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Altitude sickness is a reaction to the lower amounts of oxygen available at high altitudes (due to the lower air pressure). Your body will respond in various ways to this: some are normal, some are illnesses. The illnesses are a serious health hazard and can result in death if ignored or left untreated.
Altitude sickness is very dangerous for four reasons: it can come on suddenly and progress quickly, it can be fatal, sufferers are often some distance from medical help and are difficult to evacuate swiftly, and in many cases sufferers are reliant on their health because they're doing a lot of physical activity in dangerous environments.
Altitude sickness is a great danger for high-altitude mountaineering (above 4000 or 5000 m), a moderate danger for mountain sports (such as skiing at 3000–4000 m, notably in Colorado), and a moderate danger when flying in to a high-altitude city around 3500 m, notably Tibet (Lhasa), Peru (Cusco, especially for the Inca Trail), and Bolivia (La Paz). For moderate altitudes (such as 3500 m), the main solution is to acclimatize for a night or two at a lower altitude (near 2500 m) and take it easy for the first few days, rather than flying in and immediately going skiing or hiking. Acetazolamide (ACZ) is the most commonly used drug for prevention, and is particularly useful for flying into a high-altitude city. For higher altitudes much more care, preparation, and gradual ascent is necessary, and potent treatments are available. Particularly dangerous are tall, easy mountains, notably Kilimanjaro (5895 m) and Aconcagua (6961 m), where it's easy to get dangerously high quickly. Acclimation requires time, and rushing causes altitude sickness.
There are other risks at altitude, not covered here. One is that it may get very cold; see Cold weather. Another is that there may be danger from strong sun because there is less atmosphere above to protect you; see Sunburn and sun protection. Finally, the terrain may pose dangers such as avalanches or just falling off a mountain; see Mountaineering.
The further you move away from sea level up into higher altitudes, the lower the air pressure is. The body has two main problems with high altitude and the corresponding lower air pressure:
- Air at lower pressure has less oxygen per lungful. Your body adjusts to this by making more red blood cells to carry oxygen more efficiently. However, the process takes several days, sometimes more than a week, and in the meanwhile you may be ill.
- At lower air pressure, water evaporates faster. This can lead to dehydration.
The changes to your body at altitude are complex and can be quite dramatic. The difficulty your body has maintaining a good oxygen supply and keeping related problems under control is directly related to how high up you are, and also to recent changes in your altitude. These are the two major factors that cause altitude sickness. The altitude where you sleep is also important because most of the extra red cells needed to improve your oxygen management are manufactured while you sleep.
Hence this article talks a lot about ascent and descent. Ascending further away from sea level is the risky activity and the time you must be alert. Conversely, descending towards sea level is the single most important factor in reducing or eliminating all forms of altitude sickness.
The article also talks a lot about acclimatisation, giving your body enough time to adapt to higher altitude. This is critically important for avoiding problems.
How high is high?
Formally, high altitude is defined as:
- High Altitude: 1500–3500 m (5000–11,500 ft)
- Very High Altitude: 3500–5500 m (11,500–18,000 ft)
- Extreme Altitude: above 5500 m
However, in practice altitude sickness rarely occurs below 2500 m (8000 ft).
A minority of people, about 20%, have some symptoms of altitude sickness if they ascend to about 2500m (8000 feet) above sea level and sleep there. (This is the level of cabin pressurisation of most commercial aircraft apart from the Airbus A380 and Boeing 787). However, most people will acclimatize to 3000m (10,000 feet) with relative ease, perhaps having symptoms after the first night.
Acclimatizing to heights of 3000–5000m (10,000–16,000 feet) is much more difficult, and it is here that it is absolutely necessary to ascend slowly and return to a lower altitude to sleep if you have been travelling around at a higher altitude during the day. Over 50% of people will become ill if they ascend rapidly from sea level to 3500m (11,000 feet) without acclimatization, and everyone will if they ascend rapidly to 5000m (16,000 feet).
It is thought to be impossible to permanently acclimatize to heights above 5500m (18,000 feet). It is possible to spend several weeks sleeping as high as 6000m (20,000 feet) once acclimatized, but gradual deterioration of physical well-being will still occur.
