I hiked a mountain in the Andes to celebrate my 40th birthday. The hallucinations and violent vomiting that followed had not been part of the plan.

• Elizabeth Lavis planned to hike the ninth-highest mountain in the Bolivian Andes for her 40th birthday.
• She climbed too quickly, without ample preparation or acclimation, and got altitude psychosis.

The condors appeared at 5,000 meters. They were not real.

I got altitude psychosis a few hundred meters from high camp as I scaled the frigid face of Huayna Potosi, the ninth-highest mountain in the Bolivian Andes.

My optimism, determination, and the sugar high from my last frozen-solid Snicker's bar had all but faded into a combination of low-key anxiety, aching muscles, and vertigo by the time I started hallucinating birds with giant geometrically patterned wings alighting gently in on the ice-slicked rocks around me.

With the help of a phenomenal local guide and a lot of luck, I made it back down the mountain. I still do multi-day treks at altitude in the Andes and Caucasus mountains, but I follow some hard rules to keep myself safe.

Altitude psychosis on the mountain

I first glimpsed Huayna Potosi, a mountain near Bolivia's La Paz and El Alto, when I sailed across Lake Titicaca, the massive freshwater lake linking Peru and Bolivia. I remember thinking, naively, "I can climb that," and set forth to summit on my 40th birthday.

I climbed too quickly, without ample preparation or acclimation, and tamped down the effects of your garden-variety altitude sickness, like nausea, headaches, and fatigue with a combination of cold-brewed coca leaf tea, water, acetaminophen, and sugar.

I didn't realize I was sick until it was far too late; I started hallucinating and violently vomiting as I tenuously made my way up to high camp, crampons skidding and slipping off the packed ice as exhaustion and confusion set in.

Thanks to my excellent guide, I made it to high camp and spent the night in an orange geodesic dome perched on Potosi's precarious eastern side. I didn't sleep a wink. Instead, I spent all night prying open the triangular door of the dome with a screwdriver, vomiting spectacularly on the rocks outside, and marveling at how gorgeous the stars looked at that height.

The danger zone

High-altitude psychosis generally occurs at heights of 7,000 meters or higher, although it can strike hikers trekking at lower altitudes. Once referred to as mountain madness, it's a uniquely terrifying ailment that manifests itself in hyper-intense hallucinations and disappears once you descend.

Although high-altitude psychosis doesn't always happen in tandem with altitude sickness, my doctor speculated that my experience had a lot to do with not acclimatizing properly, overestimating my abilities, and minimizing the effects of altitude sickness through a constant regimen of over-the-counter medication and natural remedies, like coca tea.

Frontiers in Psychiatry noted that while episodes of psychosis during exposure to high altitude are frequently reported, it has still not been assigned to medical diagnoses.

Staying safe at altitude

I would have been in serious trouble if not for my guide. He rapidly identified the seriousness of the situation, carried my day pack the rest of the way, and helped me ascend slowly to high camp. Once there, he gave me spoonfuls of raw sugar and plenty of water and descended with me at first light.

I still love multi-day trekking and mountain climbing. I just recently got back from a four-day journey through the high Caucasus, where I had to scurry up steep rock scrambles, ford glacially fed rivers, and walk heel-to-toe down weaving, sharply declining cow paths for 15 miles per day.

The mountains are majestic and restorative, but they must be respected, which is why I have new rules for staying both mentally and physically safe at altitude and on extreme outdoor adventures.

Proper acclimatization is the first rule. I like to spend at least three to four days adjusting to my new altitude. I lay low the first day or two and take a tentative, exploratory, short hike on the third or fourth. Doing this allows me to assess how my body is performing and regulating at the new heights, and a shorter, less intense hike lets me know what to expect on a longer slog.

I also limit the amount of over-the-counter medicine I take, powering through the first few fuzzy days of headache and fatigue until my body resettles itself. I still drink coca tea to ward off some symptoms, but by listening to my body, I can ensure that it's functioning optimally and ready for the hike.

Finally, I've learned to be more humble. While I was fairly fit and prepared to climb Huayna Potosi, I didn't consider the mental or extreme physical toll the mountain would take. Knowing your limitations is tough but essential. Realistically, I should have trained and acclimatized for Potosi for at least a few extra months before taking my first step up the eastern side.

Life happens at the extremes, which is why it's always a good idea to climb that mountain. The responsible way to do it is to properly balance your athletic ability, stay in tune with your body's response to altitude, and observe the buddy system.

Got a personal essay about celebrating your 40th birthday? Get in touch with the editor: akarplus@businessinsider.com.