High-elevation climbing: preventing & treating altitude sickness above 4,000m

Last Updated on 8 June 2025

It’s not just the beauty of the Peruvian Andes, the majesty of Mexico’s volcanoes, or the grandeur of the French Alps that will take your breath away— it’s also the altitude.

Characterised by headaches, shortness of breath, and general fatigue, altitude sickness is the last thing you want to experience when tackling a challenging mountain, but it’s something many climbers and trekkers will contend with above 4,000m. And that’s why it really pays to be prepared and informed before you find yourself panting on a remote mountain pass with no access to potentially life-saving (or at least trip-saving) medical advice.

This climber’s guide to high-elevation adventures is a combination of personal experience and peer-reviewed scientific information that covers different types of altitude sickness, treatment options, various risk factors, and proven steps you can take to prevent altitude sickness. Whether you’re off to the Andes, the Himalayas, or Kilimanjaro, this guide will help get you ready for your next high-altitude adventure!

*I am not a medical doctor, but I do have a PhD in the medical sciences, as well as experience climbing around the world (up to 7,000m). This review is intended to provide a scientific overview of altitude sickness for climbers embarking on their own adventure, but you should always consult with a physician about your specific trip and medical history before taking any medication.

Altitude sickness in the mountains

Decreased barometric pressure at elevation reduces the availability of oxygen in the air, triggering a host of altitude-related physiological responses to occur in our bodies within a few hours to a few days of ascending above 2,500m.

At sea level, the oxygen concentration of air is 20.9%, but on a mountain like Aconcagua (7,000m), effective oxygen concentration falls to just 8.7%— the effects of hypoxia on the brain and other organs at this elevation can be extreme.

While some of the physiological changes that take place at altitude are actually positive adaptations designed to help our bodies cope with hypoxic conditions (such as rapid breathing and the production of additional red blood cells), there are also a number of maladaptive responses that can occur at altitude— and these symptoms constitute various types of altitude sickness.

“Altitude sickness” is actually a broad term for 3 specific medical conditions that occur at high altitude:

• Acute Mountain Sickness (AMS)
• High-Altitude Cerebral Oedema (HACE)
• High-Altitude Pulmonary Oedema (HAPE)

Acute Mountain Sickness (AMS)

The most common but least severe form of altitude sickness is Acute Mountain Sickness— according to recent research, about 34% of travellers who ascend above 5,000m will develop AMS. In milder cases, AMS is not too dissimilar to a hangover, and typical symptoms include a headache, nausea, general fatigue, sleep disturbances, and loss of appetite.

The good news is that all these symptoms can be resolved by relatively simple treatment measures, which often include just waiting a day and giving your body time to acclimatise. AMS itself is rarely life-threatening, but symptoms left unmanaged can progress to HACE (see High-Altitude Cerebral Oedema below), and this condition can be fatal— so it’s really important to identify the symptoms of AMS early and take appropriate steps to prevent progression of the illness.

Common symptoms of AMS

• Headache: the official diagnosis of AMS is based on a headache, plus at least one of the symptoms below
• GI symptoms: upset stomach, including nausea, vomiting, loss of appetite, and/or diarrhoea
• Fatigue or weakness: a lack of energy on the trail, beyond normal tiredness during a long day
• Dizziness/lightheadedness 
• Insomnia: some difficulty sleeping is normal at high altitudes, so this refers to extreme disturbances in your sleep

Treatment of AMS

• Rest: symptoms will often resolve on their own with a day of rest; avoid ascending further until feeling better and follow a staged ascent to limit the risk of further complications
• Descend: if symptoms are severe enough, it may be necessary to descend 300-1,000m and sleep at a lower elevation for one night before re-ascending
• Dexamethasone (Decadron): anti-inflammatory corticosteroid considered to be an extremely effective treatment for AMS; take 4mg every 6hrs 
• Acetazolamide (Diamox): primary preventative for AMS, but you can also take a higher dose of Acetazolamide (250mg every 12hrs) to treat AMS if you don’t have Dexamethasone

High-Altitude Cerebral Oedema (HACE)

If symptoms of AMS progress untreated, it’s possible to develop cerebral oedema, which is essentially swelling of the brain that begins to affect cognition, coordination, and behaviour. This is a very serious condition that, left untreated (or if you continue ascending through symptoms), can rather quickly result in coma or death— just highlighting again why it’s so important to treat AMS before it becomes HACE.

