Cannabis use is associated with worse outcomes in symptom severity and violent behavior in patients with posttraumatic stress disorder.

Objective: An increasing number of states have approved posttraumatic stress disorder (PTSD) as a qualifying condition for medical Cannabis, although little evidence exists evaluating the effect of Cannabis use in PTSD. We examined the association between Cannabis use and PTSD symptom severity in a longitudinal, observational study. Method: From 1992 to 2011, veterans with DSM-III/-IV PTSD (N = 2,276) were admitted to specialized Veterans Affairs treatment programs, with assessments conducted at intake and 4 months after discharge. Subjects were classified into 4 groups according to Cannabis use: those with no use at admission or after discharge ("never-users"), those who used at admission but not after discharge ("stoppers"), those who used at admission and after discharge ("continuing users"), and those using after discharge but not at admission ("starters"). Analyses of variance compared baseline characteristics and identified relevant covariates. Analyses of covariance then compared groups on follow-up measures of PTSD symptoms, drug and alcohol use, violent behavior, and employment. Results: After we adjusted for relevant baseline covariates, Cannabis use was significantly associated with worse outcomes in PTSD symptom severity (P < .01), violent behavior (P < .01), and measures of alcohol and drug use (P < .01) when compared with stoppers and never-users. At follow-up, stoppers and never-users had the lowest levels of PTSD symptoms (P < .0001), and starters had the highest levels of violent behavior (P < .0001). After adjusting for covariates and using never-users as a reference, starting Cannabis use had an effect size on PTSD symptoms of +0.34 (Cohen d = change/SD), and stopping Cannabis use had an effect size of -0.18. Conclusions: In this observational study, initiating Cannabis use after treatment was associated with worse PTSD symptoms, more violent behavior, and alcohol use. Cannabis may actually worsen PTSD symptoms or nullify the benefits of specialized, intensive treatment. Cessation or prevention of use may be an important goal of treatment.