Medical marijuana researchers are a step closer to being able to start a study on whether the drug helps treat post-traumatic stress disorder (PTSD), after the Public Health Service gave its approval to the study. The Drug Enforcement Administration (DEA) must still approve the research.

Suzanne Sisley at the University of Arizona told USA Today there is a “mountain of anecdotal evidence” that marijuana helps with PTSD. She wants to conduct a controlled trial to see how marijuana suppresses symptoms including insomnia, flashbacks and anxiety.

Sisley received approval for her study from the Food and Drug Administration in 2011, and approval from the Public Health Service on Friday. It is unclear how long the DEA will take to make a decision about the study, the article notes.

The study is designed to last 10 weeks and would include 50 veterans with moderate to severe PTSD symptoms. They would use marijuana grown at the federal government’s only sanctioned marijuana farm at the University of Mississippi. Participants would receive either a placebo or one of four different amounts of THC, the active ingredient in marijuana. The researchers would also study the differences between smoking and vaporizing marijuana.

Six states list PTSD as a qualifying condition to receive medical marijuana: Connecticut, Delaware, Maine, Nevada, New Mexico and Oregon.

The Department of Veterans Affairs estimates 11 to 20 percent of troops who served in Iraq or Afghanistan have PTSD.