In a carpeted office suite, Alex Beck settled onto a mattress and, under the watch of a trained guide, began chomping through a handful of “Pumpkin Hillbilly” mushrooms.

A Marine Corps veteran who was sexually assaulted during his time in the armed forces, Mr. Beck had long been searching unsuccessfully for a way to put those nightmarish years behind him. Now he was ready for a different kind of journey, a psychedelic trip through the nether regions of his own mind.

As he felt his thoughts starting to spin, his “facilitator,” Josh Goldstein, urged him to surrender and let the mushrooms guide him.

“It’s like the idea of planting a seed and then letting it go,” he said.

Stigmatized in law and medicine for the past half-century, psychedelics are in the midst of a sudden revival, with a growing body of research suggesting that the mind-altering compounds could upend psychiatric care. Governments in several places have cautiously started to open access, and as Oregon voters approved a broad drug decriminalization plan in 2020, they also backed an initiative to allow the use of mushrooms as therapy.

This summer, the state debuted a first-of-its-kind legal market for psilocybin mushrooms, more widely known as magic mushrooms. Far from the days of illicit consumption in basements and vans, the program allows people to embark on a therapeutic trip, purchasing mushrooms produced by a state-approved grower and consuming them in a licensed facility under the guidance of a certified facilitator.

Mr. Beck, 30, was one of the first clients at a facility in the central Oregon city of Bend that began conducting sessions this summer in a building that on other days of the week offers chiropractic services.

In his youth, Mr. Beck had experimented with psychedelics for recreation. But as he struggled with his lingering post-traumatic stress in adulthood, he learned about what seemed to be promising new research into plant-based psychedelics for mental health issues that did not respond to other treatments. He wondered if they could help him clear his head from the horrors of the past.

“I’m trying to reset my brain to where I can look at life in a new way,” he said.

Plants and fungi with psychoactive properties have been used for thousands of years. More modern uses in the United States grew in the 1950s with promising research on LSD and psilocybin, and the substances soon became a signature of the counterculture movement, so much so that political leaders moved to criminalize their use and halt research into their effects.

In altering the normal activity of the brain, psilocybin has the power to distort perceptions, transform senses and bend emotions. Researchers see the possibility of bestowing the brain with new elasticity, allowing people a chance to escape mental ruts. Studies have suggested that breakthroughs may be possible for people with challenging mental health conditions, including PTSD, substance addiction and treatment-resistant depression, without the habit-forming properties of some other drugs.

For those who have long worked on psychedelics research, the sudden expansion in access in Oregon and Colorado, along with cities like Denver, Detroit, Minneapolis and Washington, D.C., have prompted a mix of elation and trepidation. Oregon has settled on a middle-of-the-road approach, requiring neither a doctor’s supervision nor a specific medical diagnosis, but providing for strict oversight of supply and use.

Dr. Janis Phelps, director of the Center for Psychedelic Therapies and Research at the California Institute of Integral Studies, said she and other researchers had been wary of the decriminalization movement. Many in the field had worked for years to remain strictly scientific, hoping to avoid government crackdowns, and to give the U.S. Food and Drug Administration time to fully review the effects of psilocybin before pressing ahead with efforts to make it legal.

“I have changed my mind,” she said. While she remains concerned that bad actors could try to enter the industry strictly for profit, or try to take advantage of vulnerable people, she has come to believe that the open door in Oregon could advance the use of psychedelics in ways that methodical approaches cannot.

Dr. Charles Nemeroff, the chair of the department of psychiatry and behavioral sciences at the University of Texas at Austin, said he continues to be wary. Psilocybin is powerful, with immediate effects lasting for hours, and uncertain outcomes for patients, he said, recalling one patient of his who has experienced protracted psychosis, losing partial connection to reality, after taking doses of mushrooms. The treatments ruined her life, said Dr. Nemeroff, who said he worried about the lack of required medical oversight in Oregon’s program.

