After more than three decades of planning and a $250 million investment, Lykos Therapeutics’ application for the first psychedelic drug to reach federal regulators was expected to be a shoo-in.

Lykos, the corporate arm of a nonprofit dedicated to winning mainstream acceptance of psychedelics, had submitted data to the Food and Drug Administration showing that its groundbreaking treatment for post-traumatic stress disorder — MDMA plus talk therapy — was significantly more effective than existing treatments.

At a pivotal public hearing last summer, two dozen scientists, doctors and trauma survivors told an F.D.A. advisory panel how MDMA-assisted therapy had brought marked relief from a mental health condition associated with high rates of suicide, especially among veterans.

Then came skeptics with disturbing accusations: that Lykos was “a therapy cult,” that practitioners in its clinical trials had engaged in widespread abuse of participants and that the company had concealed a litany of adverse events.

“The most significant harms in Lykos’s clinical trials were not caused by MDMA, but by the people who were entrusted to supervise its administration,” Neşe Devenot, one of the speakers opposed to Lykos’s treatment and a writing instructor at Johns Hopkins University, told the committee.

Dr. Devenot and six others presented themselves as experts in the field of psychedelics, but none had expertise in medicine or therapy. Nor had the speakers disclosed their connection to Psymposia, a leftist advocacy group whose members oppose the commercialization of psychedelics and had been campaigning against Lykos and its nonprofit parent, the Multidisciplinary Association for Psychedelic Studies, or MAPS.

The critics did not provide evidence to back their claims of systematic wrongdoing, but when the votes were counted that day, the panel overwhelmingly rejected Lykos’s application. Before voting, panelists cited a number of concerns, among them MDMA’s potential effects on the heart and liver, and whether trial results were influenced by the fact that most study participants correctly guessed they had received the drug and not a placebo.

Seven of the 11 panelists mentioned the allegations that Psymposia had raised.

One of them, Kim Witczak, a drug safety advocate, said in an interview that the allegations of misconduct had dampened her initial excitement about MDMA.

“There were too many things that were red flags for me,” she said.

Two months later, the F.D.A. rejected the application. It did not mention the allegations of misconduct or abuse.

In a confidential letter to Lykos, the agency said its decision was based on uncertainty about how long the treatment would be effective; concerns about positive bias, including previous use of MDMA by some participants; and Lykos’s failure to collect data on feelings of euphoria, which is considered an adverse event because it can signal a potential for abuse. The letter was described by people who had read it.

An F.D.A. spokesperson declined to comment, saying the agency does not discuss pending applications.

Dr. Javier Muñiz, the former associate director of therapeutic review at the F.D.A.’s division of psychiatry who helped Lykos design its trials, said the treatment’s talk therapy component was a challenge for the agency because it does not regulate psychotherapy.

He also cited another factor: the cultural stigma of an illegal drug commonly associated with cuddle puddles and all-night raves.

“If MDMA was a previously unknown molecule, maybe the burden of proof would be lower, but because these drugs have baggage, the science has to be above reproach,” said Dr. Muñiz, who was not involved in the final review.

The significance of Psymposia’s role in torpedoing Lykos’s bid is unclear. But Dr. Muñiz and other experts said the group’s incendiary allegations made approval that much harder.

The rejection came as a shock to many in the field. It punctured the air of inevitability about the future of psychedelic medicine and led to a management shake-up and mass layoffs at Lykos and other psychedelic companies.

Some have directed their anger at Lykos and MAPS — for fostering unbridled optimism about federal approval and for failing to submit an airtight application to the F.D.A.

But in recent months, the story of how a small band of anticapitalist activists helped sink the first psychedelic compound to come before the F.D.A. has captivated scientists, therapists and investors in the field.

It has also generated fear.

Buoyed by the F.D.A.’s rejection, Psymposia and its allies have expanded their attacks, including against veterans groups that defended Lykos’s application and psychedelic researchers at Johns Hopkins University.

Lykos’s application for MDMA-assisted therapy is not dead. The company met in mid-January with F.D.A. officials to discuss a path forward. Executives said that would most likely include an independent review of its data and another clinical trial that could add years and millions of dollars to the process.

Some advocates hope that the Trump administration will take a friendlier approach. They note that Elon Musk, a presidential adviser, and Robert F. Kennedy Jr., the nominee for health secretary, are vocal supporters of psychedelic medicine.

Jonathan Lubecky, a retired U.S. Army sergeant and a psychedelic medicine policy advocate, said he believed MDMA would eventually be approved. But he worried about the capacity of Psymposia and its allies to damage a field still in its infancy.

He also worries about people with PTSD who have fallen into despair since the F.D.A.’s rejection.

“I see the consequences in my friends,” he said. “Some, quite frankly, are trying to decide whether they should stick around long enough to see it happen.”

Psychedelic Rifts

Dr. Devenot has not been shy about claiming credit for derailing the approval of MDMA-assisted therapy.

“Yesterday, beyond my wildest expectations, we made international news in a David and Goliath-scale, ‘dark horse’ victory,’” Dr. Devenot wrote on X last June.

