High-altitude trekking: a complete guide to preventing & treating altitude sickness in the mountains

For anyone visiting South America with the intention of hitting the trail, it’s not just the beauty of the Peruvian Andes or the majesty of Ecuador’s volcanos that will take your breath away— it’s also the altitude. Characterised by headaches, shortness of breath, and general fatigue, altitude sickness is pretty much the last thing you want to experience when you’re tackling an already-challenging hike, but it’s still something many trekkers will contend with. 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 comprehensive guide to high-altitude trekking is a combination of personal experience and peer-reviewed scientific information, including 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 even Kilimanjaro, this guide will help get you ready for your high-altitude adventure!

* Just a quick disclaimer that I am not a medical doctor, but I do have a PhD in the medical sciences— I conducted a review of the existing literature and all of the information provided here about altitude sickness is directly from peer-reviewed medical journals. Still, you should consult with a doctor about your specific trip and medical history before taking any medication.

About altitude sickness

Decreased barometric pressure at high elevation, which in turn decreases the availability of oxygen in the air, triggers a host of altitude-related physiological responses to occur in our bodies within a few hours to a few days of ascending above 2500m. In plain English: there’s less oxygen in the air at high altitudes, which can lead to a number of unpleasant symptoms. 

While some of these physiological changes actually help our bodies adapt to hypoxic conditions (such as breathing faster and constriction/dilation of blood vessels), there are also a number of maladaptive responses that commonly occur at altitude (like those pesky headaches I mentioned earlier). These symptoms constitute various types of “altitude sickness”.

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

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

Acute Mountain Sickness (AMS)

The most common form of altitude sickness is Acute Mountain Sickness— according to recent research, about 34% of travellers who ascend above 5000m will develop AMS. In milder cases, AMS is not too dissimilar to a hangover— most people have a bad headache, are nauseous, and just generally feel exhausted.

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 such as loss of appetite, nausea, vomiting 
• 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 really refers to extreme disturbances in your sleep

Treatment of AMS

• Descend: symptoms typically resolve if descending 300-1000m, just wait until you feel completely better before ascending again
• 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; Dexamethasone is still preferred if you have it with you

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. This is a very serious condition that, left untreated, 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 >3500m after a couple days of exposure, and it is significantly less common than AMS— less than 1% of trekkers at 4200-5500m will develop cerebral oedema. Still, you need to be prepared to recognise the symptoms of HACE and respond quickly.

Common symptoms of HACE

• Severe symptoms of AMS: such as headache, nausea, vomiting, fatigue, dizziness, insomnia, etc
• Impaired mental state: people have described this as a feeling of drunkenness, which might include confusion, difficulty speaking, or poor judgement
• Ataxia: loss of control of body movements

Treatment of HACE

• Descend: descend immediately 300-1000m until you feel better; you need to 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 trekking without a guide, a passing group might be able to provide this or you need to call for help via Personal Locator Beacon

High-Altitude Pulmonary Oedema (HAPE)

Pulmonary oedema, which is basically fluid accumulation in the lungs, can often be independent of other types of altitude sickness, meaning that you can develop HAPE without any symptoms of AMS or you can develop it on top of AMS and HACE. Although not too common, this condition actually accounts for most of the deaths at altitude, 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: minor difficulty breathing is normal at altitude, but excessive trouble breathing that doesn’t resolve during rest is cause for concern
• Cough: coughing up pink mucus is usually an indication of severe HAPE
• Chest tightness: a feeling of congestion in the chest
• Crackling or wheezing: it’s common to hear crackling, wheezing, or even gurgling in the lungs
• Tachycardia: really rapid heartrate 

Treatment of HAPE

• Descend: descend immediately 300-1000m 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; if you are trekking without a guide, a passing group might be able to provide this or you need to call for help via Personal Locator Beacon

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 actually has nothing to do with it— you can be the fittest person in the world, but it doesn’t mean you’ll be more prepared to trek at altitude than a novice hiker. Here are the main factors that influence susceptibility to altitude sickness:

ELEVATION

It goes without saying that the risk of developing altitude sickness increases as the elevation increases, particularly above 4000m.

RATE OF ASCENT

Generally, experts agree that ascending more than 500m per day to altitudes above 3000m 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.

ACCLIMATISATION

If you haven’t properly acclimatised to the altitude before leaving for your trek, you’ll be at a greater risk of developing altitude sickness.

INDIVIDUAL PREDISPOSITION

Unfortunately, some people are just more prone to altitude sickness than others, but this is something you won’t know until you’ve actually travelled to high altitudes.

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 a positive reaction in the past still isn’t a guarantee that you won’t get altitude sickness this time around).

*So…what is altitude sickness?

