Altitude sickness (acute mountain sickness, or AMS) happens when you ascend to a higher elevation faster than your body can adjust to the drop in available oxygen. It matters for athletes both training at altitude for a performance edge and traveling to compete at elevation — skiing, mountain trail running, or a tournament in a high-altitude city.
Recognizing it
Symptoms typically start 6–24 hours after arriving at altitude (generally above 2,500m/8,000ft) and feel similar to a hangover: headache, nausea, fatigue, dizziness, and trouble sleeping. Two rarer but serious progressions are worth knowing by name: HAPE (high-altitude pulmonary edema — fluid in the lungs, causing breathlessness at rest and a cough) and HACE (high-altitude cerebral edema — brain swelling, causing confusion and loss of coordination). Both are medical emergencies requiring immediate descent.
Reducing your risk
- Ascend gradually — above 2,500m, limiting sleeping elevation gain to roughly 300–500m per day is a common guideline, with an extra rest day every few days.
- Avoid flying straight into hard training — give at least a day or two to acclimatize before a competition or hard session at altitude.
- Stay well hydrated and go easy on alcohol in the first days, since dehydration can mimic or worsen symptoms.
- Consider medication for high-risk trips — acetazolamide is the first-line option for speeding acclimatization on rapid ascents to significant altitude. It works by making the blood mildly acidic (via bicarbonate loss in the urine — hence the frequent peeing it causes), which drives you to breathe faster and get more oxygen, kickstarting the acclimatization your body would otherwise take days to do on its own. It’s typically started 1–2 days before ascent. Sildenafil (yes, the same drug used for erectile dysfunction) is sometimes used as an alternative or add-on — it lowers pressure in the pulmonary arteries and can help specifically with HAPE prevention in those with a prior history, but it’s generally considered less effective than acetazolamide for overall AMS symptoms and is a second-line/adjunct choice. Both are prescription decisions to make with a clinician ahead of travel, not something to start on your own.
What to do if symptoms appear
Mild AMS often improves by stopping further ascent, resting, and hydrating at the same elevation for a day. The golden rule of altitude illness: don’t ascend further with symptoms, and if they worsen or don’t improve, descend. Symptoms of HAPE or HACE mean immediate descent and emergency medical care, not a wait-and-see approach.
A performance note
“Live high, train low” is the classic altitude-training strategy — sleeping at altitude to stimulate red blood cell production while keeping hard training sessions at lower elevation where oxygen delivery (and therefore training quality) isn’t compromised. Trying to train at full intensity immediately upon arrival at altitude, before any acclimatization, typically just produces a worse session, not a training adaptation.
This page is educational and general in nature and does not replace an individualized evaluation — see our medical disclaimer.