Regions above 8000m (26,000 feet) are referred to as the death zone: you will deteriorate noticeably while you remain at such high altitudes, some of your body's major systems will shut down and climbers will only remain there for two or three days. Death rates from altitude sickness above 7000m (23,000 feet) are estimated at 4% of all people who venture that high.
If your home is significantly above sea level, you gain a definite leg up on ascending to higher elevations, but that doesn't make you immune to altitude problems; it just pushes the threshold for their onset higher. Most otherwise healthy people who live at elevations of 1500m (5000 feet) to 2500m (8000 feet), an elevation range containing quite a few major cities, experience little trouble going to 3000m (10,000 feet) or a bit higher, but even they will be at risk of altitude problems at 5000m (16,000 feet).
Past performance at altitude is the main predictor of future performance, but is a guide, not a guarantee. Beware that even if you have not suffered acute mountain sickness (AMS) in the past at altitude, you may still suffer it in future, even at lower altitudes.
Altitude sickness tends to affect men more than women, especially men between the ages of 16 and 25. It’s not clear if there’s some unknown biological reason for this, or if it’s just the demographic most likely to attempt too much, too soon. It is important to remember that just because you are young and healthy, and haven’t experienced altitude sickness in the past doesn’t mean you are immune to it on future climbs. Physical fitness is not necessarily a good indicator, and neither are strength or good health. You may react badly to altitude despite being fit, young and healthy. In fact, the fit, young and healthy have a hidden risk: their general physical capacity leads them to believe that they should handle altitude just fine, which is not always true.
Bad health, on the other hand, is a risk factor: particularly cardiac or respiratory problems. Healthy hearts and lungs have a hard enough time getting oxygen to your tissues at high altitudes. Naturally, if you have physical problems that make exertion difficult for you, you have reason to think carefully about exertion at high altitude, where it is much harder!
Scuba diving increases risk of decompression sickness. If you have recently been diving and have not fully gotten rid of the nitrogen in your blood, you should not ascend to a higher altitude (or travel in a plane). See scuba diving for recommendations on how long to wait.
Genetics play a role: some populations, notably Sherpas, have significant genetic adaptations to altitude, and at least eight genetic polymorphisms have been identified as contributing to individual variation: AMS is an environmentally mediated polygenetic disorder. However, for most people there is currently no screening, genetic or otherwise, available to assess risk for altitude sickness. Thus prior history is used instead.
Other than mountain climbing and other mountain sports, such as skiing, the main travel destinations at high altitudes are Bolivia, Peru (in the Bolivian Plateau), and the Tibetan Plateau (Qinghai and Tibet in China), all of which have significant destinations around 3500 m (Very High Altitude), high enough to pose significant risk of altitude sickness. By contrast, other mountainous destinations such as Mongolia, Nepal, Switzerland (mostly below 2000 m), and even Bhutan (around 2500 m), are mostly settled at lower elevation, especially in valleys, and pose at most mild risk of altitude sickness.
For flying into Peru for the Inca Trail, Cusco (3400 m) is quite high (over 50% risk of AMS), and immediately hitting the Inca Trail is dangerous, occasionally resulting in death. Safer is to leave Cusco and acclimate for a night or two in the Sacred Valley, before returning to Cusco, and thence the Inca Trail. Machu Picchu itself (2400 m) is not very high, however.
For flying into Lhasa (3650 m), directly flying from sea level poses a greater than 80% risk of AMS. Acclimating for a few days in Kunming (2000 m) or Xining (2300 m) helps. Taking the train into Tibet does not: it is first too low, then too high to help acclimation.
For flying into La Paz, acclimate in the low, southern part of town (such as Calacoto or Obrajes) for a few nights.
In terms of large cities, there are under a dozen large cities (at least 100,000 population) above 3000 m, of which the most significant are La Paz (Bolivia, 3650 m), Lhasa (China, 3650 m), and Cusco (Peru, 3400 m). The countries with many major cities above 2000 m are Bolivia, Peru, China (Tibetan Plateau), Ecuador, Colombia, and Mexico, while Eritrea, Ethiopia, Guatemala, and Yemen all have 1–3 major cities (capital or second city) in 2000–3000 m, and Afghanistan and India have a few minor cities at altitude.
Comparative elevations of significant travel destinations.