Research suggests that HACE typically occurs at elevations >3,500m after a couple days of exposure, and it is significantly less common than AMS— less than 1% of trekkers at 4,200-5,500m will develop cerebral oedema. Still, you need to be prepared to recognise the symptoms of HACE and respond quickly in case it does affect you or someone else in your climbing party.

Common symptoms of HACE

• Severe symptoms of AMS: such as headache, nausea, vomiting, fatigue, dizziness, insomnia, etc
• Impaired mental state: this might include confusion, difficulty speaking, or poor judgement, as well as memory loss, hallucinations, and risky out-of-character behaviour— as if the physical effects of HACE weren’t enough, some fatalities occur because a climber loses all sense of judgement and walks off the mountain << it is incredibly difficult to recognise altitude sickness in yourself once you progress to this stage, so DO NOT let it get this bad, particularly if you are solo!
• Ataxia: loss of coordination and body movements

Treatment of HACE

• Descend: descend immediately 300-1,000m until you feel better; wait until you’re completely asymptomatic for several days before ascending again
• Dexamethasone (Decadron): anti-inflammatory corticosteroid, take 8mg (with food) followed by 4mg every 6hrs to treat HACE
• Oxygen: if it’s not possible to descend, you need to be placed on supplemental oxygen; if you are climbing without a guide, a passing group might be able to provide this OR you need to call for help via satellite communicator— I never climb without my Garmin inReach!

High-Altitude Pulmonary Oedema (HAPE)

Pulmonary oedema, a dangerous fluid accumulation in the lungs, can occur independently of other types of altitude sickness, meaning that you can develop HAPE without any symptoms of AMS or in addition to AMS and/or HACE. Specific risk factors include a rapid rate of ascent, a history of pulmonary complications, excessive salt intake, cold temperatures, and being male (sorry, guys).

Although not terribly common (and typically only occurring above 5,000m), this condition accounts for a majority of the deaths at altitude— if left untreated, HAPE has a 50% mortality rate, so it’s incredibly important to understand the signs and symptoms in case you or someone in your group becomes unwell!

Common symptoms of HAPE

• Dyspnoea: heavy breathing is normal at altitude, but excessive trouble breathing that doesn’t resolve during rest is cause for concern
• Cough: dry cough with exertion OR coughing up pink mucus
• Chest tightness: a feeling of congestion or pain in the chest
• Crackling sounds: crackling, wheezing, or even gurgling in the lungs when breathing
• Cyanosis: blue lips and extremities due to hypoxia
• Tachycardia: extremely rapid heart rate (an elevated heart rate is part of the normal response to altitude, so this alone is not indicative of HAPE)

Treatment of HAPE

• Descend: descend immediately 300-1,000m until you feel better; it’s not recommended to reascend 
• Nifedipine: pulmonary vasodilator used to treat HAPE; take 20-30mg (sustained release) every 12hrs 
• Oxygen: if it’s not possible to descend, you need to be placed on supplemental oxygen

Risk factors for altitude sickness

There are several risk factors associated with developing altitude sickness, but it’s really interesting to note that physical fitness is NOT one of them— you can be the fittest person in the world, but it doesn’t necessarily mean you’ll perform better at altitude than a casual hiker. Many strong and accomplished climbers have underestimated the impact of altitude and suffered extreme consequences as a result; do not make the mistake of thinking that you are above altitude sickness, no matter your physique.

The main factors that influence susceptibility to altitude sickness include:

ELEVATION

It goes without saying that the risk of developing altitude sickness increases as the elevation increases, and above 4,000m, this risk magnifies exponentially. While AMS can occur anywhere above 2,500m (in and some rare cases, even slightly below), HACE and HAPE typically only affect climbers above 5,000m.

The graphic below (source) is an excellent illustration of the neurological symptoms commonly experienced as climbers ascend, with the most severe impairments occurring above 6,000m and the likelihood of life-threatening complications skyrocketing above 7,500m. The difference in physiological reaction between even 1,000m at extreme elevation can be stark.

