Neighbor states on guard against pot from Colo., Wash.

States neighboring Colorado and Washington are wondering how much marijuana will spill across their borders after voters in those two states legalized its recreational use in November.

They vow to arrest and prosecute marijuana possessors even if the product is purchased legally across state lines.

Possession of any amount of marijuana is a misdemeanor under federal law, and selling it in any amount is a federal felony.

President Obama said last month that "we have bigger fish to fry" than going after pot smokers in Washington and Colorado. The Justice Department has not said how it will respond or whether it is concerned about increased cross-border trafficking from the two states.

One drug-control advocate predicts that trafficking will increase into and out of Washington and Colorado, and that could drive down the cost of marijuana regionally.

"The retail marijuana stores will be in business to make as much of a profit as possible," says Tom Gorman, director of the Denver-based drug-policy group Rocky Mountain High Intensity Drug Trafficking Area. "That means selling as much marijuana as they can to the largest number of people as they can. That will create a competitive market based on quality and the price of the product."

Law enforcement officials in neighboring states are watching as Colorado and Washington regulators decide how the product can be grown, processed and sold in their states.

"Everyone is aware of the possibility that you could have an increase (in cross-border traffic), especially for some of our counties on the border of Colorado," says Lt. Josh Kellerman, a spokesman for the Kansas Highway Patrol. "People might not understand that while they bought it legally in Colorado, it is still illegal in our state."

Wyoming Attorney General Greg Phillips notes that the state's Supreme Court in 2011 ruled that marijuana bought for medical purposes in California still was illegal in Wyoming.

"I think the same rule applies" for marijuana purchased in states that have legalized it, Phillips says.

Beau Kilmer, co-director of the RAND Drug Policy Research Center, says the amount of cross-border trafficking depends on the shape of Colorado and Washington regulations.

"The decisions they make about how many producers to allow and what type of production to allow will really shape what the market will look like and shape this whole discussion of diversion" across borders, Kilmer says. "Are they going to allow four producers, or 400?"

Kilmer says the federal government could "influence whether or not large amounts of marijuana come out of these states." One example: a threat to crack down on large suppliers even in states where buying the drug is legal.

Kilmer says neighboring states' responses also will help determine whether cross-border flow of marijuana to the newly legalized states becomes an issue.

"Does Wyoming on Friday and Saturday nights increase Highway Patrol on the border?" Kilmer asks.

Eighteen states and the District of Columbia allow marijuana possession for medical purposes, and 15 states have decriminalized its possession in small amounts, generally resulting in fines but no jail time.

Colorado had done both before voters in November legalized possession of up to an ounce for anyone 21 and older. Washington allowed medical marijuana before its legalization vote.

Some drug-policy experts say they believe Colorado has the potential to have more cross-border traffic issues because of its crossroads location on the continent and because it borders seven states compared with Washington's two.

Washington is bordered on the south by Oregon, which has both decriminalized possession and allowed for medical-marijuana possession. None of Colorado's neighbors has decriminalized, and only New Mexico and Arizona allow medical marijuana.

"There is absolutely no doubt in my mind Colorado is going to be a source for marijuana in this country," says Gorman, whose office is funded through the White House's Office of National Drug Control Policy. "It has already been with medical marijuana, which is supposed to be a regulated industry."

His office reported in August that drug busts in 22 states and the District of Columbia were traced to medical marijuana purchases in Colorado.

Predictions such as Gorman's "accentuate the fact that prohibition (of marijuana), 75 years of it in the United States, is an abject failure," says Allen St. Pierre, executive director of NORML, the National Organization for the Reform of Marijuana Laws.

He compares cross-border trafficking of marijuana with that of Mexican-made Corona beer. "Americans consume a lot of that, too," St. Pierre says, "but we don't have any cross-border problems with massive amounts of Corona beer."

Washington will not allow private production. Colorado will allow each adult to cultivate six plants at a time, but they can't sell it.

Colorado is wrestling with whether to require in-state residency to buy marijuana, says Mark Couch, spokesman for a task force set up by Gov. John Hickenlooper, a Democrat.

Brian Smith, spokesman for the Washington Liquor Board, says officials there are well aware that "we have three borders" (including Canada), and that "people are mobile."