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Seeking to change that, McKenna embarked on his first trip to South America at age 20. A decade later, he returned, this time to research his dissertation. After months in the jungle, he brought plant samples back to his lab, where he showed for the first time how ayahuasca works.
To make the brew, shamans boil together two Amazonian plants for many hours, sometimes days. As they simmer, the DMT contained in one of the plants mixes with the other one, the Banisteriopsis vine, and its key ingredient: monoamine oxidase inhibitors, or MAOIs. Normally when people ingest DMT — a not-uncommon compound in nature — the monoamine oxidase in our gut knocks it out. But the Banisteriopsis allows the hallucinogen to reach the brain.
By the middle of the 20th Century, several Brazilian churches splintered off from the shamans and took ayahuasca into a formal setting. In 1991, one of these — a group called the Uniao do Vegetal, or UDV — invited McKenna to one of its twice-monthly ceremonies during which the tea is administered as a sacrament. (A New Mexico-based branch of the church won a 2006 U.S. Supreme Court case allowing it to use ayahuasca in its ceremonies. The church has about 300 members in six U.S. states, including a group in Florida.)
In a room with 500 other people, McKenna drank first one cup, then a second, and was plunged into one of the most vivid ayahuasca visions of his life: a molecule's-eye view of photosynthesis or, as he explains it, "the force on which life depends."
When McKenna returned to his body, he writes in his new book, The Brotherhood of the Screaming Abyss, "I knew that I had been given an inestimable gift."
McKenna began devising a study to look at the biomedical effects of ayahuasca, and within two years, he was back in Brazil. On this trip, he brought along a team that included Dr. Charles Grob, a psychiatrist who heads the Division of Child and Adolescent Psychiatry at UCLA's medical school.
"Nowadays, the word is out," Grob says. "But when we did this, I'd say, 'We're doing an ayahuasca study,' and people would say, 'Aya-what-sca?' "
For about a month in the summer of 1993, the team of the Hoasca Project ran tests on 15 randomly selected members of the church, all of them men who had been using ayahuasca regularly for at least ten years. The scientists ran the same tests on similarly aged men who had never been exposed to ayahuasca.
The researchers measured every biological metric they could think of — blood pressure, heart rate, pupil dilation, body temperature — and used structured psychiatric interviews to get where their instruments couldn't: inside the participants' minds.
Many of the men had struggled with alcoholism and depression prior to joining the church, Grob learned. They credited ayahuasca with transforming their outlook. "In some cases," Grob says, "they felt like it had saved their lives."
When the researchers left Brazil and started processing their data, the bloodwork came back with one of the project's most startling discoveries: The long-term ayahuasca users showed higher levels of the transporters of serotonin, the brain chemical that regulates mood.
"That's the target that antidepressants work on, and here it was significantly elevated in the drinkers [of ayahuasca]," McKenna says.
Deficits in serotonin transporters are also connected to problems like alcoholism and depression — the same issues the 15 subjects said the ayahuasca had helped cure.
"Here we have a medicine that apparently reverses these deficits, something no other medicine is known to do," explains McKenna. "And there's also a correlation to behavioral change. You can't say it caused it, but there's definitely a correlation."
Today, 20 years after the study, McKenna is preparing to revisit the findings. Within a year, he aims to raise enough money to fund a new study, this time in Peru, to look at the effects of ayahuasca on people with posttraumatic stress disorder.
He hopes that additional research will help him establish his ultimate goal: a destination medical clinic in Peru.
"If we can bring together the best of shamanism and the best of psychotherapy, I think we can offer a new paradigm for healing," says McKenna. "What we're really trying to do here is revolutionize psychiatry."
Lisa Yeo doesn't look like a junkie. The 47-year-old has shimmering blond hair and clear skin and wears a stylish tangerine shirt. It's Halloween, and her two dogs — a shih tzu and a dachshund — yap incessantly as trick-or-treaters come to the door of her house in Toronto.
Just eight years ago, she weighed 80 pounds and was missing her two front teeth.
Yeo's father gave her her first alcoholic drink at age 6, and she was drinking alone by age 11. As a teen, she developed a cocaine addiction, and in her early 20s, she set out on a path that would take her to heroin, crack, and prostitution.
On August 11, 2005, as cops walked her out of a hotel where they had found her shooting up, Yeo realized she was finally ready to change.
She went to rehab for a year, then a recovery house for another two years. But she still wasn't totally sober: For 18 years, she'd been receiving a court-ordered dose of the opiate substitute methadone. Now, she wanted off all drugs, once and for all.