Colorado's neighbors Oklahoma and Nebraska filed a lawsuit in the US Supreme Court Thursday protesting that the western state's legalization of marijuana is harming them.
"The State of Colorado has created a dangerous gap in the federal drug control system enacted by the United States Congress," said the 83-page lawsuit, published by the Denver Post.
"Marijuana flows from this gap into neighboring states, undermining (their) own marijuana bans, draining their treasuries, and placing stress on their criminal justice systems," it added.
Colorado became the first US state to legalize marijuana for recreational use in January, where it was already allowed for medicinal use.
Washington state followed a few months later, while last month voters in the US capital backed the legalization of marijuana in a referendum.
Colorado Attorney General John Suthers said the legal action was not unexpected, but questioned the motives behind it.
"Because neighboring states have expressed concern about Colorado-grown marijuana coming into their states, we are not entirely surprised by this action," he said.
"However, it appears the plaintiffs' primary grievance stems from non-enforcement of federal laws regarding marijuana, as opposed to choices made by the voters of Colorado," he added.
Mason Tvert of the Denver-based pro-legalization group, the Marijuana Policy Project, said the lawsuit was "without merit."
"These guys are on the wrong side of history. They will be remembered similarly to how we think of state officials who fought to maintain alcohol prohibition years after other states ended it," he said.