Nebraska and Oklahoma sue Colorado over legal marijuana
Nebraska and Oklahoma have asked the US Supreme Court to nullify a 2012 law that made marijuana legal in the US state of Colorado.
The two states allege that Colorado's law is in violation of federal law.
They say that they are suing just Colorado, and not Washington state where marijuana is also legal, because they do not share a border with Washington.
Colorado's attorney general said their suit was without merit.
"Federal law undisputedly prohibits the production and sale of marijuana," said Nebraska attorney general Jon Bruning in a press release.
"Colorado has undermined the United States Constitution, and I hope the US Supreme Court will uphold our constitutional principles."
Colorado's attorney general John Suthers said in a statement that the state had been expecting legal action after Nebraska and Oklahoma complained about marijuana grown in Colorado coming into their states.
But he said he would vigorously defend Colorado's law as "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".
Colorado's citizens voted to legalise marijuana in 2012, and earlier this year the state became the first in the US to offer marijuana for sale for recreational use.
Already, Colorado has collected $34m in taxes from marijuana sales, adding a valuable revenue stream to the state's coffers.
Washington state passed a similar measure in 2012, but marijuana only went on sale for recreational use there this past summer.
Alaska, Oregon, and the District of Columbia recently voted to legalise marijuana in November.
Industry trade groups criticised the legal action.
"Colorado has created a comprehensive and robust regulatory programme for the sale of marijuana in Colorado," said Mike Elliott, the director of the Marijuana Industry Group.
"If Nebraska and Oklahoma succeed, they will put the violent criminal organisations back in charge."