A psychedelic brew used in spiritual ceremonies since ancient times is now under the microscope as a possible answer for people with severe, treatment-resistant depression.

Ayahuasca, a blend of vines and leaves that grow in the Amazon, has been used by Indigenous groups in religious ceremonies for centuries. More recently, ayahuasca ceremonies in Peru, Colombia, Brazil and other countries have become tourist draws.

Recent research has shown that ayahuasca may help lift severe depression quickly when administered in a controlled environment.

A study published in Psychological Medicine found "significant antidepressant effects of ayahuasca" in 29 patients with treatment-resistant depression. Brazilian researchers made the conclusion following a randomized trial in which patients received either ayahuasca or a placebo in a hospital setting.

"Most of our patients report they were feeling much more relieved and tranquil as a whole," said Draulio Araujo, one of the authors of the study and a professor at the Brain Institute of the Federal University of Rio Grande do Norte.

"We need to do a lot more research -- are the effects real, and where they effects come from, and the optimal way of using this," he told CTV News.

Brian Rush, a scientist emeritus with the Institute for Mental Health Policy Research at the Centre for Addiction and Mental Health (CAMH) in Toronto, is so intrigued by ayahuasca that he's part of research efforts currently underway in Peru to assess the brew's therapeutic potential.

"There's something going on that brings benefits for mental health, depression, anxiety, trauma, and in particular, addiction," he said. "Top neuroscientists around the world are interested in this now."

In traditional settings, ayahuasca is administered by a shaman. The drink can trigger vivid hallucinations, along with nausea and vomiting.

There are rare reports of adverse reactions and five deaths among tourists linked to possible ayahuasca use.

In its online travel advisories, the Canadian government warns that ayahuasca ceremonies performed in Peru and elsewhere "involve taking substances that can cause medical complications and can severely impair cognitive and physical abilities."

"Ayahuasca ceremonies are not regulated and there is no way to assess the safety of any of the services, the operators or the shamans," one such advisory says.

But advocates say ayahuasca can also bring profound changes in awareness and spiritual enlightenment.

For Allan Finney, who suffered from anxiety, depression and alcohol addiction after growing up in an alcoholic household, ayahuasca is "the reset button or the reboot of your thought patterns."

"It changes your life. It is a game changer," he told CTV News. "It helps you to deal with negative thought patterns, negative behavioural characteristics that continue to plague you throughout your life."

Finney, who lives in Charlottetown, P.E.I., tried ayahuasca for the first time in Peru and "immediately saw positive changes." He said it made him less angry and more relaxed.

"My life is so much better now…I can't even begin to tell you."

However, ayahuasca is illegal in Canada because it contains prohibited hallucinogens dimethyltryptamine (DMT) and harmaline. Health Canada granted exemptions to only two religious communities in the Montreal area, which are now allowed to import ayahuasca for ceremonial purposes.

Scientists admit they don't know exactly how ayahuasca affects the brains of people who suffer from depression and addictions, but are feverishly trying to figure it out. Brain imaging has shown that the brew changes blood flow and electrical activity in test subjects. Some patients have described it as a sort of "car wash for the brain."

"A lot of patients we had in our trial would leave saying they felt as if they were in psychotherapy for a year…while they were under the influence of ayahuasca for a couple of hours, so they felt that experience as a profound psychotherapy session," Araujo said.

Rush, the Canadian scientist, is part of a group tracking people severely addicted to drugs or alcohol who have also failed all other treatments. His project has been looking at the effectiveness of ayahuasca in the treatment of addictions in Latin America.

The study subjects must participate in a nine-month program in Peru that involves counselling and therapy. The ayahuasca is administered two to three months into the program, along with guidance to interpret the insights that often come with the experience.

"We have some people in the study who are past one year to 18 months follow-up. The stories are consistent -- they are doing very well," said Rush.

He and his colleagues had to turn to crowdfunding to get the study going in South America.

"We are still going without any big grants, people are volunteering to do transcripts," he said. "It tells you people know something is going on and they want to bring this out."

Rush is hoping to help convince Canadian regulators to allow ayahuasca to be tested in Canada.

"The people interested in this are respectable, qualified, working out of top institutions. Maybe we should give it a chance," said Rush, who hopes to plan a scientific meeting on psychedelic medicines in the fall.

The World Health Organization estimates that more than 300 million people around the globe suffer from depression and about one third of them do not respond to standard anti-depressants.