Acute Mountain Sickness


Acute Mountain Sickness (AMS) is a constellation of symptoms that represents your body not being acclimatized to its current altitude.  As you ascend, your body acclimatizes to the decreasing oxygen (hypoxia).  At any moment, there is an "ideal" altitude where your body is in balance; most likely this is the last elevation at which you slept.

Extending above this is an indefinite gray zone where your body can tolerate the lower oxygen levels, but to which you are not quite acclimatized.  If you get above the upper limit of this zone, there is not enough oxygen for your body to function properly, and symptoms of hypoxic distress occur - this is AMS.  Go too high above what you are prepared for, and you get sick.

This "zone of tolerance" moves up with you as you acclimatize.  Each day, as you ascend, you are acclimatizing to a higher elevation, and thus your zone of tolerance extends that much higher up the mountain.  The trick is to limit your daily upward travel to stay within that tolerance zone.


AMS has been likened to a bad hangover, or worse.  However, because the symptoms of mild AMS can be somewhat vague, a useful rule-of-thumb is: if you feel unwell at altitude, it is altitude sickness unless there is another obvious explanation (such as diarrhea).

The diagnosis of AMS is made when a headache, with any one or more of the following symptoms is present after a recent ascent above 8000 feet (2500 meters). All of these symptoms may vary from mild to severe:

  • Loss of appetite, nausea, or vomiting
  • Fatigue or weakness
  • Dizziness or light-headedness
  • Difficulty sleeping

The exact mechanisms of AMS are not completely understood, but the symptoms are thought to be due to mild swelling of brain tissue in response to the hypoxic stress.  If this swelling progresses far enough, significant brain dysfunction occurs (See references for HACE).  This brain tissue distress causes a number of symptoms; universally present is a headache, along with a variety of other symptoms.

will i get altitude sickness?

Anyone who goes to altitude can get AMS.  It is primarily related to individual physiology (genetics) and the rate of ascent; there is no significant effect of age, gender, physical fitness, or previous altitude experience.  Some people acclimatize quickly, and can ascend rapidly; others acclimatize slowly and have trouble staying well even on a slow ascent.  

There are factors that we don't understand; the same person may get AMS on one trip and not another despite an identical ascent itinerary.  Unfortunately, no way has been found to predict who is likely to get sick at altitude.

It is remarkable how many people mistakenly believe that a headache at altitude is "normal"; it is not.  Denial is also common - be willing to admit that you have altitude illness, that's the first step to staying out of trouble.

It is OK to get altitude illness, it can happen to anyone.  It is not OK to die from it.  With the information in this tutorial, you should be able to avoid the severe, life-threatening forms of altitude illness.

how to treat acute mountain sickness

The mainstay of treatment of AMS is rest, fluids, and mild analgesics: acetaminophen, aspirin, or ibuprofen. 

These medications will not cover up worsening symptoms. The natural progression for AMS is to get better, and often simply resting at the altitude at which you became ill is adequate treatment.  Improvement usually occurs in one or two days, but may take as long as three or four days.  Descent is also an option, and recovery will be quite rapid.

Is your headache due to AMS?

A frequent question is how to tell if a headache is due to altitude. Altitude headaches are usually nasty, persistent, and frequently there are other symptoms of AMS; they tend to be frontal (but may be anywhere), and may worsen with bending over. However, there are other causes of headaches, and you can try a simple diagnostic/therapeutic test.

Dehydration is a common cause of headache at altitude.  Drink one liter of fluid, and take some acetaminophen or one of the other analgesics listed above.  If the headache resolves quickly and totally (and you have no other symptoms of AMS) it is very unlikely to have been due to AMS.

Things to avoid
Respiratory depression (the slowing down of breathing) can be caused by various medications, and may be a problem at altitude. The following medications can do this, and should never be used by someone who has symptoms of altitude illness:

  • Alcohol
  • Sleeping pills
  • Narcotic pain medications in more than modest doses

Preventative Medication

Under certain circumstances, prevention with medication may be advisable, especially for persons on forced rapid ascents (such as flying into Denver from sea level and going directly up to the mountains) or for persons who have repeatedly had AMS in the past.

The main medication for preventing Acute Mountain Sickness is Acetazolamide (Diamox┬«), but only under the specific limited conditions outlined above. Most people who have a reasonable ascent schedule will not need it, and in addition to some common minor but unpleasant side effects it carries the risk of any of the severe side effects that may occur with sulfonamides.

There is also the possibility that Ginkgo biloba extract may be helpful. Some early work with Ginkgo biloba extract was encouraging with regards to its use in preventing AMS, but some recent large, well-designed studies have shown no benefit.

*Please note that the above tips are tips only. For more details on high altitude tips, please contact you local family physician.


Basic High Altitude Advice Page