A US military veterans group with more than 2 million members has formally called on Congress to "recognize cannabis as a drug with potential medical value" and make it easier for researchers to study whether marijuana is an effective treatment for post-traumatic stress disorder (PTSD) and traumatic brain injuries.

The American Legion, the country's largest veterans organization, approved a resolution last week at its national convention that calls on US lawmakers to remove marijuana from the list of Schedule I controlled substances, a restrictive category that includes heroin, LSD, and other drugs with "no currently accepted medical use and a high potential for abuse."

In August, after five years of deliberations and review, the Drug Enforcement Administration (DEA) declined a petition to remove marijuana from the Schedule I category. The agency's chief said the decision was based on the fact that the Food and Drug Administration has not determined that marijuana is a "safe and effective medicine."

While urging Congress to overrule the DEA, the American Legion also called on the agency to "license privately funded medical marijuana production operations" in order to "enable safe and efficient cannabis drug development research." The DEA has already said it will allow more scientists to grow research-grade weed, but it has yet to actually grant anyone permission.

The nonprofit, nonpartisan veterans group, which was founded after World War I and now has hundreds of outposts across the country, is especially interested in studies that examine whether marijuana can benefit people with PTSD, a common affliction among soldiers returning from war zones.

Earlier this year, researcher Sue Sisley and the Multidisciplinary Association for Psychedelic Studies (MAPS) earned federal approval for the first-ever study of how smoking marijuana affects veterans with PTSD. Sisley reportedly spoke at several American Legion events and lobbied for the group to approve the weed resolution.

Speaking to Marijuana.com, Sisley called the American Legion's resolution a "historic shift in public policy" and said it could help shape federal policy in the coming years.

"I consider this a major breakthrough for such a conservative veterans organization," Sisley said. "Suddenly the American Legion has a tangible policy statement on cannabis that will allow them to lobby and add this to their core legislative agenda. The organization has a massive amount of influence at all levels."