(CNN)Inside an unassuming house in a small, nondescript village north of Madrid, 18 people are gathered in a room lined with mattresses.
Ayahuasca therapy boom: Risky or life-changing?
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It looks like a giant sleepover.
Only the carefully positioned blue buckets give away the true purpose of the evening.
Tonight an ayahuasca session will take place and, with uncontrollable vomiting invariably part of the proceedings, the buckets are a necessary precaution.
Ayahuasca, also known as "yage," is a mind-altering concoction made from two Amazonian plants: the Banisteriopsis caapi vine and the leaves of the chacruna plant.
It's generally taken in the Amazon jungle as part of a ritualistic ceremony, guided by a shaman singing traditional songs called icaros.
In the past many Westerners have flocked to ayahuasca centers in the jungle as it's reported to have beneficial effects on conditions such as depression and anxiety, as well as aiding people to quit serious addictions.
However, outside of the Amazon ayahuasca has an ambiguous legal status.
Chacruna contains dimethyltryptamine, or DMT, a naturally occurring psychedelic compound.
Its use is restricted by many countries and can be illegal to possess (including in the United States), supply or prescribe.
However, since 2013, ayahuasca has had an open presence outside of the Amazon, largely through the work of Ayahuasca International, a company offering retreats across the globe where the substance is consumed.
Based in Spain and founded by Argentine Alberto Jose Varela, the business openly promotes itself across 50 pages on Facebook in 10 languages, claiming more than 500,000 fans and more than a million visits per month.
In 2009, Varela spent 14 months in a Spanish prison for charges related to ayahuasca consumption.
He was later absolved when, according to Varela, it was proven that ayahuasca is not in fact a drug, thus setting a legal precedent for the use of ayahuasca in Spain.
While in prison, the Argentine developed a plan to bring a hybrid of psychotherapy and ayahuasca to the masses, forming Ayahuasca International in 2013.
The organization currently has a 50-member staff operating in 10 countries and offers about 20 retreats worldwide each month, mostly in Europe.
It doesn't use shamans, but facilitators trained over six months at its own European Ayahuasca School.
Instead of traditional icaros, recorded music is played.
However, the company's Western approach and "aggressive marketing tactics" have provoked uproar by many in the ayahuasca community, who view the ancient shamanic traditions as integral to the Amazonian plant's consumption.
Last year 100 anthropologists and academics signed an open letter warning of health risks and expressing support for the indigenous Cofan tribe in Colombia who had themselves previously issued a formal complaint against Ayahuasca International and its practices.
Varela didn't respond to specific CNN questions about the complaint, but said his group wasn't trying to replicate existing ceremonies.
"We are a team of people sharing the goal of inner evolution, without attachment to any particular techniques, traditions or belief system," he said.
"We put our trust in the total freeing of consciousness so that people can discover their own inner master."
Seemingly undeterred by the controversy, Ayahuasca International continues to expand.
The retreat in the village north of Madrid is aimed at English-speakers, with most making the journey from the UK, where consumption of ayahuasca remains illegal.
Participants come from many walks of life: among them a psychotherapist, a filmmaker, a city worker, an IT engineer; all with their own reasons for trying the Amazonian brew.
"I'm a psychotherapist," says Angela, 41, from Liverpool. "I've always been interested in professional development and keeping myself clean for clients."
She's already taken ayahuasca 12 times, mostly in the Peruvian jungle.
"Through it I've become a lot more relaxed in myself and more present," she says.
Many of the participants are here on a spiritual quest to "expand their consciousness."
But a few have come with more immediate, health-related goals.
Dean, 32, who works in property in Manchester, England, says he's been addicted to ketamine since his mid-20s, resulting in health problems.
"I was scared before I came here," he says. "I've never done it before, although I shouldn't be because I've taken plenty of drugs in the past and this is a controlled environment.
"But you see a lot of people coming face to face with their demons, and maybe because of that, I was afraid of what I was going to see."
On the opposite side of the room is Fabienne, 51, a French citizen now living in England who hopes ayahuasca will ease the multiple sclerosis she was diagnosed with 12 years ago.
"I believe that somehow, unconsciously, I created my symptoms and by unraveling my experiences and going into a deeper understanding it might free my physical body," she says.
Fabienne describes her experience so far at the retreat as "surprising, shocking, sacred, beautiful, difficult and ongoing ... It [ayahuasca] gives me hope as a human being."
Tonight's session is being facilitated by Ramon, a Mexican economics graduate who quit his well-paid bank job after taking ayahuasca for the first time just over a year ago.
"I felt empty and I went on an Ayahuasca International retreat as a participant and afterward I said I wanted to become part of the team," he says.
After going to the jungle with Ayahuasca International founder Alberto Varela, Ramon resigned from his job, moved to Spain and took part in a six-month course to become an ayahuasca facilitator.
Tonight is the first time he'll lead a session unassisted by others.
Unaware of his lack of experience, the participants put their hopes and fears in Ramon's hands.
The session starts with the ingestion of a powdered tobacco blown into the nasal cavities with the purpose of bringing calm to the mind.
After some initial coughing and spluttering, the intense sensations begin to ease and the moment approaches for taking the first dose of ayahuasca.
One by one the participants nervously approach Ramon who fills a small shot glass with the bitter, treacly substance, which most down in one large gulp.
With the ayahuasca administered and everyone resuming their position on their mattresses, it's a waiting game until the plant mixture begins to take effect.
For many, the first sign that something is happening is the visual appearance of brightly colored geometric patterns (a hallucination), often accompanied by feelings of nausea.
Some relive traumatic moments from their childhoods, finding peace, acceptance and forgiveness, while others receive insights on the nature of reality.
Throughout the five or six hours the ayahuasca session lasts, there's a cacophony of noises ranging from retching and vomiting, through to sobbing and hysterical laughter.
And then as suddenly as it began, Ramon announces that the session has ended, leaving many of the participants to sleep off their experience on their mattresses while others head back to their rooms.
It's clear that Ayahuasca International's growing presence worldwide represents a heightened interest in trying the Amazonian brew, despite its ambiguous legal status.
Fabienne offers her own conclusions.
"More and more people are searching for greater meaning in their lives and I think people are bored of consumerism and want experiences that enhance their growth," she says. "That's why it's becoming more popular and needed."
Although only partway through the retreat, most of the participants have found the experience worthwhile.
"I'd definitely recommend it," says Fabienne. "But you have to take responsibility to realize that it's serious. It's not the '80s or '90s trip; it's to be used much more as a growth experience.
"I've gone from absolute dread of ayahuasca to starting to having a relationship with it and trusting myself in that experience and that whatever arises is OK."
Some names have been changed at the request of the interviewees.