- Mount Kilimanjaro (5895 m) — the summit of Africa's highest mountain can be reached by just hiking; in fact the record for an ascent and descent is less than seven hours! Hence, it’s easy to get dangerously high in a short time.
- Thimphu (2250–2650 m)
- Mount Fuji (3776 m) – Japan’s highest mountain, generally climbed in a single overnight hike, high enough to cause sickness
- Tibet, China
- Lhasa (3500–3650 m)
The Swiss and French Alps do have some ski resorts and viewpoints at risky altitudes with fast cable car or train access from the valley (usually around 1000 m), resulting in extremely rapid ascent to significant altitudes, for instance Aiguille du Midi (3842 m) or Jungfraujoch (3454 m).
- North America
- Many high ski resorts, e.g., Breckenridge (~3000–4000 m)
- Aspen (2400 m)
- South America
- Aconcagua (6961 m) – highest mountain outside of Asia. Technically easy to climb, so can get dangerously high in a short time.
- La Paz (3650 m) – highest capital city
- El Alto (4150 m) – highest major metropolis
- Bogotá (2600 m)
- Cusco (3400 m)
- Inca Trail: Warmiwañusqa “Dead Woman Pass” (4200 m – highest point, daytime), Pacaymayo (3500–3600 m – usually highest night, similar to Cusco)
- Machu Picchu (2400 m)
Effects of altitude
Altitude has some physiological effects on all people who are at high altitudes. These effects are not in and of themselves symptoms of illness, although they are signs of the increased difficulty that the body has getting at altitude. Normal acclimatization consists of:
- Decreased blood oxygen saturation, which increases over time
- Increased heart rate
- Increased ventilation
- Increased urination
- Shortness of breath during exertion
- Changed breathing pattern at night
- Awakening frequently at night
It is therefore important to drink water to handle the increased urination, and to avoid anything that decreases breathing. Notably, alcohol both dehydrates and decreases breathing (it is a depressant), so it should be avoided or consumed moderately. If you are not urinating more than usual, you are either dehydrated or not acclimatizing well: try drinking more. The unusual night breathing can be scary and can disturb companions (like snoring), but is normal.
You will naturally breathe faster at higher altitudes to compensate for the lower air pressure. It’s possible you won’t notice this: a similar effect happens during air travel. This is called “hypoxic ventilatory response” (HVR); it is frequently misnamed “hyperventilation”.
Increased urine output is a response to hypoxia: increased breathing reduces CO2 in the blood, resulting in more output of bicarbonate, which increases urination. This will make you urinate a lot at altitude. If you aren’t urinating much more than you usually would, then you might actually be dehydrated, or not acclimatizing.
Because of the disruption to oxygen and carbon dioxide levels in your blood due to the body chemistry changes and hyperventilation that occurs at altitude, your body’s “when to breathe” chemical signals become confused. While you’re awake you will remember to breathe, but when you sleep it is common to have interrupted breathing: holding your breath for up to fifteen seconds and then breathing very rapidly when you start breathing again.
This can be very alarming when you wake up knowing that you weren’t breathing or were short of breath; or when you notice someone else has stopped breathing. But it is a normal response to altitude, and happens to almost everyone. Acclimatisation only improves it a little.
Illnesses at altitude
As well as the less dangerous physiological effects, altitude makes you susceptible to actual illnesses, several of them very dangerous. While not all the effects of altitude can be avoided, you should take sensible steps to avoid actual illness, and take it very seriously if it does occur.
High altitude headache is the most common symptom, and first warning sign, affecting about 80% of people who ascend. In itself this headache is not dangerous, but other symptoms should be monitored. If other symptoms arise, or the headache does not resolve with a litre of fluids, mild analgesics, and a day or two of acclimatization, you are likely suffering mild AMS and are at increased risk of more serious illness.
You need to increase your fluid intake at high altitudes. The loss of appetite, a precursor to nausea, can lead you in to a dehydration headache. Unfortunately, it is easy to mistake dehydration headaches for acute mountain sickness (AMS) headaches (below) and vice versa. If a headache does not improve after drinking a litre of fluids it should be considered as an AMS effect.
