Veterans Medical Marijuana Safe Harbor Act This bill authorizes activities related to medical marijuana use by veterans and provides for research on medical marijuana use by veterans. Specifically, the bill authorizes (1) a veteran to use, possess, or transport medical marijuana in accordance with applicable state or Native American tribal law; (2) a Department of Veterans Affairs (VA) physician to discuss with a veteran the use of medical marijuana as a treatment if the physician is in a state or on tribal land that authorizes such treatment; or (3) a VA physician to recommend, complete forms for, or register veterans for participation in a medical marijuana treatment program in accordance with applicable state or tribal law. These authorizations expire after five years. The bill also requires the VA to report on (1) the use of medical marijuana by veterans to manage pain and to treat diseases and disorders (e.g., post-traumatic stress disorder); and (2) the relationship between state-approved medical marijuana treatment programs, program access, and opioid use and misuse reduction.