RATE OF ASCENT

Rate of ascent, or the speed at which you travel from lower to higher elevations, is among the greatest predictors of altitude sickness. This explains why flying from sea level to 3,500m illicits an immediate response in most travellers and therefore the importance of resting at that elevation prior to any hiking or climbing at higher altitudes, as well as the need to plan a staged ascent.

Experts agree that ascending more than 500m per day to altitudes above 3,000m puts you at risk of developing altitude sickness, so it’s a good idea to plan a staged ascent, especially during your first few days on the trail. Most high-altitude expeditions move at what would be a snail’s pace for fit climbers— but your body can often only handle a few hundred metres a day and it’s important not to rush, particularly at extreme altitude.

ACCLIMATISATION

In addition to rate of ascent, acclimatisation is the single greatest predictor of physiological response to altitude! Regardless of your fitness, it is wholly unreasonable to expect to go from sea level to even a moderately high elevation without substantial complications, and potentially life-threatening ones.

INDIVIDUAL PREDISPOSITION

Due to a host of genetic and biological factors, some people are simply more prone to altitude sickness than others, but this is something you won’t know until you’ve travelled to high altitudes. Again, this has absolutely nothing to do with your physical fitness— an elite athlete may experience heightened symptoms at altitude compared to a weekend trekker, even with identical acclimatisation programs.

HISTORY OF AMS, HACE, OR HAPE

If you’ve had altitude sickness before, you’re as much as 12x more likely to develop it again (but interestingly, a positive experience in the past still isn’t a guarantee that you won’t get altitude sickness this time around).

Preventing altitude sickness

There are a few simple, common-sense ways to prevent serious altitude sickness from ruining your trip (or far worse). Some of these need to be considered at the trip planning stage, such as adding extra days to your itinerary, but there are also medications and specific things to avoid that can help prevent altitude sickness during your expedition.

Acclimatisation

The importance of proper acclimatisation for high-altitude pursuits cannot be understated. This includes high summits and time at elevation in the weeks (and possibly even months) immediately preceding your climb, as well as a carefully crafted itinerary that builds altitude gradually over consecutive days to allow additional acclimatisation time.

The exact acclimatisation schedule will depend entirely on your objective, timeline, and previous experience, but the most important thing here is to simply listen to your body and give yourself the best chance of success through adequate preparation. Days spent in a high-altitude environment in town can also be beneficial— before climbing in Peru, take a few days to walk around Cusco (3,400); before ascending Pico de Orizaba, hang out and eat tacos in Mexico City (2,240m). Even a few thousand metres above sea level will give you a physiological advantage as you ascend higher.

As acclimatisation for Aconcagua (6,962m), I climbed 4 of Mexico’s 6 highest volcanoes (4,461-5,636m) and arrived fairly well adapted to handle the elevation. All of my hikes in the Cusco region (Salkantay, Ausangate, Rainbow Mountain) were acclimatisation hikes for the 10-day independent Huayhuash Circuit, where it was extremely important to be adapted to the conditions. And even in Chamonix, I allowed several days to climb neighbouring peaks before jumping straight onto Mont Blanc at 4,808m.

Though still never a guarantee, my acclimatisation program was a massive contributor to my success, and in all cases, I saw other climbers succumb and eventually retreat off the route due to altitude sickness. Do not underestimate the importance of acclimatisation!!

Gradual ascent

One of the most important risk factors for developing altitude sickness is the rate of ascent, so limiting your net elevation gain to 500m or less per day (after reaching 3000m) will decrease the likelihood of getting sick. Most guided tours take this into consideration already, but if you’re embarking on a solo trek, it’s really important to sit down and carefully plan out your route to avoid climbing too quickly.

In some cases, this won’t be practical— research suggests that, for a majority of individuals, acclimatisation is NOT possible above 5,600m and that prolonged time spent at these elevations actually has a deteriorating effect on the body rather than contributing to positive adaptations. There is a delicate balance between ascending slowly at high altitudes and limiting time actually spent at those altitudes.

As an example, many expeditions sleep at (and summit from) the 3rd High Camp on Aconcagua at 6,000m, but I chose instead to climb from the 2nd High Camp at 5,560m. Although my summit push was then 1,400m in a single day, I had the benefit of breathing more oxygen at the lower camp and therefore suffering less fatigue and altitude symptoms prior to my ascent. What works for one person doesn’t necessarily work for another, but there’s a complex interplay of factors at elevation and ALL need to be considered when designing your itinerary.