Dehydration headache can also be recognised by comparing pulse rates: if your pulse rate goes up more than 20% when standing up after lying down for five minutes, you need more fluids.
Acute mountain sickness
Acute mountain sickness (AMS) is the most common unhealthy response to altitude: it's a collection of signs that your body is becoming ill and has not adapted successfully to a higher altitude.
For your own safety, assume any illness at altitude is AMS – denial of AMS is very frequent and dangerous. The most common reasons that people fail to descend as soon as they should are bad assumptions. They assume that having AMS is a sign of weakness; that their level of fitness means they can’t have AMS; or mistake their symptoms for the flu or another illness. Too-aggressive ascent schedules are another cause: if not enough time is budgeted, acknowledging AMS and slowing down may prevent one from successfully summiting a peak, though worsening AMS may well force this anyway.
Assume AMS first: it happens to healthy strong people, and if it turns out you are indeed sick with something else, descending to a lower altitude will make it easier for your body to heal anyway.
In particular, if you’ve recently ascended, and you have a headache and any other symptom, you have AMS. The other signs of AMS vary for different people, but include:
- loss of appetite
- nausea or vomiting
- difficulty walking (called gait ataxia)
- rattling breath
- feeling generally extremely ill
The last three signs in particular are signs that you are becoming quite ill, but you should not wait for the onset of these symptoms before acknowledging you have AMS: they’re fairly reliable indicators of the onset of more serious problems, namely high altitude cerebral edema (HACE) or high altitude pulmonary edema (HAPE).
You and your party should keep an eye on each other for signs of AMS, and if you have AMS, for signs of it worsening. Very sick people can become confused and not realise how ill they are. Loss of appetite is a particularly good sign: anyone who has been walking or climbing at altitude for a day should be hungry for a good meal in the evening.
If you have symptoms of AMS, do not ascend further. Consider descending, or wait a few days to acclimatize and for symptoms to resolve before ascending further.
If you have signs of HACE or HAPE, descend immediately. Your life may depend on it.
High altitude cerebral edema
High altitude cerebral edema (HACE) is the end-stage of AMS (conversely AMS can be thought of as the mild form of HACE). When you have HACE, your brain swells and stops working properly.
HACE symptoms include a number of signs of mental functions failing: confusion, fatigue and weird behaviour. But the most reliable one is gait ataxia, and you can test it by walking heel to toe along a straight line on the ground. Healthy people can pass this test easily, anyone who has difficulty balancing while they do it is showing signs of HACE.
HACE is extremely serious, and you may only have a few hours to help someone with HACE. The main treatment is descent, but a person experiencing these symptoms will need significant help. Dexamethasone is one drug that can be used to relieve symptoms, but it is just a temporary bridge to give more time for descent.
A 2008 medical study Why Climbers Die On Mount Everest shows HACE as the leading cause of death.
High altitude pulmonary edema
High altitude pulmonary edema (HAPE) is another severe altitude illness. It sometimes occurs in conjunction with AMS or HACE, but sometimes on its own — it's thought to have different causes. When you have HAPE, your lungs fill with fluid. Signs include extreme fatigue; breathlessness (when not due to interrupted breathing — give yourself 30 seconds to recover upon waking); a cough, especially if it is wet and has blood in it; rattling or gurgling breath; chest congestion; very fast heart rate; very fast breathing; and blue extremities. A fever is sometimes present. It most commonly sets in at night.
HAPE is another extremely serious illness, and like HACE should be treated as a critical emergency. Nifedipine is the drug of choice for the treatment of HAPE, but it can only provide temporary relief and rapid descent is very important.
Cheyne Stokes breathing
Above 3000m (10,000 feet), some people experience a periodic breathing during sleep known as Cheyne-Stokes respirations. The pattern begins with a few shallow breaths and increases to deep sighing respirations then falls off rapidly. Respirations may cease entirely for a few seconds and then the shallow breaths begin again. During the period when breathing stops the person often becomes restless and may wake with a sudden feeling of suffocation. This can disturb sleeping patterns, exhausting the climber.
Acetazolamide is helpful in relieving the periodic breathing. This type of breathing is not considered abnormal at high altitudes. However, if it occurs first during an illness (other than altitude illnesses) or after an injury (particularly a head injury) it may be a sign of a serious disorder.
