AMS stands for acute mountain sickness, a condition caused by reduced oxygen levels at high altitude. It typically strikes within 2 to 12 hours of arriving at elevations above roughly 8,000 feet (2,500 meters) and is the most common form of altitude illness. Most cases are mild and resolve on their own within one to three days, but AMS can progress to life-threatening complications if warning signs are ignored.
What Happens Inside Your Body
At high altitude, every breath you take contains less oxygen than at sea level. Your body responds in several ways at once. Blood vessels in the brain widen to push more oxygen-rich blood through, and this dilation can increase cerebral blood flow by roughly 24% within hours of a rapid ascent. At the same time, you start breathing faster to compensate for the thin air, which lowers carbon dioxide levels in your blood and triggers the opposite effect: vessel constriction.
These two opposing forces create a tug-of-war. In the first hours and days at altitude, the oxygen-driven dilation tends to win, which can cause mild swelling (edema) in brain tissue. That swelling, combined with increased pressure inside the skull and heightened stress-hormone activity, produces the hallmark symptoms of AMS. Over the next few days, if you stay at the same elevation and don’t climb higher, the balance shifts back toward normal and symptoms fade.
Symptoms and How They’re Scored
Headache is the defining symptom. Without a headache, it isn’t considered AMS. Beyond that, the most common complaints are nausea or loss of appetite, fatigue, weakness, and dizziness or lightheadedness. Symptoms usually appear during or after the first night at a new altitude.
Doctors and researchers use the Lake Louise Acute Mountain Sickness Score to formally diagnose the condition. It rates four symptoms on a 0-to-3 scale:
- Headache: from none (0) to severe and incapacitating (3)
- Gastrointestinal symptoms: from good appetite (0) to severe nausea and vomiting (3)
- Fatigue or weakness: from none (0) to severe and incapacitating (3)
- Dizziness or lightheadedness: from none (0) to severe and incapacitating (3)
A total score of three or more, with at least one point coming from headache, confirms AMS. The scoring system also includes a functional question: whether symptoms forced you to stop your ascent, descend, or be evacuated.
How Long It Lasts
If you stop gaining elevation, AMS generally resolves within 12 to 48 hours as your body acclimatizes. The whole cycle of onset, peak symptoms, and resolution typically plays out over one to three days. Descending even a few hundred meters usually brings faster relief. The key variable is whether you keep climbing. Pushing higher with active symptoms extends the illness and raises the risk of dangerous complications.
Risk Factors
The single biggest risk factor is how fast you ascend. Flying or driving directly to a high-altitude destination, rather than gaining elevation gradually, dramatically increases your chances of developing AMS. A personal history of altitude sickness is another strong predictor: if you’ve had it before, you’re likely to get it again under similar conditions.
Living at low elevation matters too. Someone who normally lives near sea level is more vulnerable than someone who lives at moderate altitude. Interestingly, physical fitness does not protect you. Elite athletes get AMS at the same rates as anyone else because fitness doesn’t change how quickly the body adjusts to lower oxygen levels. Age plays a small role: younger adults tend to report AMS more frequently than older adults, though the reasons aren’t entirely clear.
Prevention Through Pacing
Controlled ascent is the most effective prevention strategy. Above 8,000 feet, you should gain no more than 1,000 to 1,600 feet (300 to 500 meters) of sleeping elevation per day. When you first reach 8,200 feet (2,500 meters), spending a full rest day before climbing higher gives your body a head start on acclimatization. After that, building in a rest day every third day of ascent is standard guidance. Above 14,000 feet (4,267 meters), the recommendation tightens to a two-day rest for every 1,000 feet gained.
Prescription medications can also help prevent AMS when taken before ascent, particularly for people with a history of the condition or those whose itinerary forces a rapid gain in elevation. Your doctor can advise whether preventive medication makes sense for your specific trip.
Treatment
For mild cases, the first-line treatment is simple: stop ascending and rest at your current altitude. Over-the-counter pain relievers can help with the headache, and staying well hydrated supports the acclimatization process. Most people feel significantly better within a day or two.
For moderate symptoms that aren’t improving, descending 500 to 1,000 meters is the most reliable fix. Supplemental oxygen, when available, provides rapid symptom relief by directly addressing the underlying oxygen deficit. In remote settings where descent isn’t immediately possible, portable pressure bags can simulate a lower altitude by increasing air pressure around the person inside.
When AMS Becomes Dangerous
Altitude illness exists on a spectrum. AMS sits at the milder end, but it can progress to two serious, potentially fatal conditions: high-altitude cerebral edema (HACE) and high-altitude pulmonary edema (HAPE).
HACE is essentially severe brain swelling. The earliest and most specific warning sign is ataxia, a loss of coordination that may look like stumbling or difficulty walking in a straight line. As it progresses, you may notice slurred speech, confusion, increasing drowsiness, or a declining ability to think clearly. Any of these signs in someone with AMS symptoms demands immediate descent.
HAPE affects the lungs rather than the brain. It causes fluid buildup that makes breathing progressively harder, even at rest. A persistent dry cough, breathlessness that worsens when lying down, and unusual fatigue during minimal exertion are its hallmarks.
The critical point is that both conditions can develop from what initially seems like ordinary AMS. Recognizing the shift, particularly the subtle onset of unsteady walking or worsening mental clarity, is what separates a manageable altitude headache from a medical emergency.