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Marijuana at a medical marijuana dispensary in Los Angeles. Twenty-three states and the District of Columbia have legalized medical marijuana. David McNew/Getty Images hide caption

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Using chemicals to control bugs or mold is common among commercial cannabis growers. But with no federal oversight, experts are concerned growers may be using dangerous pesticides. Ted S. Warren/AP 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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Workplace drug testing for marijuana may need updating in light of changing laws, a case before the Colorado Supreme Court suggests. Kai-Huei Yau/MCT/Landov hide caption

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Robert Calkin, the president of the Cannabis Career Institute, spoke at an exhibition called Hempcon held in San Bernardino, Calif., last year Courtesy of Robert Calkin/AP hide caption

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One of three marijuana plants growing in the backyard of a 65-year-old retiree from Pompano Beach, Fla. He grows and smokes his own "happy grass" to alleviate pain. Carline Jean/MCT/Landov hide caption

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Marijuana grown for medical purposes is shown inside a greenhouse at a farm in Potter Valley, Calif. Eric Risberg/AP hide caption

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Vaporizer pens look like the e-cigarettes that dispense nicotine. But these devices are optimized for a potent marijuana resin with high concentrations of THC. Courtesy of Grenco Science hide caption

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C. Nash smokes after possession of marijuana became legal in Washington state on Dec. 6, 2012. Ted S. Warren/AP hide caption

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Marilyn Budzynski takes care of her 20-year-old son, Michael, in Eustis, Fla., in September. Michael suffers from Dravet syndrome, a severe form of epilepsy. Tom Benitez/MCT /Landov hide caption

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There's data to support the notion that pot, or a drug based on its active ingredient, could help ease the fears of PTSD. Ted S. Warren/AP hide caption

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A new free market for medical marijuana in Canada will replace small growers with large-scale indoor farms. Menahem Kahana/AFP/Getty Images hide caption

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A sign outside a medical marijuana dispensary in Manitou Springs, Colo. Voters amended the state's constitution to legalize marijuana for recreational use in November 2012. Eric Whitney hide caption

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A man pulls out a bag of marijuana to fill a pipe at Hempfest in Seattle on Aug. 16. Thousands packed a waterfront park for the opening of a three-day marijuana festival, an event that is part party, part protest and part victory celebration after the legalization of pot in Washington and Colorado in 2012. Elaine Thompson/AP hide caption

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