Maryland residents shouldn't both own firearms and have MMJ IDs. Legal gun owners shouldn't use any Federally controlled substance. Doing so violates the Federal Gun Control Act, which states that an "unlawful user" of a Scheduled substance cannot be considered a law-abiding gun-owner. Since cannabis is regarded as a Schedule I substance, Maryland MMJ patients are still seen as "unlawful users."

While medical patient info is technically confidential, health information subject to HIPAA protections, MMJ patient status is still accessible by the Maryland State Police. In the event of a Maryland individual seeking to buy a firearm, Maryland law enforcement will ask if the individual is an MMJ patient. By law, Maryland residents must answer these queries to the police truthfully. Otherwise, they are violating federal law and could be subject to severe criminal penalties. In those cases in which a Maryland resident discloses that they are, in fact, an MMJ patient in the state, the police will block any lawful gun transaction for that individual.