WASHINGTON — An Arizona study hoping to provide definitive answers on the medical benefits of marijuana for wounded veterans is in danger of collapsing because researchers can’t find enough participants.

They say Veterans Affairs officials could help, but so far have shown no interest in talking to local doctors or veterans about the work.

“It’s so frustrating,” said Dr. Sue Sisley, principal investigator for the study. “They say they want more data on ways to help veterans. If they want more data, this is it.”

But a spokesman for VA said the study’s problems are the fault of the research team, not the federal bureaucracy.

“Federal law restricts VA’s ability to conduct research involving medical marijuana, or to refer veterans to such research projects,” said VA Press Secretary Curt Cashour. “The researcher is free to work with veterans service organizations and state veterans officials who may not face such restrictions to identify candidates for her study.”

The struggles come just days after the U.S. Food and Drug Administration announced plans to redesignate the main ingredient of the psychedelic drug ecstasy as a “breakthrough therapy” for treatment of post-traumatic stress disorder, a move hailed by advocates as increasing opportunities for helping veterans.

But investigation into similar potential uses for cannabis — long praised by supporters for pain management and mood stabilization — has proven much more difficult, largely because of more restrictive federal rules over its handling and experimentation.

It’s classified as a schedule 1 drug, alongside heroin and cocaine, severely limiting how it can be purchased and used for scientific studies. Congressional efforts to change that and broaden medical research have stalled in recent years.

Sisley’s effort was formally launched two years ago after getting FDA approval and $2.1 million from the Colorado Department of Public Health and Environment. It’s located in Arizona but sponsored by the California-based Multidisciplinary Association for Psychedelic Studies.

The study has been billed as potentially the first definitive U.S. research on whether marijuana can help manage veteran’s PTSD, and has received high-profile backing from the American Legion and other veterans organizations.

But it has faced numerous problems. Getting federal approval took five years. Officials at Johns Hopkins University in Maryland withdrew from the study shortly after its formal start, leaving Sisley with a single Arizona site to monitor participants.

She says researchers have screened more than 4,000 state veterans but managed to enroll only 22 of the 76 needed for a full study. Criteria for applicants is restrictive: veterans with service-connected chronic post-traumatic stress disorder available for a 14 weeks of initial monitoring and six months of follow-up.

“For the last two years, we’ve reached out to every veterans service organization and every post in the state,” Sisley said. “We’ve depleted that source. And it’s still not enough.”

She believes that dozens of veterans who would qualify for the study are among the thousands of patients at the nearby Phoenix VA Medical Center, and has lobbied administrators for two years to let her discuss the work with physicians there.

They’ve consistently denied those requests. Letters to top VA health officials and VA Secretary David Shulkin have not received any response.

VA officials have consistently cited federal rules barring their physicians from discussing marijuana as a possible treatment alternative, even in states where the drug is legal for medicinal or recreational purposes.

Sisley regularly responds with a Veterans Health Administration memo from March 2012 which states that none of the rules “preclude VA clinicians, in the normal course of their clinical duties, from discussing specific research studies with their patients where appropriate, and referring them to a non-VA investigator for more information about a non-VA study.”

At a White House press conference in May, Shulkin was asked about easing the rules for medical research on cannabis to help broaden treatment options for veterans.

“Right now, federal law does not prevent us at VA to look at that as an option for veterans,” he said. “I believe that everything that could help veterans should be debated by Congress and by medical experts.

“If there is compelling evidence that this is helpful, I hope the people take a look at that and come up with the right decision, and then we will implement that.”

Sisley insists she is trying to provide that evidence, and has sharply criticized local and federal VA officials for not offering more assistance. Cashour responded that VA is not to blame.

“If the researcher is truly interested in finding veterans for her study, she should spend more time recruiting candidates and less time writing letters to the media,” he said.

The study has employed a full-time recruiter and again petitioned local veterans groups for assistance, but Sisley believes VA cooperation could solve the problem much more quickly.

If new participants can’t be found by Oct. 1, she said, researchers will either have to shut down the work or expand the study parameters to include non-veterans. Neither of those moves will provide the veteran-specific PTSD data she was hoping to collect.

“It was a seven-year saga with federal regulations just to get the study to this point,” she said. “I don’t want to see that lost.”

For more information on the study, visit the MAPS web site