Climb high, sleep low

A key tenet of high-altittude expeditions is “climbing high, sleeping low”, or camping at a lower altitude than your high point for the day to help minimise altitude sickness.

The physiological benefit of this is two-fold:

• it allows for a gradual acclimatisation to higher altitudes by pushing upwards with minimal net gain per day
• because of changes in respiration that occur during sleep, the altitude at which a climber sleeps tends to have a greater impact on the risk of AMS than maximum altitude throughout the day

On a trek, you might employ this principle by camping at a lower spot than your highest mountain pass for the day, while on an expedition, you might climb up to a higher point as an acclimatisation exercise before returning to a lower camp to sleep.

Dietary strategies

AVOID ALCOHOL & PAIN MEDS

There is some research to suggest that avoiding alcohol and opiate pain medications (like hydrocodone/ Vicodin) might also help prevent altitude sickness. At the very least, cutting out alcohol will probably keep dizziness and headaches to a minimum.

STAY HYDRATED

Drinking lots of water doesn’t do anything to help prevent altitude sickness per se— but many symptoms that mimic altitude sickness (e.g. headaches, dizziness) are actually related to dehydration. And, as a result of the altitude, we tend to pee more frequently and lose more liquids to sweat evaporation than usual, so it’s extra important to stay hydrated on a high-altitude climb.

EAT A CARB-HEAVY DIET

Especially for those who are experiencing stomach upset, it’s important to eat lots of carbs while keeping fats to a minimum. Not only are carb-y foods (like potatoes, pasta, rice, bread, and fruit) easier for your body to digest and therefore less likely to cause indigestion, there’s also limited evidence to suggest that these foods can help reduce the symptoms of altitude sickness.

Medication

Even if you don’t think you’ll need it, I strongly recommend going to the doctor and getting a prescription for altitude sickness medication prior to a high-altitude expedition. Far better to have it on hand and never use it than to be seriously ill without it!

These are the two meds commonly used to prevent altitude sickness:

DIAMOX (ACETAZOLAMIDE)

There is considerable scientific evidence to support the use of Diamox (Acetazolamide) for the prevention of altitude sickness. Essentially, the drug works by increasing the acidity of your blood, which in turn stimulates the respiratory system to improve oxygenation. Acetazolamide can actually help you acclimatise quicker to high altitudes— and this drug doesn’t mask any symptoms of altitude sickness, so if you feel well, it means you’re well! I’d estimate about a quarter of the climbers around Base Camp on Aconcagua were taking Diamox and noted no symptoms of altitude sickness.

It’s typically recommended that you begin taking Acetazolamide the night before ascending to high altitude and continue taking 125 or 250mg 2x a day until you reach your final elevation. There is some debate about the best dosage, but your doctor will help you make a decision. Common side-effects are tingling in the fingers or a frequent need to urinate, but thankfully nothing too serious.

DEXAMETHASONE

There is also some support for taking Dexamethasone (Decadron), which is a corticosteroid used to treat AMS and HACE, for preventative purposes. Unless your doctor specifically recommends this based on your medical history, it’s generally believed to be less effective and less safe than Acetazolamide. That’s primarily because it can mask the symptoms of AMS, which might encourage you to keep ascending when your body actually needs a rest.

It’s not safe to use Dexamethasone for more than 2 weeks and it’s also sometimes necessary to gradually ween yourself off the drug to avoid rebound AMS (basically developing symptoms because you stopped taking the medicine too quickly), but obviously your doctor will discuss this with you. The typical dose is 2mg very 6hrs.

Herbal remedies

COCA LEAVES

Andean natives have been using coca for more than 3,000 years to alleviate symptoms related to the altitude, and bags of coca leaves, coca tea, and even coca candy are now ubiquitous in the high-altitude cities of South America. There’s no concrete scientific evidence to suggest that coca leaves are an effective prevention strategy for altitude sickness, but they can definitely help with many of the symptoms, particularly headaches and nausea.

By all means, drink coca tea and eat coca candy on your high-altitude trek, but just understand that coca is no substitute for acclimatisation and gradual ascent, nor is it actually proven to prevent AMS like Acetazolamide has been. The best idea is probably using coca leaves in combination with other evidence-based prevention strategies.

