In a newly published study by Jane Metrik and colleagues, marijuana use was found to exacerbate symptoms of post-traumatic stress disorder (PTSD) in veterans with cannabis use disorder (CUD). This confirms the results of previous studies in veterans that found an association between marijuana use and worsening symtpoms of PTSD. A common issue with many studies on the effect of marijuana use on mental health outcomes is the bi-directionality of the relationship. In other words, it is difficult to definitively say whether it is marijuana use that leads to mental illness or if those susceptible to mental illness tend to [...]