Ayahuasca, used for centuries in South American jungles, is booming in the U.S. Illustration by Bjørn Lie

The day after Apollo 14 landed on the moon, Dennis and Terence McKenna began a trek through the Amazon with four friends who considered themselves, as Terence wrote in his book “True Hallucinations,” “refugees from a society that we thought was poisoned by its own self-hatred and inner contradictions.” They had come to South America, the land of yagé, also known as ayahuasca: an intensely hallucinogenic potion made from boiling woody Banisteriopsis caapi vines with the glossy leaves of the chacruna bush. The brothers, then in their early twenties, were grieving the recent death of their mother, and they were hungry for answers about the mysteries of the cosmos: “We had sorted through the ideological options, and we had decided to put all of our chips on the psychedelic experience.”

They started hiking near the border of Peru. As Dennis wrote, in his memoir “The Brotherhood of the Screaming Abyss,” they arrived four days later in La Chorrera, Colombia, “in our long hair, beards, bells, and beads,” accompanied by a “menagerie of sickly dogs, cats, monkeys, and birds” accumulated along the way. (The local Witoto people were cautiously amused.) There, on the banks of the Igara Paraná River, the travellers found themselves in a psychedelic paradise. There were cattle pastures dotted with Psilocybe cubensis—magic mushrooms—sprouting on dung piles; there were hammocks to lounge in while you tripped; there were Banisteriopsis caapi vines growing in the jungle. Taken together, the drugs produced hallucinations that the brothers called “vegetable television.” When they watched it, they felt they were receiving important information directly from the plants of the Amazon.

The McKennas were sure they were on to something revelatory, something that would change the course of human history. “I and my companions have been selected to understand and trigger the gestalt wave of understanding that will be the hyperspacial zeitgeist,” Dennis wrote in his journal. Their work was not always easy. During one session, the brothers experienced a flash of mutual telepathy, but then Dennis hurled his glasses and all his clothes into the jungle and, for several days, lost touch with “consensus reality.” It was a small price to pay. The “plant teachers” seemed to have given them “access to a vast database,” Dennis wrote, “the mystical library of all human and cosmic knowledge.”

If these sound like the joys and hazards of a bygone era, then you don’t know any ayahuasca users—yet. In the decades since the McKennas’ odyssey, the drug—or “medicine,” as many devotees insist that it be called—has become increasingly popular in the United States, to the point where it’s a “trendy thing right now,” as Marc Maron said recently to Susan Sarandon, on his “WTF” podcast, before they discussed what she’d learned from her latest ayahuasca experience. (“I kind of got, You should just keep your heart open all the time,” she said. “Because the whole point is to be open to the divine in every person in the world.”)

The self-help guru Tim Ferriss told me that the drug is everywhere in San Francisco, where he lives. “Ayahuasca is like having a cup of coffee here,” he said. “I have to avoid people at parties because I don’t want to listen to their latest three-hour saga of kaleidoscopic colors.”

Leanna Standish, a researcher at the University of Washington School of Medicine, estimated that “on any given night in Manhattan, there are a hundred ayahuasca ‘circles’ going on.” The main psychoactive substance in ayahuasca has been illegal since it was listed in the 1970 Controlled Substances Act, but Standish, who is the medical director of the Bastyr Integrative Oncology Research Center, recently applied for permission from the F.D.A. to do a Phase I clinical trial of the drug—which she believes could be used in treatments for cancer and Parkinson’s disease. “I am very interested in bringing this ancient medicine from the Amazon Basin into the light of science,” Standish said. She is convinced that “it’s going to change the face of Western medicine.” For now, though, she describes ayahuasca use as a “vast, unregulated global experiment.”

Most people who take ayahuasca in the United States do so in small “ceremonies,” led by an individual who may call himself a shaman, an ayahuasquero, a curandero, a vegetalista, or just a healer. This person may have come from generations of Shipibo or Quechua shamans in Peru, or he may just be someone with access to ayahuasca. (Under-qualified shamans are referred to as “yogahuascas.”) Ayahuasca was used for centuries by indigenous Amazonians, who believed that it enabled their holy men to treat physical and mental ailments and to receive messages from ancestors and gods. Jesse Jarnow, the author of “Heads: A Biography of Psychedelic America,” told me, “It’s a bit less of a to-do in many of its traditional uses—more about healing specific maladies and illnesses than about addressing spiritual crises.” Now, though, ayahuasca is used as a sacrament in syncretic churches like the Santo Daime and the União do Vegetal (“union of the plant”), both of which have developed a presence in the United States. The entire flock partakes, and the group trip is a kind of congregational service.

The first American to study ayahuasca was the Harvard biologist Richard Evans Schultes, who pioneered the field of ethnobotany (and co-authored “Plants of the Gods,” with Albert Hofmann, the Swiss scientist who discovered LSD). In 1976, a graduate student of Schultes’s brought a collection of the plants back from his field research to a greenhouse at the University of Hawaii—where Dennis McKenna happened to be pursuing a master’s degree. Thanks to McKenna, some B. caapi cuttings “escaped captivity,” he told me. “I took them over to the Big Island, where my brother and his wife had purchased some land. They planted it in the forest, and it happened to like the forest—a lot. So now it’s all over the place.”

Terence McKenna died in 2000, after becoming a psychedelic folk hero for popularizing magic mushrooms in books, lectures, and instructional cassette tapes. Dennis McKenna went on to get a doctorate in botany and is now a professor at the University of Minnesota. When we spoke, he was on a book tour in Hawaii. He had been hearing about ayahuasca use in a town on the Big Island called Puna, where people call themselves “punatics.” “Everybody is making ayahuasca, taking ayahuasca,” he said. “It’s like the Wild West.”

If cocaine expressed and amplified the speedy, greedy ethos of the nineteen-eighties, ayahuasca reflects our present moment—what we might call the Age of Kale. It is a time characterized by wellness cravings, when many Americans are eager for things like mindfulness, detoxification, and organic produce, and we are willing to suffer for our soulfulness.

Ayahuasca, like kale, is no joy ride. The majority of users vomit—or, as they prefer to say, “purge.” And that’s the easy part. “Ayahuasca takes you to the swampland of your soul,” my friend Tony, a photographer in his late fifties, told me. Then he said that he wanted to do it again.

“I came home reeking of vomit and sage and looking like I’d come from hell,” Vaughn Bergen, a twenty-seven-year-old who works at an art gallery in Chelsea, said of one ayahuasca trip. “Everyone was trying to talk me out of doing it again. My girlfriend at the time was, like, ‘Is this some kind of sick game?’ I was, like, ‘No. I’m growing.’ ” His next experience was blissful: “I got transported to a higher dimension, where I lived the whole ceremony as my higher self. Anything I thought came to be.” Bergen allows that, of the nine ceremonies he’s attended, eight have been “unpleasant experiences.” But he intends to continue using ayahuasca for the rest of his life. He believes that it will heal not only him but civilization at large.