Want to skip all my scientific mumbo jumbo and just hear it in plain English? Here’s the deal:

• There’s less oxygen in the air at high altitudes, which can lead to “altitude sickness”
• Altitude sickness is a blanket term for 3 different medical conditions that are related to altitude: Acute Mountain Sickness (AMS), High-Altitude Cerebral Oedema (HACE), and High-Altitude Pulmonary Oedema (HAPE)
• Some of the symptoms associated with altitude sickness are just annoying or unpleasant (headaches, nausea, fatigue), but HACE and HAPE can actually be life-threatening if left untreated for even a day
• It’s important to recognise the symptoms of altitude sickness so you can take appropriate measures (e.g. taking medication, descending to a lower altitude, evacuating the mountain)
• Acclimatisation and rate of ascent are the most important risk factors for developing altitude sickness, so take it slow!

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 you can take and specific things to avoid that will all help prevent altitude sickness.

Acclimatisation

Spending time at a moderate altitude prior to beginning your trek is a great way to give your body some time to get used to hypoxic conditions. It’s usually recommended that you spend at least 2 days in Cusco (or another high-altitude town) before departing on a nearby trek. During this time, keep physical exertion pretty low— just walk around town, visit some of the markets, and let yourself adjust to the elevation.

Before a particularly high trek, you should also complete a few short acclimatisation hikes in the area. 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. I found that all the acclimatisation made an enormous difference and I didn’t have any issues with the altitude, even at 5100m.

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.

Trek high, sleep low

There’s definitely something to be said for “trekking high, sleeping low”, or camping at a lower altitude than your high point for the day to help minimise altitude sickness. You should also try to increase your sleeping altitude gradually, no more than 600m per day if possible.

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. Far better to have it on hand and never use it than to be seriously ill without it! These are the two meds used to prevent altitude sickness:

Acetazolamide

There is a considerable amount of scientific research to support the use of Acetazolamide (Diamox) for preventing 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 does not mask any symptoms of altitude sickness, so if you feel well, it means you are well. My parents took Acetazolamide on our recent Peru trip to aid in their acclimatisation and felt that it really helped, especially given that they only had 1 day in town before we hit the trail.

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 3000 years to alleviate symptoms related to the altitude, and bags of coca leaves, coca tea, and even coca candy are now ubiquitous in all the high-altitude cities. 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.

Other common-sense 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, going dry will probably keep dizziness and headaches to a minimum, as these symptoms can still occur in healthy people at elevation.

Stay hydrated

Drinking lots of water doesn’t do anything to help prevent altitude sickness per say— but many symptoms that mimic altitude sickness (e.g. headaches, dizziness) are actually related to dehydration. 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 trek.

Eat a carb-heavy diet

Especially for those who are experiencing some 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 is also limited evidence to suggest that these foods can help reduce the symptoms of altitude sickness.

*So…what does this mean for your trip?

The most important information you need to remember when planning your own high-altitude trek:

• Include several days in your itinerary to hang around town and get used to the altitude before hitting the trail
• Try to ascend no more than 500m each day after reaching 3000m
• Taking Acetazolamide (125 or 250mg 2x daily) is a scientifically proven way to promote acclimatisation and prevent altitude sickness
• Drinking coca tea might help you reduce some of the symptoms associated with altitude
• While on your trek, be sure to keep hydrated and eat regular meals, even when you’re feeling unwell
• 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

My experience with high-altitude trekking

Living on the coast in Australia (a country whose mountains would probably classify as hills in South America), I was as far from adapted to elevation as a person could be. As a result, I was pretty nervous about how my body was going to react on a high-altitude trek! I knew from my first trip to Peru that I felt tired and unfit during my first few days at altitude in Cusco (3400m), so what about 5000m+?!

As it turns out, I was totally fine on all of my high-altitude treks and hardly felt any different than normal, which I would largely attribute to the 5 days I spent walking around Cusco and doing short day hikes in the area before setting out on the Salkantay Trek (4650m). This trek also helped me acclimatise for the higher Ausangate Circuit/Rainbow Mountain trek (5050m) and, finally, the 10-day Huayhuash Circuit (5100m).

I can’t stress the importance of acclimatisation enough— I met plenty of people who battled on the uphill sections of these treks due to headaches and terrible nausea while I zipped by, full of energy. But even though I didn’t end up needing it, I was still glad to have altitude sickness preventatives and treatments in my backpack (Acetazolamide, Dexamethasone, and Nifedipine). My parents did end up taking some medication and it was reassuring to know that we were totally prepared, just in case things took a bad turn.

Hopefully this info has been helpful for your next high-altitude adventure. Happy trails!

References & resources

A PRACTICAL GUIDE TO ALTITUDE SICKNESS IN SOUTH AMERICA

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

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.  

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.