Preparing for high altitude starts weeks before your trip, not at the trailhead. Most people begin feeling the effects of reduced oxygen above 8,000 feet (2,400 meters), and roughly 25% of travelers to moderate altitude experience some form of altitude sickness. The good news: with the right preparation timeline, you can dramatically reduce your risk and feel stronger when you arrive.
Why Altitude Affects Your Body
At sea level, oxygen makes up about 21% of the air you breathe. That percentage stays the same at altitude, but the air pressure drops, which means each breath delivers less oxygen to your bloodstream. At 10,000 feet, you’re getting roughly 30% less oxygen per breath than at sea level. Your body compensates by breathing faster, increasing heart rate, and eventually producing more red blood cells, but that last adaptation takes days to weeks.
This lag between arrival and adaptation is where altitude sickness lives. Headache, nausea, fatigue, and poor sleep are the hallmark symptoms, typically appearing 6 to 12 hours after arrival and peaking around day two or three.
Start Preparing Two to Four Weeks Out
The single most effective preparation strategy is pre-acclimatization: spending time at progressively higher elevations before your target altitude. If you live near mountains, weekend trips to moderate elevations help your body start building the red blood cells it needs. Even sleeping one or two nights at 7,000 to 8,000 feet in the weeks before a bigger trip gives you a head start.
If pre-acclimatization isn’t practical, focus on aerobic fitness. You don’t need to be an elite athlete, but a strong cardiovascular base means your heart and lungs work more efficiently with limited oxygen. Prioritize sustained cardio like running, cycling, or hiking with a loaded pack. Interval training is especially useful because it mimics the oxygen stress you’ll face at elevation. Aim for at least three to four weeks of consistent training before your trip.
Check Your Iron Levels
Your body needs iron to build the extra red blood cells that carry oxygen at altitude. If your iron stores are low, your ability to acclimatize suffers. Research suggests getting a baseline ferritin test and supplementing if levels fall below 30 micrograms per liter for women or 40 for men. This is worth doing three to four weeks before your trip so supplementation has time to work. A simple blood test from your primary care provider covers it.
Adjust Your Diet for Altitude
At altitude, your body shifts toward burning carbohydrates for fuel instead of fat. This happens because carbohydrates require less oxygen to convert into energy, which is a real advantage when oxygen is scarce. Traditional sports nutrition guidelines for altitude recommend getting at least 60% of your total calories from carbohydrates, roughly 6 to 8 grams per kilogram of body weight per day. For a 150-pound person, that translates to about 400 to 550 grams of carbs daily.
Start increasing your carbohydrate intake a few days before you ascend. Focus on easily digestible sources: rice, pasta, oatmeal, bread, potatoes, and fruit. Many people lose their appetite at altitude, so eating calorie-dense foods you actually enjoy matters more than optimizing macros perfectly. Pack snacks you’ll reach for even when you don’t feel hungry.
Hydration Before and During Your Trip
You lose water faster at altitude. The dry air pulls moisture from your lungs with every exhale, and your increased breathing rate amplifies this effect. You also urinate more frequently as part of the body’s natural acclimatization process.
Start hydrating deliberately the week before your trip, aiming to drink 25 to 50% more water than your normal daily intake. Maintain that increased intake once you arrive at elevation. If you typically drink eight glasses a day, bump it to 10 to 12. Carry a water bottle and sip consistently rather than chugging large amounts at once. Adding electrolyte tablets or packets helps your body retain the fluid rather than simply flushing it through.
A quick way to monitor hydration: your urine should stay pale yellow. Dark urine at altitude is a clear signal to drink more.
Prescription Medication for Prevention
Acetazolamide (sold as Diamox) is the most widely used prescription medication for preventing altitude sickness. It works by changing the acidity of your blood, which stimulates faster, deeper breathing and helps your body take in more oxygen. The CDC recommends starting it the day before you ascend and continuing for the first two days at your target altitude, or longer if you’re still climbing.
Common side effects include tingling in your fingers and toes, increased urination, and a metallic taste when drinking carbonated beverages. These are annoying but harmless. Despite being chemically related to sulfa drugs, acetazolamide does not cause cross-reactions in people with sulfa antibiotic allergies, according to the CDC. However, anyone with a history of severe allergic reactions to multiple medications should use caution.
Talk to a provider about a prescription before your trip. Acetazolamide isn’t available over the counter, and it’s worth doing a trial run at home to see how your body responds before relying on it in the field.
Pace Your Ascent
How fast you go up matters more than how fit you are. The general rule above 8,000 feet: don’t increase your sleeping elevation by more than 1,000 to 1,500 feet per day. For every 3,000 feet of elevation gain, take a rest day where you sleep at the same altitude as the previous night. You can climb higher during the day as long as you come back down to sleep. This “climb high, sleep low” approach is one of the most reliable acclimatization strategies.
If you’re flying directly from sea level to a high-altitude city like Cusco (11,150 feet) or La Paz (11,940 feet), you’re skipping gradual acclimatization entirely. In these cases, plan to do very little for the first 24 to 48 hours. Light walking is fine, but avoid strenuous activity, and rest when your body tells you to. Some travelers build in an intermediate stop at a moderate elevation for a night or two before continuing higher.
What to Avoid in the First 48 Hours
Alcohol is a poor companion at altitude. It disrupts sleep quality, contributes to dehydration, and can mask early symptoms of altitude sickness. Skip it entirely for your first two days at elevation, and go easy after that. Sleeping pills present similar risks because they can suppress your breathing rate during sleep, which is exactly what your body is trying to increase.
Overexertion on your first day is one of the most common mistakes. The excitement of arriving at a beautiful destination makes people push too hard before their body has adjusted. Even if you feel fine, keep your activity level low for at least the first full day.
Recognize Dangerous Warning Signs
Mild altitude sickness, with its headache and mild nausea, is uncomfortable but manageable. Two serious conditions require immediate action.
High-altitude pulmonary edema (HAPE) happens when fluid leaks into the lungs. It typically develops two to four days after arrival. Warning signs include a dry cough that progresses to coughing up pink or frothy mucus, unusual breathlessness even at rest, a racing heart rate, and blue or gray discoloration of the lips or fingernails. HAPE can be fatal if ignored.
High-altitude cerebral edema (HACE) involves swelling in the brain. Symptoms include severe confusion, loss of coordination (unable to walk a straight line), altered behavior, and extreme drowsiness. HACE often accompanies HAPE and escalates quickly.
Both conditions demand immediate descent. Moving even 1,000 to 3,000 feet lower often produces rapid improvement. The cardinal rule: if you feel increasingly sick at altitude, go down. Never ascend further with worsening symptoms.
A Quick Preparation Timeline
- 4 weeks before: Start or intensify aerobic training. Get a ferritin test and begin iron supplements if needed.
- 1 week before: Increase water intake by 25 to 50%. Shift your diet toward more carbohydrates.
- 1 day before ascent: Begin acetazolamide if prescribed. Pack electrolyte supplements and high-carb snacks.
- Days 1 to 2 at altitude: Take it easy. Stay hydrated. Avoid alcohol and sleeping pills. Monitor for symptoms.
- Days 3 and beyond: Gradually increase activity. Continue hydrating aggressively. Follow the 1,000-foot rule if climbing higher.