On Friday, drugmaker Lykos Therapeutics withdrew notified from the Food and Drug Administration that the agency has decided not to approve MDMA-assisted treatment for PTSD at this time. Instead, the agency asked the drugmaker to further study the safety and effectiveness of the treatment.

The long-awaited announcement came in the form of a “complete response letter” from the FDA to Lykos, according to a press release from the company.

It represents a significant blow to the company and the wider movement to bring psychedelics into the mainstream of mental health. Lykos said it plans to request a meeting with the FDA to ask for reconsideration of the decision.

Lyko CEO Amy Emerson called the FDA’s request for another Phase 3 study “deeply disappointing” and said conducting the study “would take several years.”

She added that many of the agency’s requests “can be addressed with existing data, post-approval requirements or by reference to the scientific literature.”

“The FDA’s decision to request another Phase 3 study is a major setback for the field,” said Jennifer Mitchell, a professor at the University of California, San Francisco and author of the MDMA studies, in the Lykos press release.

The treatment had received great support from patients, leaders in the field of mental health and psychedelics, and politicians from both parties. The past few weeks, advocates exerted great pressure from the public campaign, which emphasizes the urgent need for new and effective treatments for veterans and the millions of people affected by PTSD.

Many in the psychedelic industry had seen this as a defining moment. The FDA decision on MDMA can be seen as a wake-up call for other drugs in the pipeline such as psilocybin and LSD, both of which are considered “classic psychedelics” unlike MDMA.

The wave of enthusiasm about the drug’s therapeutic potential has also come from researchers at top academic centers and even the Department of Veterans Affairs.

Still, the treatment’s fate was far from certain.

Opposition to Lyko’s application had gathered momentum in recent months, culminating in a controversial public meeting when advisers to the FDA voted against the evidence support the treatment, largely based on concerns about flaws in the clinical research.

In his press release, Lykos said the FDA reiterated the issues raised by the committee.

That vote put the FDA in a tricky spot, given that it has historically sided with the advisers. Those who hoped to see MDMA make it to market were rebuffed that concerns about the quality of the research were overblown and, in some cases, focused on study design problems that were not unique to MDMA.

The push to establish MDMA-assisted therapy as a legitimate treatment for PTSD stretches back two decades.

Researchers affiliated with the nonprofit Multidisciplinary Association for Psychedelic Studies, or MAPS, pioneered the early studies and developed the therapy protocol embedded in the treatment. That work ultimately set the stage for the drugmaker, which was incubated by MAPS, to move forward with larger human trials and seek approval from federal regulators.

Lykos has up more than $100 million pending FDA approval of its MDMA treatment.

Optimism about the treatment stems in large part from the company’s promising Phase 3 clinical trials, which collectively enrolled about 200 people. The latest, published last year, showed roughly 70% of the participants no longer met diagnostic criteria for PTSD after three sessions of MDMA, compared with about 48% who had the same talk therapy protocol but received a placebo.

Follow-up research from the company showed that participants were still benefiting from the treatment at least six months after their last dosing session.

“While disappointing, this move by the FDA highlights the importance of conducting rigorous research into the safety and efficacy of MDMA-assisted therapy,” says Alan Davisdirector of the Center for Psychedelic Drug Research and Education at Ohio State University, “Despite this setback, I am hopeful that there will be a better outcome down the road after more research is completed in the coming years.”

The debate on the merits of Lyko’s application has become increasingly divided, with accusations that misconduct and bias in the clinical trials compromised the results and undermined its safety. The drug manufacturer and many of the therapists involved in the research have strongly denied these claims.

It is unclear to what extent any of this factored into the FDA’s consideration of the treatment.

In response to Friday’s decision, Emerson reiterated the company’s plan to push the research forward, saying it would “work tirelessly and use all available regulatory avenues to find a reasonable and expeditious path forward for patients.”

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