Colorado's Board of Health has voted against adding post-traumatic stress disorder to the list of ailments eligible for treatment with medical marijuana.

The 6-2 vote Wednesday came despite the backing of the state's chief medical officer. The board has voted at least three times against adding PTSD to the list of eight debilitating conditions that qualify for medical pot.

Colorado hasn't added an ailment to its medical pot list since voters approved the eight existing conditions in 2000. Board members cited a lack of research showing marijuana can treat PTSD.

Nine other states currently consider PTSD a condition eligible for treatment with medical marijuana.