built into the trip. The Wilderness Medical Society recommends staged ascent and pre-acclimatisation as a means of prevention. The CDC says that two or three nights at 2,450 to 2,750 m before going higher is markedly protective. This is what the good itineraries above do.

Hypoxic tents and altitude masks. The 2024 guideline is unambiguous: hypoxic tents should not be relied upon for facilitating acclimatisation, preventing altitude sickness or increasing the odds of summit success, a strong recommendation based on moderate-quality evidence. They may help elite athletes with weeks of use and careful monitoring; for a trekker, they are an expensive way to sleep badly. Intermittent hypoxic breathing sessions have limited evidence.

Timing. Acclimatisation fades quickly: the gains are largely lost within one to two weeks, so schedule the pre-trip altitude as close to departure as you can and re-ascend within a week if you want to keep them.

Fitness, water, alcohol, sleep and supplements: the myths

Fitness does not protect you. The CDC states that training and fitness do not affect the risk of altitude sickness, and the Edinburgh altitude charity says the same. Fit people climb faster, which is why they often get sicker. Get fit for the workload, then go up slowly anyway.

Do not force water. The 2024 guideline says forced hydration has never been shown to prevent altitude illness and may cause hyponatraemia, a dangerous fall in blood sodium; UK Climbing's expedition doctor calls over-hydration a potentially life-threatening illness. The sensible rule from Medex is to drink enough to pass clear urine at least four times a day and no more. Dehydration mimics altitude sickness, which is where the "drink four litres" advice came from.

Avoid alcohol for the first 48 hours, and ideally throughout: it depresses breathing and its hangover is indistinguishable from altitude sickness.

Sleeping pills. Alcohol, opiates and older sedatives depress breathing and are not recommended; the CDC considers short-acting sleep medicines generally safe when there are at least eight hours before activity, but if the poor sleep is itself a symptom of altitude sickness, the UIAA says sleep medication is not the answer. Ask your doctor before you travel rather than a trekking forum.

Supplements. There is no good evidence that ginkgo, coca, iron (unless you are deficient), vitamin C or "altitude" gummies prevent altitude sickness. Ibuprofen has weak evidence for prevention and treats the headache. Caffeine withdrawal causes headaches that get blamed on altitude, so keep your usual coffee going.

Acetazolamide, and why it is a conversation with your doctor

Acetazolamide (Diamox) is the drug the Wilderness Medical Society, the CDC, the UIAA and the NHS name for preventing acute mountain sickness when a slow ascent is not possible or you have had altitude sickness before. It is prescription-only in the UK, so the route is a GP or a travel clinic. The guidelines describe a preventive dose of 125 mg twice a day started the night before you begin ascending and continued for two days at your highest altitude, and they recommend trying a dose at home first because of side effects (tingling fingers and lips, passing more urine, a strange taste from fizzy drinks) and because people with a sulphonamide allergy may not be able to take it. It speeds acclimatisation rather than masking symptoms, so a headache on acetazolamide still means altitude sickness. Dexamethasone treats altitude sickness and cerebral oedema but does not improve acclimatisation, is limited to short courses and is an emergency drug on guided expeditions, not a prevention strategy. Nifedipine and similar drugs are for pulmonary oedema only. None of this replaces descent.

Symptoms and red flags

Acute mountain sickness is a headache plus one or more of nausea or loss of appetite, fatigue, dizziness and poor sleep, arriving six to twelve hours after a new altitude and usually settling in a day or two if you stay put. A Lake Louise score of three or more counts; a persistent headache that does not respond to rest and simple painkillers means do not go higher. The two emergencies are high-altitude cerebral oedema (confusion, clumsiness, inability to walk heel to toe, drowsiness) and high-altitude pulmonary oedema (breathlessness at rest, a cough that becomes wet, frothy or pink sputum, blue lips, extreme weakness). Both are treated by immediate descent of at least 300 to 1,000 m, oxygen if available, and the emergency drugs in a guide's kit; a portable hyperbaric bag buys time when descent is impossible. The person who is sick is the worst judge of it, so on any trip agree in advance that the group decides.

Appetite, weight and how to eat at altitude

This is the part almost every acclimatisation guide misses. Above 3,000 m appetite falls, and it keeps falling the higher you go, independently of sickness. In Operation Everest III, a 31-day chamber simulation to the height of Everest, the subjects' energy intake dropped by around 1,000 kcal a day and they lost about 5 kg, mostly because meals felt bigger sooner; even at 5,000 and 6,000 m, with almost no sickness symptoms, they ate less. A ten-day study at a simulated 3,000 m found intake 13 percent lower, and the research points to the appetite hormone leptin rising in the first days at altitude and to carbohydrate absorption weakening above about 5,500 m. Loss of appetite is also one of the defining symptoms of altitude sickness itself.

