Physician Sue Sisley battles to study cannabis and psilocybin mushrooms for pain, addiction and PTSD.

From the earliest days of her career, physician Sue Sisley has been passionate about caring for US military veterans. Back then, many of the people she treated were self-medicating with black-market cannabis because, unlike prescription drugs, marijuana allayed nightmares and other symptoms of post-traumatic stress disorder (PTSD). A few puffs helped them to fall asleep.

“Initially, I discouraged them and rolled my eyes thinking about it,” says Sisley, whose training taught her to view only approved drugs as medicines. “I lacked sympathy for their claims and thought they were drug seekers.”

But over time, Sisley saw how the ineffectiveness of mental-health treatments could fuel hopelessness. Currently, 17 US veterans die by suicide daily, on average. The cannabis users among Sisley’s patients were often the ones who maintained a will to live.

“It made me realize that I was very misled, by the government and our training programmes, to believe that cannabis was dangerous,” she says. “I didn’t learn about any medical benefits.”

The early lessons from her patients influenced Sisley. Over the next two decades, she challenged US federal agencies, navigated a legal and regulatory maze and creatively secured funding to investigate and develop treatments, based on cannabis and psychedelics, that the US government had blocked for decades.

A physician-researcher is born

After the US Congress passed the Controlled Substances Act of 1970, cannabis was made illegal and classified as a Schedule I drug, defined as having no accepted medical use. That put marijuana in the same category as heroin and most psychedelic drugs: possession or use of the drug, and growing cannabis without a Schedule I research licence, could land someone in prison.

In the mid-1990s, several US states began defying federal law, allowing adults to purchase cannabis for medicinal purposes. In 2008, when medical marijuana was still off limits in Arizona, Sisley was one of the few physicians who spoke publicly about legalizing it. Her advocacy caught the attention of Rick Doblin, who sought to get certain psychedelic drugs and cannabis taken off Schedule I so that they could be studied and any medical benefits reaped.

Sisley (centre) visits the Texas state capitol to advocate for research on psychedelics.Credit: Andrew Neugebauer/STOKE Creative

Doblin founded a US non-profit body, the Multidisciplinary Association for Psychedelic Studies (MAPS), to raise money from private donors for clinical trials. He approached Sisley to lead a cannabis study.

Although she lacked a background in research, Sisley was an associate professor of psychiatry and assistant director of telemedicine at the University of Arizona College of Medicine in Phoenix, and Doblin saw her potential. “What really appealed to me is that Sue looks to where science is being blocked by politics, and she’s willing to stand up for what she thinks is right,” he says.

In 2014, Sisley and MAPS received nearly US$2.2 million from the state of Colorado. With approval from the US Food and Drug Administration (FDA), they were poised to lead the world’s first randomized controlled trial of smoked cannabis as a treatment for PTSD in veterans.

But then the University of Arizona declined to renew Sisley’s contract. Although Sisley says that the university simply stated that her services were no longer needed, she thinks there were political pressures involved.

“Even though it was legitimate research” and Arizona voters had approved the medical use of marijuana in 2010, Sisley says, “I believe some legislators did not want anything related to cannabis to have a place at the university.”

In the end, however, the termination of her contract “turned out to be a huge gift”, she says. She learnt that she didn’t need a university or hospital to lead the research. In fact, she now thinks the bureaucracy might have slowed her down. In the following two years, Sisley co-founded the Scottsdale Research Institute, purchased a building for it in Phoenix, Arizona, and built a laboratory to begin her study.

In 2016, as she prepared to enrol participants, the cannabis for the study arrived. At the time, the University of Mississippi in Oxford was the sole US government-approved provider of cannabis for use in human research. But when Sisley opened the box, she was horrified. The cannabis was mouldy, pulverized into powder and littered with twigs. It was nothing like the buds sold at dispensaries, and some participants had difficulty in inhaling it because it irritated their throats.

Sisley’s team found that participants who smoked cannabis showed improvement over the placebo group, but not enough to demonstrate statistical significance (M. O. Bonn-Miller et al. PLoS ONE 16, e0246990; 2021). Sisley thought that if participants had access to the cannabis available in dispensaries, they might have tolerated it better and experienced more benefits.

To raise awareness about barriers to cannabis research, Sisley went on a speaking tour (together with Dodger, her guide dog). In 2019, at the South by Southwest conference on music, film, culture and technology in Austin, Texas, she described what was, by then, a ten-year battle to complete her clinical trial, and a move by the US Drug Enforcement Administration (DEA) that might help. In 2016, the agency had announced a policy that aimed to broaden the government’s cannabis supply, so Sisley applied for a Schedule I licence to grow cannabis for a phase IIb trial. But, after two-and-a-half years, the DEA still had not processed her application.

Suing the government for science

Matthew Zorn, an attorney based in Houston, Texas, attended Sisley’s talk. He had become interested in marijuana as a pain treatment while caring for his partner, who later died of bone cancer, and was angered by the red tape surrounding the drug supply for trials. “All Sue was trying to do was advance research. She did everything by the book,” says Zorn. If the cannabis used in a trial “is garbage, of course you can’t prove something is safe and efficacious, and then it’s not going to be available for patients”.

Zorn recruited his colleague Shane Pennington, then also based in Houston, who had experience of bringing cases against federal agencies. Together, representing Sisley pro bono, they helped her to sue the DEA in 2019.

Sue Sisley speaks at the 2025 South by Southwest festival in Austin, Texas.Credit: Andrew Neugebauer/STOKE Creative

The lawsuit prompted the US Office of Legal Counsel to release a memo that revealed why the DEA had been sitting on Sisley’s cannabis application. The US government had handed over control of the cannabis supply for medical trials to the University of Mississippi when, under the terms of an international treaty, it was supposed to ensure that a single government agency possessed and distributed the supply.

With the memo’s release, “they realized they had to do something”, says Anthony Coulson, former assistant special agent in charge of the DEA’s district office in Tucson, Arizona. “Sisley’s lawsuit forced the DEA to the table.”

Two months after the lawsuit was filed, the DEA began processing Sisley’s application and dozens from other US scientists, allowing them to grow medical-grade cannabis for research studies. This was crucial, because cannabis used in phase III trials must match what’s on the market for patients, whereas the government’s supply of cannabis did not.

“Because of Sue’s years of sacrifice, she built a foundation for every single cannabis researcher,” says Coulson.

Sisley’s case was settled in 2020, and she feels the struggle was worth it (see ‘Quick-fire Q&A’). “There are so many people suffering who might get their lives back and benefit from this research.”