Dr. Radowitz said that he sees “no difference whatsoever” between esketamine and ketamine, contrary to the F.D.A.’s assessments. Even so, he doesn’t think two hours is “necessary.” He acknowledges that Nushama’s practices differ from F.D.A. protocols for administering esketamine, but said he is not worried about potential risks or legal liability. “It doesn’t concern me,” he said. “I have no problem using this medication.”

What’s Next?

For some patients, the promise of ketamine’s benefits overshadows its risks, legal status and cost. Maria Kennedy, 30, who works in public relations, had the first of her six “journeys” at Nushama in October 2021. She had previously tried talk therapy and selective serotonin reuptake inhibitors for anxiety and depression, she said, but during the pandemic felt herself spiraling, isolated and anxious in a studio apartment. Her therapist, who knew Dr. Radowitz, referred her to Nushama.

Ms. Kennedy said that during a few treatments she felt as though she were floating through space, nestled beneath the snug eye mask and hovering beyond her body. In others, the ketamine triggered precise, specific visions — once she saw her mother wrapping presents before a birthday party.

By the time the I.V. was removed, Ms. Kennedy said she would feel mostly back to normal. She would stay at Nushama, taking her time to peel herself from the “cozy” chair. “The only thing that I can compare it to is waking up after a really awesome sleep,” she said. Afterward, she would take her dog to a cafe and read with a coffee or a beer.

Across the country, ketamine clinics have seen increased interest. Since SoundMind opened in August 2021, they have had over 100 people a month sign up on average. Boise Ketamine Clinic in Idaho is booked until the end of April for ketamine-assisted psychotherapy treatments. In San Diego, a clinic called South Coast TMS and Ketamine had a 40-person wait-list for months, until the center raised its prices to $1,500 per session, a representative said.

Dustin Robinson, a founder of the venture capital fund Iter Investments, which concentrates on the psychedelics space, estimated that a typical ketamine clinic with, say, five rooms makes $75,000 to $100,000 per month, and potentially double that if it’s fully booked. Profit margins, he added, can be more than 30 percent, which according to industry reports is far higher than most health care services. “There is not a huge amount of staff and the medicine is very cheap — almost negligible — the staff is the main cost,” he said.

Mr. Robinson knows Mr. Godfrey, but is not an investor in Nushama, which charges $4,000 for six sessions; insurance rarely covers ketamine for mental health, but might if there is also a pain diagnosis. Nushama does not provide single sessions. “It’s hard to get in shape going to the gym once,” said Mr. Meloff.