Decompression sickness (DCS, also known as the bends or caisson disease) is a severe illness in which bubbles of nitrogen form in your blood, blocking blood supply to parts of your body. Symptoms include persistent tingling or joint pain, fatigue, itching, rashes, confusion and collapse. Decompression sickness is brought on by extremely sudden changes in air pressure (effectively an increase in altitude), such as loss of cabin pressure in a plane you are flying in. Even a fast ascent to most altitudes (such as by plane) would not normally cause decompression sickness. The exception is for anyone who has recently been scuba diving, who should avoid ascents above the altitude that their dive took place at for between 12 and 24 hours depending on dive activity. See the Scuba diving article for more information.
Acclimatize to altitude gradually
Acclimatization is the process of getting your body to adapt to the lower oxygen levels by ascending slowly into higher altitudes, spending some time at each one to adapt. It is essential to budget sufficient time, and have a realistic ascent profile: ascend gradually, and leave extra days in case it is necessary to spend extra time acclimatizing. Over-aggressive schedules, such as 6-day ascent/descent of Kilimanjaro, pose very high risk of AMS, and significant risk that you will not successfully complete the expedition due to not having enough time to acclimatize, instead being forced to turn back.
The most important factor is to increase your sleeping elevation (the altitude where you’re spending the night) slowly. If you’re on a hiking or climbing holiday, a typical strategy is to spend a day (or initially part of a day) at a higher altitude and return to a lower altitude to sleep: “climb high, sleep low”. This is particularly used on summit day for a high summit (such as Kilimanjaro), or over a high pass (such as the Inca trail). This also works for people doing winter sports at high altitudes: ski at the top of the resort and sleep at the bottom.
Here are the recommended maximum increases in your sleeping elevation which will stop most people from proceeding to AMS:
- Go no higher than 2400m (8000 feet) the first night.
- Increase your sleeping elevation by 300m (1000 feet) per night after 3000m (10,000 feet).
- Every 1000m (3000 feet), you should spend a second night at the same altitude. This will be every fourth night if you have been ascending at the maximum pace recommended above.
These guidelines are conservative; the CDC gives somewhat more aggressive guidelines, such as 2800 m (9100 feet) the first day and increase of 500 m per night, particularly for lower elevations (below 3500 m), though it increases the risk. You can, of course, ascend more gradually than these rates. Many people ascending from sea level choose to spend several nights at 2500m (8000 feet) to 3000m (10,000 feet) before beginning acclimatization to higher altitude.
During acclimatisation, drink a lot of non-alcoholic drinks, due to increased urination.
The drug Acetazolamide (ACZ) jump-starts and speeds acclimatisation, via the same biological pathway, and is effective in preventing AMS and reducing its severity. However, it is no substitute for a reasonable ascent schedule.
Hypoxic preconditioning (spending time in a hypobaric chamber to simulate altitude) is also possible. While still rare, it has been gaining popularity in recent years.
Be particularly wary with oxygen equipment: some tourists have died at altitude when their equipment failed and they were utterly unacclimatized.
Avoid rapid ascents
Rapid ascents are the opposite of acclimatization; you make a rapid ascent when you're gaining altitude faster than recommended. This may mean climbing and camping higher than recommended, but you can also make an even more rapid ascent by driving to a high altitude location, and flying from low altitude to high altitude is an even more rapid ascent. For example, flying from sea level to Lhasa, Tibet, which is 3700m (12,000 feet) high, is distinctly unwise. Consider spending a week or so at an intermediate altitude; see Overland to Tibet for some possibilities. If you are going to travel around Tibet — where some inhabited areas are over 5000m (16,000 feet) and some mountains over 8000m (26,000 feet), do not set out until you are thoroughly acclimatised in Lhasa. The same is true if you travel to the Andes; destinations such as Cuzco, La Paz or the Inca Trail are located well over 3000m above sea level.
Where possible, avoid ascents more rapid than recommended above, particularly any sudden ascent to 3000m (10,000 feet) or higher. Even if you are taking Acetazolamide (below) a rapid ascent makes it more likely you'll get AMS and makes AMS progress to serious illness faster, so you will have less time to respond and descend.
