Deuel County Sheriff Adam Hayward shows off a container of confiscated marijuana in Chappell, Neb., in July. Nikki Kahn/The Washington Post/Getty Images hide caption

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A marijuana bud displayed in Denver. Don't legalize pot, the pediatricians say, but don't lock teenagers up for using it, either. Seth McConnell/The Denver Post/Getty Images hide caption

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Adam Eidinger, chairman of the D.C. Cannabis Campaign, puts up Initiative 71 posters in October. The measure to legalize possession of up to 2 ounces of marijuana was approved by a wide margin of local voters. Jacquelyn Martin/AP hide caption

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Nebraska and Oklahoma say Colorado's marijuana law is unconstitutional, in a challenge to the law in the Supreme Court. Earlier this month, visitors from Texas smell marijuana at the Breckenridge Cannabis Club. Brennan Linsley/AP hide caption

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Volunteers with the DC Cannabis Campaign (left and center) talk to a voter on Election Day about the ballot initiative to legalize marijuana. The measure was approved, but its fate remains uncertain. Allison Shelley/Getty Images hide caption

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Trimmers prepare the marijuana flower, or bud, to make it more appealing to consumers. They use scissors to snip off the leaves and stems. Brett Myers/Youth Radio hide caption

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Nabor, a small-scale marijuana grower in the northwestern Mexican state of Sinoloa, checks his plants. As legal pot increasingly becomes available in the U.S., Americans appear to be buying more that is grown domestically. Prices for marijuana from Mexico have fallen sharply. John Burnett/NPR hide caption

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Early results showed more than a 2-1 lead for a measure to make recreational marijuana use legal in Washington, D.C. A sign promoting the initiative is seen on a corner in the Adams Morgan neighborhood Tuesday. Allison Shelley/Getty Images hide caption

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Low-dose pot-infused cookies, called the Rookie Cookie, sit on the packaging table Sept. 26 at The Growing Kitchen in Boulder, Colo. The state legalized recreational pot use last year. Brennan Linsley/AP hide caption

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This human-scale lab rat cage is parked near a skate park in Denver, Colo., to make a point about the lack of science on marijuana. Richard Feldman Studio/Sukle Advertising and Design hide caption

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Edibles available at LoDo Wellness Center, a retail marijuana and medical marijuana dispensary and grow facility in downtown Denver. Matthew Staver/Landov hide caption

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Amber McGowan, left, and Krystal Klacsan work Monday at Cannabis City in Seattle, a day before the store is to begin legal pot sales on Tuesday. The store will be for now the only one in Seattle to sell recreational marijuana. Ted S. Warren/AP hide caption

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People smoke marijuana, presumably, because it affects their brains, not despite that fact. Above, people in Sao Paulo, Brazil, campaign for the legalization of marijuana. Nelson Almeida/AFP/Getty Images hide caption

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Former Supreme Court Associate Justice John Paul Stevens likens making pot illegal to Prohibition. In his new book, Six Amendments, he proposes constitutional changes including a curb on an individual's right to bear arms. Chip Somodevilla/Getty Images hide caption

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With the Colorado state capitol in the background, Cannabis Cup attendees dance and smoke pot at the annual 4/20 marijuana festival in Denver. Brennan Linsley/AP hide caption

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