“I’m really uncomfortable with this,” he said, adding that it could erase progress the field has made. “We’ll end up back in the Nixonian era in which psychedelics could not even be studied.”

While some form of legalized marijuana is authorized in all but 12 states, creating a huge, multi-billion-dollar industry, the psilocybin market remains small, with an uncertain financial outlook for those entering it. Only five businesses are approved to manufacture the therapeutic-use fungi in Oregon, with 13 sites approved to host dosing sessions.

Bend is home to two of them. One offers a treatment experience that costs as much as $15,000, including several days spent getting to know the facilitator and the townhome-like space where the treatment takes place. Mr. Beck, who lives in Bend, connected with another organization known as Bendable, a nonprofit that helps coordinate treatment and asks clients to pay what they can afford.

A single session costs about $3,000, which includes a preparatory meeting, a guided session with the mushrooms that lasts several hours, and a follow-up appointment a few days later in which the client discusses lessons from the session and how to integrate them into their other therapy.

Amanda Gow, the executive director of Bendable, said she opens her email each day to messages from all over the country: a woman in Kentucky desperate for help with her husband’s PTSD, a father in western Oregon willing to try anything to help his adult son’s depression, a single mother in Bend struggling with childhood trauma.

Many described years of therapy, medical visits and antidepressants but little progress. The wait list includes hundreds of people.

Officials in other states are watching what happens in Oregon. Voters in Colorado approved a measure last year to decriminalize psilocybin and to set the state on the path to a legal therapeutic market. In other states, including Texas, lawmakers have authorized studies of psilocybin for treating ailments such as PTSD. The F.D.A. has granted the drug “breakthrough therapy” status, which allows for expedited review of substances that have demonstrated substantial promise.

But there is uncertainty about the best path forward. California lawmakers approved a bill this year to decriminalize several hallucinogens, but Gov. Gavin Newsom vetoed the measure, saying the state needs to first set up regulated treatment guidelines. The American Psychiatric Association has urged caution, saying treatments should be limited to research studies for now.

Mr. Goldstein, who works with Bendable and guides sessions for clients, had his first psychedelic experience more than three decades ago and spent recent years facilitating underground mushroom sessions. He does not have a medical degree but previously worked as an academic director at a therapeutic boarding school.

Not all of the sessions he has supervised have been pleasant. One client, Mr. Goldstein said, recently had her first psychedelic session and hated it, asking for it to end soon after it began. It lasted six hours. He said such experiences highlight the importance of guided sessions, with someone able to help people navigate the experience. Even a difficult session, he said, can help clients gain insight into why they were struggling so much with what was on their mind.

“Those can be better than the people who just see rainbows and unicorns,” he said.

For his treatment session, Mr. Beck arrived in the morning to the office suite. Mr. Beck lit a candle and Mr. Goldstein put on a playlist of music that traces an arc of a psilocybin experience, beginning with calming tracks with titles such as “Flute Traveller” and “Unlocking the Doors of Eternity.”

Once the mushrooms took hold, as Mr. Beck described it later, he felt himself beginning to thrash, but Mr. Goldstein said he had remained largely calm. Mr. Beck recalled visions of colorful strands of ribbon floating through his mind, wrapping themselves around different issues that he had prepared himself to tackle — the sexual assault, the PTSD, various difficult relationships.

As the mushrooms began to wear off after several hours, he was tearful. He told Mr. Goldstein about realizing how important family was to him. For the first time, he said, he decided he wanted to have children of his own some day.

The following day, Mr. Beck and Mr. Goldstein met at a park to discuss the experience and how to integrate it into Mr. Beck’s more traditional therapy.

“I had been holding on to so many traumas and issues,” Mr. Beck said. “It was like a massive weight had been released.”

Still, there is more to do. Mr. Beck continues in his traditional therapy and plans to increase the frequency of those sessions, which he has found productive with more clarity about what needs to be discussed. The treatments have complemented each other, he said.

“I wouldn’t say it’s a ‘one and done, I’m completely cured,’” he said. “It takes work.”