GIGANTE, Costa Rica – There was a ghostlike quality to Rudy Gonsior, a U.S. former Special Forces sniper, on the morning he arrived at a jungle retreat to see if a vomit-inducing psychedelic brew could undo the damage years of combat had done to his mind.

Glassy-eyed and withdrawn, he barely spoke above a whisper and was much quieter than the six other veterans who had come to dredge up painful memories of comrades fallen in battle, thoughts of suicide and the scar that taking a life leaves on the psyche.

“I have traveled across continents to come to the jungle to do psychedelics,” marveled Gonsior, who had steered clear from drugs his whole life. “I guess this is what might be considered a Hail Mary.”

They had come to western Costa Rica to try ayahuasca, a substance people in the Amazon rainforest have imbibed for centuries. Some Indigenous communities regard the brew, which contains the hallucinogen DMT, as a powerful medicine that keeps them spiritually resilient and in harmony with the natural world.

The lodge the Americans visited was a far cry from that, with a swimming pool and a sprawling deck that anchors well-appointed cabanas featuring ocean views. Charging from $3,050 to $7,075 per person for weeklong retreats, the lodge is among the newest and priciest additions to a booming alternative healing sector.

Until relatively recently, only a few botanists, hippies and spiritual seekers gained access to the world of Amazon shamanism, which missionaries drove underground during colonization.

But now, thousands of people from around the world make pilgrimages each year to the more than 140 ayahuasca retreat centers in Latin American countries where the substance’s use in ceremonial settings is legal or, as in Costa Rica, not explicitly outlawed.

Besides psychedelic ceremonies, which are often physically and emotionally draining, retreat organizers offer group therapy sessions, yoga classes, art therapy, meditation circles and warm floral baths.

Collectively, these centers have become an unlicensed and unregulated mental health marketplace for people searching for an alternative to antidepressants and other widely prescribed pharmaceuticals.

The draw of psychedelics has surged amid a growing body of scientific research that builds on promising studies from the 1960s and 1970s. Much of that research was shut down after psychoactive substances were outlawed during the Vietnam War era.

But in the past few years, the Food and Drug Administration designated psilocybin, the psychedelic component in what are commonly called magic mushrooms, and MDMA, known as ecstasy, as “breakthrough therapies.” That rare designation allows scientists to fast-track larger studies that could pave the way to administering psychedelics as medicine.

Drinking ayahuasca can be dangerous, especially while taking antidepressants and hypertension drugs. It can also set off psychotic episodes for people with serious mental health conditions, like schizophrenia.

And while some retreats have strict rules and protocols that have been developed in consultation with medical professionals, the ayahuasca boom has sometimes been exploited by scammers and charlatans, and it has come under scrutiny for instances of sexual assault on vulnerable or impaired participants.

“You have to recognize that there’s a Wild West element” to ayahuasca retreats, said Dr. Matthew Johnson, a professor of psychiatry and behavioral sciences at Johns Hopkins University.

In a controlled setting, he said, unleashing the brain can help patients revisit repressed trauma and generate insights. So the medical establishment, once deeply skeptical of the therapeutic potential of psychedelics, is embracing “what is essentially a new area of medicine,” he added.

But he worried that psychedelic retreats in general may be ill-equipped to screen people for whom trips can be dangerous. “They can put people in a very vulnerable place,” Johnson said. “That is not to be underestimated.”

Still, the growing buzz around psychedelic-assisted healing has put places like the Soltara Healing Center, where the veterans went, at the forefront of a push to challenge conventional mental health care.

Co-founder Melissa Stangl said responsibly run ayahuasca centers could be the seeds of a transformation. “We are on the cusp of bringing psychoactive medicines into the mainstream health care system,” she said.

Before their first ayahuasca ceremony, the veterans met individually with two Peruvian “maestros,” or healers. “Their hearts are hardened,” said Teobaldo Ochavano, who helps run the nighttime ceremonies alongside his wife, Marina Sinti. “They seemed unable to experience love or joy.”

Like many service members of his generation, Gonsior said he enlisted in the Marine Corps to avenge the attacks of Sept. 11, 2001. In 2006, he said, he deployed to Iraq for the first of multiple combat tours. He and his men were constantly ambushed with roadside bombs and shot at by snipers, he said, and 17 service members he deployed with returned home in body bags.

In 2007, Gonsior said, he joined the Army Special Forces, where he served as a sniper. It left him feeling that he had joined a “cult of death,” he said.

He said he didn’t allow himself to process the guilt until years later, when he was gripped by depression and bouts of rage.

By the time he and the other veterans filed into the ceremony room, they had signed a lengthy hold-harmless agreement.

It was quiet when the maestros blew out the candles. But the silence was short-lived.

When ceremonies reach a crescendo, bouts of loud vomiting pierce the singing. There is sometimes audible weeping in one corner and ecstatic laughter from across the room.

As dawn approaches and the ayahuasca starts wearing off, participants emerge looking gaunt and dazed as the rational mind struggles to regain control.

“These experiences have a way of completely blasting people out of the mental ruts they’re stuck in and to look at a broader set of possibilities,” said Johnson at Johns Hopkins, one of several universities conducting clinical trials. But he and other experts who cite the psychiatric promise of psychedelics worry about their use without adequate controls.

“The room for error is not having adequate medical support,” said Collin Reiff, a psychiatrist at New York University.

Still, Jesse Gould, 33, a former Army Ranger who brought the veterans to Soltara, said the benefits of the retreat experience outweigh the risks. His first few ceremonies were brutal, he said, calling them “an all-out war” in which he felt like he was pushed “to the edge of sanity.”

But in the months that followed, he said, his depression mellowed, his crippling social anxiety melted away and his mood swings, which had felt like a “tug of war in my brain,” ceased. “It seemed to almost rewire my brain,” Gould said.