Multiple states have authorized cannabis as an opioid substitution agent and as a treatment for posttraumatic stress disorder (PTSD).

This study sought to investigate the relationship between cannabis use, non-medical opioid use, and PTSD symptoms among U.S. veterans.

From 1992–2011, veterans admitted to specialized intensive PTSD treatment participated in a national evaluation with assessments at intake and four months after discharge. Participants with non-medical opioid use ≥ 7 days during the 30 days preceding admission were divided into two groups: those with cannabis use ≥ 7 days, and those without cannabis use. These two groups were compared on measures of substance use and PTSD symptoms at baseline and 4-months outpatient follow-up. We hypothesized that, at both assessments, the group with baseline cannabis use would show less non-medical opioid use and less severe PTSD symptoms.

Of 1,413 veterans with current non-medical opioid use, 438 (30.3%) also used cannabis, and 985 (69.7%) did not. At baseline, veterans with concurrent non-medical opioid and cannabis use had slightly fewer days of non-medical opioid use (p < .005; d = −0.16), greater use of other substances (p < .0001) and more PTSD symptoms (p = .003; d = 0.16), compared to veterans who used non-medical opioids but not cannabis. At follow-up, substance use or PTSD symptoms did not significantly differ.

Cannabis use was not associated with a substantial reduction of non-medical opioid use, or either improvement or worsening of PTSD symptoms in this population. Hence, these data do not encourage cannabis use to treat either non-medical opioid use or PTSD.