The practical advice from the expedition medicine bodies is to eat a light but high-calorie diet, little and often, weighted towards carbohydrate in the first days, and to choose food you actually want to eat, because on a big mountain the meal you like is the one that gets eaten. That is the design brief for our high calorie energy bars: 120 g, 527 to 557 kcal, 60 to 66 g of carbohydrate and around 18 g of protein each, not very sweet (sweetness is the first thing people go off up high), soft to eat with cold hands at minus 45, and quick to turn into porridge with hot water in a tent. They are the same high calorie protein bars people use for training at home, and the same product customers buy as high calorie bars for weight gain when the problem is simply eating enough, which is exactly the problem at 5,000 m: one bar replaces the meal you cannot face. A pocket of high calorie snack bars on the summit day, eaten a piece at a time every hour, is how a lot of our customers get through the night without the stove. My mountaineering nutritionguide covers the full plan for camps and summit days, high calorie porridge and high calorie drinks are the warm and liquid options for mornings when chewing is too much, what to eat when your appetite is low has the small, dense ideas that work at any altitude, and the best energy bars for expeditions comparison ranks the options by calories per gram and cold performance. Salty and savoury foods stay palatable longest; pack more variety than you think you need; and if you want a mountain to aim all this at, the best mountains to climb guide grades the world classics by difficulty.

A planning checklist for a UK-based expedition member

Book an itinerary that sleeps no more than 500 m higher each night above 3,000 m with a rest day every three or four days, and refuse the five-day Kilimanjaro. Get a couple of nights at real altitude in the fortnight before you fly if you can (the Alps are two hours away), and do not buy a tent. See your GP or a travel clinic six to eight weeks out to discuss acetazolamide, vaccinations and your insurance's altitude limit. Get fit for the days, not for the altitude. Pack the expedition kit list, a pulse oximeter if your guide uses one, and food you will eat when you do not want to eat. Learn the three golden rules and agree with your group that anyone can call a descent. Drink to clear urine, skip the beer at base camp, keep your coffee, and walk slower than feels natural on every uphill for the first week.

Frequently asked questions

How to acclimatise to altitude at home? You cannot, in the UK. The only things with good evidence are real nights at altitude within two weeks of your trip (an Alpine or Pyrenean hut, the Toubkal refuge) and building acclimatisation days into the trip itself. The 2024 Wilderness Medical Society guideline specifically recommends against relying on hypoxic tents, and altitude masks do not simulate altitude at all.

How long does it take to adapt to altitude? One to three days to feel comfortable at a new altitude up to about 3,000 m, roughly a week to adapt to 5,000 m, and three to six weeks for the full increase in red blood cells. Following the 300 to 500 m a day rule builds the time in automatically.

At what altitude do you need to acclimatise? Altitude sickness becomes possible above about 2,500 m and common above 3,000 m, which is where the sleeping-altitude rules start. Most people can go to 2,500 m in a day with little more than breathlessness; the NHS advises spending a few days below 2,500 m before going higher.

Why am I so sensitive to altitude changes? Susceptibility is largely genetic and does not track fitness; if you have had altitude sickness before you are more likely to get it again. Fast ascent, a high first-night altitude, dehydration or over-hydration, alcohol and a cold all make it worse. If you know you are sensitive, ascend more slowly than the rules and talk to a doctor about acetazolamide.

What is the fastest way to adjust to high altitude? There is no fast way that is also safe. The closest things are recent real altitude exposure, a staged itinerary, and, on medical advice, acetazolamide, which speeds the body's adjustment rather than hiding symptoms.

What supplements can help prevent altitude sickness? None with good evidence. Ginkgo, coca leaf, vitamin C and iron (unless you are anaemic) have not been shown to prevent it. Acetazolamide is a prescription medicine, not a supplement, and ibuprofen has weak evidence.

How quickly does altitude acclimatisation wear off? Most of it within one to two weeks of returning to low altitude, and probably about as quickly as it was gained. Re-ascending within a week keeps most of the benefit.

What are the 3 stages of acclimatisation to high altitude? The usual description is immediate (faster breathing and heart rate within hours), short-term (the kidneys adjust blood chemistry over two to five days so breathing can stay high) and long-term (more red blood cells and capillaries over weeks).

Is VO2 max worse at altitude? Yes: maximal oxygen uptake falls roughly 1 percent for every 100 m above about 1,500 m, and in Operation Everest III it was 59 percent lower at 7,000 m. This is why everyone is slower up high and why pacing, not fitness, decides summit days.

How long does it take to acclimatise to 5,000 feet (1,500 m)? Most people feel nothing beyond mild breathlessness at 1,500 m and adjust within a day or two; this is below the altitude at which sickness normally starts.

Does Garmin's altitude acclimation feature mean anything? It estimates your recent exposure from barometric data and time spent above 800 m; it is a reasonable reminder of the two-week decay but not a medical measure. A pulse oximeter reading and how you feel are better guides.

Flaming Phoenix is not affiliated with the Wilderness Medical Society, the CDC, the UIAA, the NHS, Medex or any trekking company. This page reports their published guidance as of September 2026 for general information; it is not medical advice, and decisions about medication and fitness to travel belong with a doctor.