A person who actually uses marijuana—medicinally or otherwise—is an "unlawful user of ... [a] controlled substance," because marijuana is defined as a controlled substance and can't legally be prescribed under federal law. If a person admits marijuana use to a licensed firearm dealer, the dealer is prohibited from transferring a firearm to that person because the dealer will have a "reasonable cause to believe" that the person is an unlawful user.
A customer who presents a state-issued marijuana card might or might not actually be a user, just as a person with a driver's license may or may not actually drive a car, but the dealer would still have "reasonable cause to believe" the person is an unlawful user.
For more information about the use marijuana for medical purposes and its applicability to Federal firearms laws, please consult the Bureau of Alcohol, Tobacco, Firearms and Explosives' "Open Letter to all Federal Firearm Licensees"by cliicking here.