The single most effective way to prevent altitude sickness is controlling how fast you gain elevation, particularly your sleeping altitude. Most cases occur when people ascend too quickly above 2,500 meters (8,200 feet), outpacing their body’s ability to adjust to thinner air. The good news: with the right pacing, hydration, nutrition, and sometimes medication, altitude sickness is largely preventable.
Why Altitude Sickness Happens
At higher elevations, air pressure drops and each breath delivers less oxygen. Your body compensates by breathing faster, producing more red blood cells, and adjusting blood flow, but these changes take time. When you climb faster than your body can adapt, fluid can leak into the brain or lungs, producing the headaches, nausea, and fatigue known as acute mountain sickness (AMS). In rare cases, this progresses to dangerous swelling in the brain or fluid in the lungs.
The risk starts around 2,500 meters (8,200 feet) and increases sharply with altitude and speed of ascent. It affects fit and unfit people alike. Previous experience at altitude helps, but it doesn’t guarantee immunity on the next trip.
Control Your Ascent Rate
Pacing is the cornerstone of prevention. The Wilderness Medical Society’s 2024 guidelines recommend that once above 3,000 meters (9,800 feet), you should not increase your sleeping altitude by more than 500 meters (1,650 feet) per day. Build in an extra rest day, where you don’t gain any sleeping elevation, every three to four days.
Critically, avoid ascending to a sleeping altitude above 2,750 meters (9,000 feet) in a single day from low elevation. This means if you’re flying into a high-altitude city like Cusco (3,400 meters) or La Paz (3,640 meters), you’re already at risk from the moment you land. Plan at least one or two easy days before any physical exertion.
Climb High, Sleep Low
This is the classic mountaineering rule, and it works because your sleeping altitude matters more than the highest point you reach during the day. You can hike or climb well above 500 meters in a single day as long as you descend to sleep at a lower elevation. For trekkers above 3,000 meters, a practical approach is to hike to the next camp, continue upward for an hour or two, then return to camp to sleep. This gives your body a preview of the altitude it will face the next day.
A commonly cited version of this rule: above 3,000 meters, increase your sleeping altitude by no more than 300 meters (1,000 feet) per day, with a rest day every 900 meters (3,000 feet) of cumulative gain. The Wilderness Medical Society’s slightly more generous 500-meter guideline reflects updated evidence, but if you’re prone to altitude sickness, the more conservative approach is safer.
Hydration and Nutrition at Altitude
Your body loses water faster at altitude through increased breathing and lower humidity. Experts recommend drinking 4 to 5 liters of fluid per day during altitude training and activity, with 400 to 800 milliliters per hour during active climbing or trekking. Adding a small amount of sodium (roughly half a gram per liter) helps your body retain fluid more effectively.
Carbohydrates deserve special attention. Your body preferentially burns carbs in low-oxygen environments because they require less oxygen to metabolize than fat. Research on high-altitude nutrition suggests that at least 60% of your total calories should come from carbohydrates, translating to about 6 to 8 grams per kilogram of body weight per day during active trekking. For a 70-kilogram person, that’s 420 to 560 grams of carbs daily. At base camp with lighter activity, 3 to 5 grams per kilogram is sufficient. In practical terms, this means prioritizing rice, pasta, bread, oatmeal, dried fruit, and energy bars over fatty or protein-heavy meals.
Medication for Prevention
When a gradual ascent isn’t possible, or if you have a history of altitude sickness, medication can help. The most widely used preventive drug is acetazolamide (sold as Diamox). It works by making your blood slightly more acidic, which tricks your body into breathing faster and deeper, speeding up acclimatization. Common side effects include tingling in the fingers and toes, increased urination, and a metallic taste with carbonated drinks. These are annoying but harmless.
A steroid called dexamethasone is another option, typically reserved for moderate-to-high-risk situations. The standard preventive dose is 2 mg every six hours or 4 mg every twelve hours. It reduces brain swelling effectively but, unlike acetazolamide, doesn’t actually help your body acclimatize. It masks the problem rather than fixing it, so symptoms can return when you stop taking it. Higher doses (4 mg every six hours) are sometimes used in extreme scenarios, such as military or rescue teams airlifted above 3,500 meters who must immediately perform physical work.
For people with a history of high-altitude pulmonary edema (fluid in the lungs), a blood pressure medication called nifedipine can be taken preventively. The extended-release form is started the day before ascent and continued for four to seven days after reaching target elevation. This is a targeted prevention strategy for a specific, serious condition, not something most trekkers need.
Pre-Acclimatization Before Your Trip
If you have access to altitude before your trip, spending time at moderate elevation in the weeks beforehand helps. The most studied approach involves living at 2,000 to 3,000 meters for at least 12 hours per day while training at lower elevations, sustained for a minimum of 21 days. This “live high, train low” method is well-established in athletic performance research and translates directly to altitude sickness prevention.
For most recreational trekkers, a 21-day pre-acclimatization protocol isn’t realistic. A more practical version is to spend a few nights at moderate altitude (2,000 to 2,500 meters) in the week before heading higher. Even brief altitude exposure primes your body’s red blood cell production and breathing responses. If your trip starts with a flight directly to high altitude, arriving a day or two early and staying at an intermediate elevation makes a measurable difference.
Check Your Iron Levels
Your body’s primary response to altitude is producing more red blood cells to carry oxygen more efficiently. Iron is essential for this process. Iron deficiency, even without full-blown anemia, can impair your ability to acclimatize and may increase your risk of altitude sickness. If you have risk factors for low iron (heavy menstrual periods, vegetarian or vegan diet, frequent blood donation, or endurance training), getting a ferritin blood test before a high-altitude trip is a simple, worthwhile precaution. Low stores can be corrected with supplementation in the weeks before departure.
Alcohol and Sleep Aids
Alcohol is a respiratory depressant, meaning it slows your breathing. At altitude, where your body compensates for low oxygen by breathing faster, this works against you, especially during sleep. The first few nights at altitude are when your body is working hardest to adjust. Avoiding alcohol during this period gives your respiratory system the best chance to do its job. Sedative sleep medications carry the same concern. If you’re struggling to sleep at altitude, know that poor sleep is common and is a direct effect of low oxygen on your brain rather than a sign of altitude sickness. The Lake Louise scoring system for altitude sickness was actually revised in 2018 to remove disturbed sleep as a symptom, because research showed it’s a normal response to thin air rather than a warning sign.
Recognizing Early Symptoms
Prevention also means knowing when your strategy isn’t working. The hallmark of acute mountain sickness is a headache at altitude combined with at least one other symptom: nausea, fatigue, or dizziness. These typically appear 6 to 12 hours after arrival at a new elevation. Mild symptoms often resolve with rest and time, but the critical rule is simple: do not ascend to a higher sleeping altitude while symptomatic. If symptoms worsen despite resting at your current elevation, descend. Even dropping 300 to 500 meters typically brings rapid relief.
More concerning signs include severe headache unresponsive to pain medication, confusion, loss of coordination (test by walking heel-to-toe in a straight line), persistent vomiting, or a wet cough with breathlessness at rest. These suggest the dangerous forms of altitude illness and require immediate descent.