Posttraumatic stress disorder (PTSD) is a debilitating disorder experienced by a subset of individuals following life-threatening trauma, such as sexual and physical assault, natural disasters, and military combat. Despite there being a number of treatments for PTSD, there is a need to help people who have treatment-resistant PTSD and do not benefit from these treatments as many individuals continue to experience marked PTSD symptoms following treatment. In response to overwhelming demand, several U.S. states have passed laws allowing the medical use of cannabis (marijuana) by individuals with PTSD. Unfortunately, there has been very little scientific evidence on suitability of cannabis as an alternative therapy for PTSD. The purpose of this study is to find out if cannabis can reduce PTSD symptoms in 76 military veterans with treatment-resistant PTSD. Four different types of smoked cannabis will be evaluated using a "triple-blind" cross-over placebo controlled design. Study participation will last 8.5 months. Participants will select a community observer, a friend or relative who can be contacted by the researchers, who will report on the participant's health and well-being throughout the study. After screening and meeting study requirements, participants will be randomly assigned to receive one of four types of cannabis that vary based on THC and CBD content. During the study, each participant will smoke two of the four types of cannabis, up to 1.8 grams per day, for a 3-week period. Participants can smoke their daily 1.8g cannabis at any time, using as much as they need. After each 3-week cannabis use period, they will stop smoking marijuana/cannabis for two weeks where no cannabis use will be allowed. Study findings will help researchers better understand the risks and benefits associated with cannabis use among veterans with PTSD, information that can be used to aid in shared decision-making among people with PTSD and their doctors or therapists.