There is a little known, and rarely acted on, federal firearm law which says that cannabis users can’t own guns. The law makes no differentiation between legal and illegal cannabis users, which makes sense given that the federal government doesn’t acknowledge legal cannabis users.

Usually, however, law enforcement agencies don’t go checking every medical marijuana patient to make sure they don’t also have a gun. But apparently the Honolulu Police Department doesn’t do things the usual way.

The Honolulu Police Chief has sent letters to the city’s medical marijuana patients informing them that they have 30 days to “voluntarily surrender [their] firearms, and ammunition to the Honolulu Police Department or otherwise transfer ownership.”

In other words, Honolulu MMJ patients can have legal weed or a legal gun, but they can’t have both. According to Leafly, who reported the story, this “may represent the first time a law enforcement agency has proactively sought out state-registered medical marijuana patients and ordered them to surrender their guns.”

Hawaii has only had medical marijuana dispensaries for three months.

Though the federal law technically applies, Honolulu PD is actually using a state law to enforce its whole weed-and-guns-don’t-mix move, one that says that, “No person who is a fugitive from justice or is a person prohibited from possessing firearms or ammunition under federal law shall own, possess, or control any firearm or ammunition therefore.” Since federal law makes cannabis users “a person prohibited from possessing firearms,” the state law applies.

Honolulu’s pothead gun nuts are far from the first people to be unhappy about the law. It was challenged in the 9th US Circuit Court of Appeals last year. The court upheld the gun ban, while giving a pretty wishy-washy justification for it.

“It may be argued that medical marijuana users are less likely to commit violent crimes, as they often suffer from debilitating illnesses, for which marijuana may be an effective palliative,” the federal ruling said. “But those hypotheses are not sufficient to overcome Congress’s reasonable conclusion that the use of such drugs raises the risk of irrational or unpredictable behavior with which gun use should not be associated.”

Photo via Flickr user Peretz Partensky