An increasing number of veterans are treating their post-traumatic stress disorder (PTSD) with marijuana, according to the Associated Press.

Marijuana is not approved by the Department of Veterans Affairs. Research on marijuana’s effect on PTSD is contradictory and limited, the article notes. Some studies indicate marijuana may help people manage symptoms of PTSD in the short term, while one study suggested it may worsen symptoms.

The Marijuana Policy Project says 10 states have listed PTSD among conditions for which medical marijuana can be prescribed. A few more states allow doctors enough discretion to recommend marijuana to patients suffering from PTSD.

The U.S. Senate passed an amendment in November that would have allowed VA physicians to recommend medical marijuana to veterans in states where it is legal, but the measure failed to pass the House.

In order for VA doctors to be able to recommend a drug, federal law requires that randomized, controlled trials prove it is effective. Among research currently being conducted on marijuana and PTSD are two studies funded by Colorado.

“There surely is not enough scientific evidence to say marijuana helps PTSD,” said Marcel Bonn-Miller, a University of Pennsylvania professor who is leading the Colorado studies. “But we’ll get a heck of a lot closer to getting to know the answer in two to three years.”

The percentage of veterans with PTSD who have been diagnosed with marijuana dependence has increased since 2002 from 13 percent to almost 23 percent—translating to more than 40,000 veterans, according to VA data.

Dr. Karen Drexler, the VA’s Deputy National Mental Health Program Director for Addictive Disorders, says marijuana may initially provide some relief for veterans with PTSD. She cautioned that for these veterans, “It’s very hard to stop it once you start it,” she said. “It gets into this vicious cycle.”