Founded in 2014 as a nonprofit media organization offering “leftist perspectives on drugs, politics and culture,” according to its website, Psymposia has been widely credited for bringing attention to sexual abuse, especially in underground settings, within the nascent field of psychedelic medicine.

The group has no paid staff and operates as an informal collective of psychedelic industry watchdogs united by their “desire to disrupt the status quo,” Brett Greene, a former member of the organization and one of its founders, said on a podcast in 2016.

In an interview, Dr. Devenot, the group’s most high-profile member, said Psymposia was largely focused on “making things safer” for those who use psychedelics and highlighting abuses that others in the field were unwilling to address.

Dr. Devenot, a self-described expert in psychedelic bioethics who uses gender neutral pronouns, often refers to their experience as a sexual assault survivor whose healing was aided by psychedelics. After being “bullied out of the mainstream” psychedelic movement, Dr. Devenot said they connected with other “very marginalized” individuals at Psymposia.

Dr. Devenot’s writings paint a dark portrait of the field. In a recent article, Dr. Devenot argued that “global financial and tech elites are instrumentalizing psychedelics as one tool in a broader world-building project that justifies increasing material inequality.”

For many Psymposia contributors, Lykos is Public Enemy No. 1, in part because of the company’s origins as a for-profit arm of MAPS, an organization whose founder, Rick Doblin, has long promoted psychedelics as a tool for healing humanity.

For Psymposia, MAPS’s decision in 2014 to create a corporate entity betrayed those values. Dr. Doblin has said the organization could no longer rely on philanthropy to fund MDMA’s regulatory review and a post-approval marketing process that can cost hundreds of millions of dollars.

Despite Psymposia’s modest resources, its members have become feared for their ability to use social media to damage reputations and careers, according to more than four dozen academic researchers, clinicians, industry executives, mental health advocates and former Psymposia members who were interviewed for this article.

Many asked not to be named for fear of retaliation.

“Even the name Psymposia causes a pang of anxiety,” said Robin Carhart-Harris, a leading psychedelics researcher at the University of California, San Francisco. “Doing this interview, I’m worried: Am I kicking the hornet’s nest?”

Another Psymposia activist, David Nickles, describes himself as an underground researcher and an anarchist. Mr. Nickles, whose legal name is David Maliken, according to court documents, has written critically about veterans and the police.

In an interview, Mr. Nickles declined to discuss the use of a different name.

Ido Hartogsohn, a historian and sociologist of psychedelic science at Bar-Ilan University in Israel, served as a peer reviewer for a paper written by members of Psymposia. He said that the group early on played an important role highlighting abuses in the field but that he had become disenchanted by its tactics.

“Psymposia makes some valid points,” he said. “But their work is glaringly political, and biased, and it relies too much on shock effect, bad-faith readings of others and questionable assumptions and assertions.”

In a 2018 Facebook post that has since been deleted, Mr. Nickles outlined strategies for damaging psychedelic companies and nonprofits through persistent, critical media coverage and sabotaging “business operations in ways designed to raise the costs of operating,” according to a screenshot of the post.

The group has become known for its take-no-prisoners approach.

In 2019, Psymposia activists criticized Beatriz Labate, executive director of the Chacruna Institute for Psychedelic Plant Medicines, an educational nonprofit, after her organization published a series of interviews about sexual transgressions in the psychedelics community and included a man seeking forgiveness for past violations.

Psymposia accused Dr. Labate of giving a platform to an “abuser,” she said, adding that Mr. Nickles published private emails between them in what she said was an effort to paint her in a bad light.

The fallout was immediate, she said, with speakers and sponsors pulling out of a conference she had been organizing, and disinviting her from other events.

“I really felt my whole career was finished,” Dr. Labate said.

Oriana Mayorga, Psymposia’s former director of community engagement, said she also experienced the group’s wrath not long after leaving the organization.

Ms. Mayorga, who is of Latin American and Caribbean descent, said Psymposia’s leaders sought retribution after she criticized on social media a post by Mr. Nickles that accused MAPS of perpetuating “white supremacy, capitalism and imperialism.”

Days later, Mr. Nickles, Dr. Devenot and Lily Kay Ross, who is married to Mr. Nickles, sent a 28-page letter to administrators at the university where Ms. Mayorga was enrolled, accusing her of “discrimination, bullying and intimidation.” The 2020 complaint included transcripts of Ms. Mayorga’s public talks, screenshots from her social media accounts, and text and email messages between Ms. Mayorga and her former colleagues.

In an interview, Dr. Ross said that they had contacted Ms. Mayorga’s university to provide her an opportunity “for education and growth.”

The letter did not result in disciplinary action, but Ms. Mayorga said the experience was devastating. She largely withdrew from the field and no longer has an online presence.

“They’ve hurt people like me 10 times more than the good work they believe they’ve done,” she said.

Psymposia’s reputation was elevated in 2021, when a podcast it produced with New York magazine on abuses in the world of underground psychedelic therapy became popular on Spotify.

The podcast highlighted an ethical violation that occurred in an early Lykos trial that was not part of the company’s F.D.A. application, when a husband-wife therapy team in Canada spooned and cuddled a participant, Meaghan Buisson, during her MDMA session.