Consider road or rail travel rather than flying directly to somewhere with a very high altitude — but remember that the surface option often involves a lot higher altitudes: the Manali-Leh road for example will take you from below 2000m (7000 feet) to 5000m (16,000 feet) in less than a day. Or fly in stages, stopping somewhere at moderate altitude in between. If you must fly to any destination about 3000m (10,000 feet) at least spend a few days at some intermediate destination en route. If flying to a more moderate altitude above 2500m (8000 feet), you will still want to spend several nights at that altitude before setting off into higher country.
Remember to drink adequately – around one litre extra of fluids per day. Pushing large volumes of water does not protect against AMS, and can give the same symptoms (headaches, nausea, vomiting and more) as severe AMS from electrolyte imbalance.
Rest stepping and pressure breathing
Some advocate behavior modifications to prevent AMS, notably rest stepping and pressure breathing. The effectiveness in preventing AMS is unclear, but these are widely practiced.
Rest stepping consists of taking one quick step uphill, then locking the downhill knee and putting one’s weight on the lower leg, resting the muscles before the next step. Breathing is also regular: inhale during the step and exhale during the rest. Progress should be slow but steady, adjusting the rest period, rather than taking breaks. Beyond simply slowing the pace and increasing breathing, this is particularly useful during steep ascents, as it reduces endurance demands on the quadriceps.
Pressure breathing consists of forcefully exhaling through pursed lips, and is typically done at a regular pace (every few steps, or indeed every step).
Check blood oxygen and pulse
You can use a pulse oximeter to measure your blood oxygen saturation and pulse rate, which can help you detect problems before you get symptoms. These are inexpensive and easily available, with good accuracy. However, interpreting the numbers is complicated: normal readings vary between individuals and change with altitude. As a rule of thumb, within a group, individuals with lower SpO2 (either resting or after exercise) at a given altitude are more likely to develop AMS at higher altitudes, though precise cutoff values are difficult to give. Medical staff at altitude typically carry these, but it is prudent to purchase one (or two, as a backup) yourself.
Refrain from smoking (due to ) and alcohol (due to dehydration) when you arrive by plane in a high altitude area from lower altitudes.
As soon as the symptoms of AMS appear, your first priority is recovering. You must not ascend any further until the symptoms have disappeared – “don’t go up until the symptoms go down”. This may take up to 48 hours – if it takes longer, descend! You could also descend on the onset of symptoms; this will make them disappear much faster, probably within hours, and even minor descents (100 m) can help significantly.
If you are getting sicker or showing signs of HACE or HAPE, you must descend to a lower altitude as quickly as possible. If it is night time, do not wait for morning if you have a choice at all. You should descend at least as far as you were the last night you had no AMS symptoms. You may need to seek hospital care.
People with HACE and HAPE are frequently confused or exhausted, and are likely to need help with the descent. Help them down!
For cases of AMS, particularly flying into a high city (such as Cusco or Lhasa), supplemental oxygen can alleviate symptoms, particularly on arrival or first night. Oxygen is available at some airports, hotels, and even restaurants, and an oxygen tank is a frequent part of medical packs on high-altitude treks such as the Inca trail.
There is some equipment available to treat people with HACE or HAPE at high altitudes, including hyperbaric bags in which the sufferer can lie in a higher pressure atmosphere. Likewise, because the main cause of these illnesses is a lack of oxygen, breathing oxygen from a tank will slow their onset and may provide some temporary relief of symptoms. Either treatment buys some time if it is too dangerous to descend, but they are not a substitute for descent.
Sufferers of DCS need to be hospitalised and treated in a recompression chamber: descent to sea level is not sufficient to alleviate DCS symptoms. As with HACE and HAPE, breathing oxygen may provide some temporary relief of symptoms allowing for rescue. Scuba diving organisations can advise further.
Acetazolamide (ACZ, AZM, sold as Diamox) stimulates your breathing. The drug was originally designed as a treatment for glaucoma, but a side effect of increased breathing rates and depth have proven useful to climbers. It jump-starts and speeds up acclimatization rates, improves periodic breathing, and helps people recover from AMS more quickly. ACZ is primarily taken preventatively (as a prophylaxis: starting a day or two before ascent, and continuing at altitude and during further ascent), and also has some use for treatment.