AGUA FLORIDA

Agua Florida is another popular Andean herbal remedy for altitude sickness that can be purchased in high-altitude towns like Cusco for a few soles. If you’re on a guided trek, they will usually have a bottle for you to use— pour a couple drops in your hand, clap, and then inhale deeply a few times. My parents used it frequently on our high-altitude treks in Peru and felt that it helped with their headaches! Like coca, though, Agua Florida isn’t so much a preventative measure as it is a way to manage altitude symptoms.

Summary: what you need to know about altitude sickness

• At high altitudes, there’s effectively less oxygen available in the air and these hypoxic conditions can lead to altitude sickness. In its more minor forms (AMS), altitude sickness is fairly common above 5,000m and thankfully easy to manage through rest, hydration, and a safe ascent plan.
• Some of the symptoms associated with altitude sickness (AMS) are rather minor, such as headaches, nausea, and fatigue, but cerebral (HACE) or pulmonary oedema (HAPE) can be life-threatening if left untreated. It’s important to recognise the symptoms of altitude sickness so you can take appropriate measures, such as descending, taking medication, or evacuating.
• Acclimatisation and rate of ascent are the most important risk factors for developing altitude sickness, so prepare for high-altitude trips by hiking and climbing at lower elevations and, once on the mountain, take it slow— the general advice is to ascend no more than 500m each day above 3,000m.
• Taking Diamox (Acetazolamide; 125 or 250mg 2x daily) is a scientifically proven way to promote acclimatisation and prevent altitude sickness that might be worth considering— but medication should NOT replace acclimatisation altogether, nor should it justify risky speed-ascents without preparation. Diamox is a diuretic, so drink additional water to often the fluid loss.
• While at high altitudes, be sure to keep hydrated and eat regular meals, even when you’re feeling unwell. Dehydration can exacerbate symptoms of altitude sickness and lead to additional complications.
• Carry Dexamethasone and Nifedipine in your backpack in case you or anyone in your group begins to show symptoms/signs of HACE or HAPE (and know what these symptoms are, so you can react quickly!). If symptoms of AMS worsen or HACE/HAPE are suspected, descend immediately and do not re-ascend until you feel completely better without the use of medication.

References & resources

Biondich AS, Joslin JD. Coca: High altitude remedy of the ancient Incas. Wilderness & Environmental Medicine. 2015;26(4):567-71. 

Jensen JD, Vincent AL. High Altitude Pulmonary Edema. National Library of Medicine: StatPearls. 2024.

Luks AM, McIntosh SE, Grissom CK, Auerbach PS, Rodway GW, Schoene RB, Zafren K, Hackett PH. Wilderness Medical Society consensus guidelines for the prevention and treatment of acute altitude illness. Wilderness & Environmental Medicine. 2010;21(2):146-55. 

Luks AM, Swenson ER, Bärtsch P. Acute high-altitude sickness. European Respiratory Review. 2017;26(143):160096. 

Mehta SR, Chawla A, Kashyap AS. Acute mountain sickness, high altitude cerebral oedema, high altitude pulmonary oedema: The current concepts. Medical Journal, Armed Forces India. 2008;64(2):149. 

Parise I. Travelling safely to places at high altitude: Understanding and preventing altitude illness. Australian Family Physician. 2017;46(6):380.  

Prince TS, Thurman J, Huebner K. Acute Mountain Sickness. National Library of Medicine: StatPearls. 2024.

Richalet JP, Larmignat P, Poitrine E, Letournel M, Canouï-Poitrine F. Physiological risk factors for severe high-altitude illness: a prospective cohort study. American Journal of Respiratory and Critical Care Medicine. 2012;185(2):192-8.

Roach RC, Hackett PH, Oelz O, Bärtsch P, Luks AM, MacInnis MJ, Baillie JK, Lake Louise AMS Score Consensus Committee. The 2018 Lake Louise acute mountain sickness score. High Altitude Medicine & Biology. 2018;19(1):4-6. 

Vardy J, Vardy J, Judge K. Acute mountain sickness and ascent rates in trekkers above 2500 m in the Nepali Himalaya. Aviation, Space, and Environmental Medicine. 2006;77(7):742-4.