Medical Marijuana and Cannabidiol Research Expansion Act This act establishes a new, separate registration process to facilitate research on marijuana. Specifically, the act directs the Drug Enforcement Administration (DEA) to follow procedures specified in the act to register (1) practitioners to conduct marijuana research, and (2) manufacturers to supply marijuana for the research. The act allows certain registered entities (including institutions of higher education, practitioners, and manufacturers) to manufacture, distribute, dispense, or possess marijuana or cannabidiol (CBD) for the purposes of medical research. Additionally, the act directs the DEA to register manufacturers and distributors of CBD or marijuana for the purpose of commercial production of an approved drug that contains marijuana or its derivative. Finally, the act includes various other provisions, including provisions that require the DEA to assess whether there is an adequate and uninterrupted supply of marijuana for research purposes; prohibit the Department of Health and Human Services (HHS) from reinstating the interdisciplinary review process for marijuana research; allow physicians to discuss the potential harms and benefits of marijuana and its derivatives (including CBD) with patients; and require HHS, in coordination with the National Institutes of Health and relevant federal agencies, to report on the therapeutic potential of marijuana for various conditions such as epilepsy, as well as the impact on adolescent brains and on the ability to operate a motor vehicle.