Acetazolamide is not an absolute preventative measure, particularly in the case of forced ascents. A prescription is necessary, and a doctor should be consulted about proper dosages.
ACZ should be begun prior to leaving town: severe allergic reactions are rare but possible, even with no prior history, and it is safer to be near proper medical facilities.
There are some side effects. Firstly, the drug acts as a diuretic, causing increased urination, and can cause easy dehydration, so drinking plenty of water is important. Secondly, it can cause tingling (pins and needles) of the fingers and toes.
This drug can be useful for people who have had AMS in the past; people on a forced ascent, particularly to a Very High Altitude (for example, flying into Tibet or La Paz); and anyone who has AMS, particularly if they are choosing not to descend.
Current CDC guidelines are 125 mg, taken twice a day (every 12 hours), starting the day before ascent, and continuing the first 2 days at altitude, or longer if ascent continues. A 250 mg dose is more effective, but side effects are more likely and more severe: it’s recommended if you are higher risk. If you have enough 125 mg pills, you can double the dose to 250 mg if necessary, so if in doubt, it’s prudent to get more than the minimum.
Simple preventative drugs
Ibuprofen 600 mg every 8 hours is reasonably effective for prevention of AMS; not as effective as ACZ, but it is cheap, widely available over-the-counter, and is well-tolerated (few/mild side effects). Gingko biloba has some effectiveness for prevention in some trials, for 100–120 mg every 12 hours, taken before ascent.
Other than supplemental oxygen, one can relieve symptoms of AMS via usual means: treat headaches with headache medication (non-opiate analgesics, like aspirin, acetaminophen (Tylenol), NSAIDs, etc.) and treat nausea and vomiting with anti-nausea drugs (antiemetics, like ondansetron (Zofran)).
ACZ is moderately effective for treating symptoms, but it's more for prevention. Dexamethasone is most effective for rapidly treating moderate to severe symptoms.
Coca leaves, available primarily in Andean regions of Peru and Bolivia (in coca tea, chewing, or in candies), are a mild stimulant and alleviate symptoms to some degree, particularly headaches (like the caffeine in coffee or tea) though they do not speed acclimatisation. Some people find that vegetarian or starchy food helps them somewhat.
Antacids may help with nausea, but do not help with acclimatization. There is the occasional misconception that antacids have some impact on acclimatization, presumably due to confusion between blood acidity (which is related to acclimatization) and stomach acidity (which is not).
If symptoms get worse while staying at the same altitude, you are in danger: descend immediately.
Oxygen and hyperbaric chambers
Supplemental oxygen (2 L/minute) will relieve AMS headaches quickly, and resolve AMS over hours; it is also lifesaving in cases of HAPE, and important for HACE. Oxygen typically isn't available in the field, but is available at hospitals, and at some airports, such as Cusco, for arriving passengers. An alternative field treatment is a hyperbaric chamber (high atmospheric pressure tent), which increases the amount of oxygen available in the air.
Other drugs, which are significantly more potent, include dexamethasone, nifedipine, salmeterol (Serevent), sildenafil, temazepam (Temaze), and tadalafil. Dexamethasone prevents and treats AMS and HACE, but primarily used for treatment (with ACZ preferred for prevention), as adjunct to descent, but is also used for summit day on high peaks such as Kilimanjaro and Aconcagua, to prevent abrupt altitude sickness. Nifedipine prevents and ameliorates HAPE, and is generally reserved for people who are susceptible to the condition. Salmeterol (in conjunction with oral therapy), sildenafil, and tadalafil are all used for HAPE prevention.
Some of these drugs are found in capsules sold in China e.g. Gao Yuan Kang (高原康), which contains dexamethasone. Some herbal preparations are also purported to prevent/treat high altitude illness, such as gingko biloba and a combination capsule called Gao Yuan Ning (高原宁), sold in China. The effectiveness of these preparations remains scientifically unproven, although Gao Yuan Ning (高原宁) is used by Chinese military personnel in cases of rapid ascent.
It is extremely important to note that all these drugs can have significant side effects, especially dexamethasone, a potent steroid medication. Tourists are advised to consult their doctor prior to obtaining these medications. Foreign tourists should procure any necessary medications in their home countries and note the ingredients contained in the medications.