The Department of Veterans Affairs says it will not conduct research into whether medical marijuana could help veterans suffering from post-traumatic stress disorder and chronic pain, as veterans groups are pushing for the use of the drug as an alternative to opioids and anti-depressants.

In a letter to U.S. Rep. Tim Walz, D-Minn., Department of Veterans Affairs Secretary David Shulkin said VA’s ability to research medical marijuana is hampered by the fact that the drug is illegal federally. Shulkin’s letter came in response to an inquiry by 10 Democrats on the House Veterans’ Affairs Committee. The letter asks Shulkin to commit the VA to investigating whether medical marijuana can help veterans suffering from PTSD and chronic pain and identify barriers to doing so.

“VA is committed to researching and developing effective ways to help Veterans cope with post-traumatic stress disorder and chronic pain conditions,” Shulkin wrote in a response to the members of Congress. “However, federal law restricts VA’s ability to conduct research involving medical marijuana, or to refer veterans to such projects.”

The response comes as at least 29 states, plus the District of Columbia, Guam and Puerto Rico, have legalized the use of medical marijuana in some form. Veterans groups, including the American Legion, have been pushing for the drug to be studied and used to help ease the effects of PTSD, chronic pain and other disorders.

“What America’s veterans need prioritized right now is for cannabis to be treated as a health policy issue,” said Nick Etten, founder and executive director of the Veterans Cannabis Project. “We’re desperate for solutions for the conditions we’re dealing with.”

According to a 2017 VA review, about 15 percent of veterans treated at outpatient PTSD clinics reported using marijuana in the previous six months. According to an American Legion phone survey released in November, 22 percent of veteran household respondents said they used cannabis to treat a medical condition. Ninety-two percent of veteran households surveyed for the Legion said they support researching whether marijuana can effectively treat mental and physical conditions and 82 percent said they want to have medical cannabis as a legal treatment option.

Last month, the Veterans Health Administration urged patients to discuss medical marijuana use with their doctors. The shift will allow doctors and patients to determine what, if any, effect marijuana use might have on treatment plans.