Posttraumatic stress disorder (PTSD) is a disabling psychiatric disorder that affects 20%-30% of U.S. Veterans. PTSD is strongly associated with increased risk for substance abuse comorbidity, including cannabis use disorder. Multiple states now include PTSD as a condition for which patients can be legally prescribed medical marijuana, despite the fact that there has not been a single large-scale randomized clinical trial demonstrating the efficacy of cannabis to treat PTSD to date. The overall objective of the current proposal is to study the impact of reduced cannabis use on functioning among Veterans with PTSD. The investigators will evaluate the relationship between cannabis use and daily functioning among cannabis users and heavy cannabis users. The central hypothesis is that reductions in cannabis use will lead to positive changes in the functional outcomes of Veterans. The rationale for this research is that it will provide the first and only real-time data concerning the impact of reduced cannabis use on daily functioning among Veterans with PTSD.