How can high altitude climbers survive with so little oxygen?

Imagine unzipping yourself from the cozy cocoon of your sleeping bag, forcing your feet into numbingly frosty boots, and then propelling yourself out of the tent into the subzero night just so you can stagger a few feet away to relieve your bursting bladder.

Many high altitude climbers have to endure this “bathroom” ordeal many times a night, but you won’t hear them complaining. To them, it’s worth it. The constant need to pee is a minor hassle compared to the benefit offered by a drug they take to help acclimatize to extreme elevations.

When climbers and trekkers travel to high altitudes, between 9,000 and 20,000 feet, any extended stay leaves them at risk of developing acute mountain sickness, or AMS. At these heights in the mountains there is less than half the amount of oxygen in the air as there is at sea level, which can lead to AMS symptoms of headaches, nausea, dizziness and fatigue. If you travel upward from sea level slowly enough, gaining only a few thousand feet every day, you can avoid AMS by giving your body the chance to adjust.

According to Robert Roach, the director of the Altitude Research Center at the University of Colorado Denver, the hallmark of acclimatization is breathing more than you would at sea level. If a climber travels upward slowly enough, the body adjusts on its own and breathing speeds up, which tricks the body into thinking it isn’t at elevation.

But when a slow ascent and natural adjustment isn’t an option — like when you only have a long weekend to fly up to the mountains for a ski vacation — a boost can be found in pill form.

“The best we can do is try to mimic acclimatization,” says Roach, “and acetazolamide does that.” Also known as Diamox, this drug really gets your breathing going.

Acetazolamide works by interrupting the flow of carbon dioxide (CO₂) within your body, so that you quickly build up an excess of CO₂. In response, your body naturally tries to get rid of it. So, letting your lungs do the work, you begin to breathe more frequently. And because you’re breathing more quickly, you’re also inhaling more oxygen, slowing the onset of AMS.

Breathe out — bye-bye, CO₂. Breathe in — hello, precious O₂. But there’s a catch.

As we exhale all that CO₂, the pH level in the bloodstream begins to change, forcing the body to make more fine-tuned adjustments. In order to bring balance back to the blood, bicarbonate — a simple compound that helps maintain a steady and neutral pH in our body — is shuttled to the kidneys. From there, we can easily get rid of it … by peeing it away. Having to urinate every few hours, even throughout the night, is part of what makes acetazolamide work. As long as we keep flushing away the bicarbonate, we can continue to inhale and exhale at a faster pace.

High altitude climbers have been taking acetazolamide to help adapt their breathing for over 40 years, but dosage varies depending on how much each person needs, because everyone reacts differently at altitude, says Roach. Not everyone suffers AMS to the same degree, and some of the world’s best big mountain climbers are so good at adjusting to altitude on their own that the drug doesn’t really help much.

Because there is no easy way to predict who might develop AMS, Roach says people can climb without acetazolamide to find out how the body reacts in that extreme environment, as long as they are prepared to take the drug or head back down the mountain if they get sick. For most mountaineers, that’s what the spirit of climbing high is all about: testing your own limits.

“One of the totally cool things is learning how your body does [at elevation],” Roach says, “and it’s a shame if people never, ever have that.”