After the trial concluded, the male therapist, Richard Yensen, began a sexual relationship with Ms. Buisson. In 2018, Ms. Buisson filed a civil claim in British Columbia saying that Mr. Yensen had sexually assaulted her. The case was settled out of court.

After learning of the violation, MAPS notified health authorities in the United States and Canada and barred the two therapists from its programs. The organization publicly addressed the incident in 2019 in a statement.

The podcast did not provide evidence of systemic problems in Lykos’s trials, but it helped fuel rumors of rampant misconduct. Psymposia’s approach had another impact, too: It cleaved the small, close-knit psychedelics community.

“If you don’t agree with their view on a particular issue or say anything that deviates from the narrative they’re pushing, you’re automatically labeled as supporting sexual assault or being ethically questionable,” said Manesh Girn, a neuroscientist at the University of California, San Francisco.

Dr. Ross said the problem was not Psymposia’s approach, but the psychedelic community’s reluctance to engage with the issues that Psymposia was highlighting.

‘Very Disturbing’ Allegations

As the F.D.A.’s advisory panel meeting approached, Psymposia ramped up efforts to thwart Lykos’s application.

It found an audience at the Institute for Clinical and Economic Review, or ICER, an independent nonprofit that evaluates the clinical and cost effectiveness of new medical interventions.

The opening pages of the institute’s report on Lykos’s application detailed many of the ethical concerns raised by Psymposia.

Days before the committee meeting, Dr. David Rind, ICER’s chief medical officer, emailed several members a link to five public testimonies, four provided by Psymposia affiliates. He described the allegations as “very disturbing.”

In an interview, Dr. Rind said that the institute had not conducted its own investigation but was hoping that the F.D.A. would follow up.

Around the same time, Dr. Devenot submitted a petition to the F.D.A. urging it to extend the public session to accommodate speakers who they said would detail data fraud, systematic misreporting of adverse events and of enabling “entrapment, sexual abuse and coercive control” by Lykos.

“If the F.D.A. again prioritizes industry interests over public health,” the petition said, “the outcome could mirror the trajectory of OxyContin, which was also once promoted as a wonder drug offering relief from chronic suffering.”

The F.D.A. agreed to extend the hearing.

Of the 32 speakers, 10 opposed Lykos’s application. Seven of those 10 were affiliated with Psymposia, though none mentioned their connection to the group.

During the daylong meeting, panelists repeatedly raised questions about Psymposia’s misconduct claims.

One advisory member voted in favor of Lykos’s application — the sole panelist with expertise in psychedelic medicine.

Even though Psymposia did not provide evidence to back up its allegations of widespread wrongdoing, Amy Emerson, the former chief executive of Lykos, said the speakers succeeded in shaping the narrative.

“They were able to prey on the fears of people in government who care about reputational risk,” she said. Ms. Emerson resigned shortly after the F.D.A. denied approval.

In their public testimony, Dr. Devenot repeated an explosive accusation they had shared with ICER: One of the therapists who took part in Lykos’s clinical trials, Veronika Gold, had admitted to pinning down a screaming patient.

But the incident, detailed in a book chapter Ms. Gold wrote, involved ketamine, not MDMA. And rather than being “pinned down,” Ms. Gold said the patient was consensually pushing against her hands, which were passively raised.

Dr. Devenot also testified that Ms. Gold had used a similar practice with a clinical trial participant. Ms. Gold said the incident did not happen, a claim backed up by Lykos, which said it reviewed videos of her therapy sessions.

The accusations, repeated in the media, were damaging, she said. “People have expressed concerns about my ethics and practice,” Ms. Gold said.

Concerns about the organization’s ability to disrupt the field have mounted in recent months after a public relations firm began amplifying Psymposia’s and Dr. Devenot’s allegations of malpractice against Lykos. Dr. Devenot declined to say who was funding the group’s work.

Another longtime Psymposia ally, Sasha Sisko, has been pressuring academic journals to retract studies based on Lykos’s clinical trials. In August, the journal Psychopharmacology retracted three studies that contained data from the session with Ms. Buisson.

Lykos disagreed with Psychopharmacology’s decision, saying a correction to the papers would have sufficed.

Mx. Sisko, who uses gender-neutral pronouns, has also criticized Lykos trial participants who have spoken favorably about their experiences.

Becca Kacanda, who posted about her treatment on X, said Mx. Sisko criticized her on the platform and wrote in a direct message that she had undergone a “whack-a-doodle nonsense ‘therapy.’”

Ms. Kacanda said Mx. Sisko seemed to be fishing for information to use against Lykos and trying to “gaslight” her about her trial experience.

“I am not trying to silence cases of abuse or constructive critiques,” Ms. Kacanda said. “But Psymposia does not have the good faith intentions that they are presenting themselves to have.”

Mx. Sisko declined to be interviewed on the record for this article.

After the F.D.A. decision, Mr. Nickles and Dr. Ross made a surprising announcement of their own: They were starting their own group.

The reason: Psymposia, they said, had engaged in undisclosed unethical behavior.

Rachel Nuwer is a longtime freelance science writer for The Times.