Foreigners in the U.S. should not visit dispensaries, even if the use of marijuana is legal in the state in which they are located.  Some people who were in the U.S. on valid visas and who provided their passports at dispensaries and then left the US have been denied admission when seeking to return to the US.  Unbeknownst to them, their visas were revoked.  

Despite the fact that marijuana may be legal in a state, its use is considered a Schedule 1 drug under federal law, which governs immigration.  A Schedule 1 drug is among the most serious crimes. Possession or even in some cases the admission of possession of any amount of a Schedule 1 drug can result in a bar from the U.S.  

Even if a visa was not revoked, foreigners  are being questioned at a border check when it is learned they visited a State in which possession of marijuana is legal.  An admission to a border agent that the foreigner possessed marijuana in any amount results in a permanent bar from the U.S.  

Bar waivers are possible in very narrow circumstances involving hardships to U.S. family members when trying to immigrate to the U.S., or based on equitable principles when trying to visit the U.S.  Unfortunately, in the current political climate, the Department of Homeland Security is searching for ways to deny entry to the U.S.