Officials in Aspen have put out a brochure on how to use marijuana safely and legally. Marci Krivonen hide caption

itoggle caption Marci Krivonen

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

itoggle caption Brennan Linsley/AP

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

itoggle caption Brett Myers/Youth Radio

Brooke Gehring, CEO of Patients Choice and Live Green Cannabis, stands in one of her company's grow houses in Denver. Kirk Siegler/NPR hide caption

itoggle caption Kirk Siegler/NPR

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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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

itoggle caption Kai-Huei Yau/MCT/Landov

Outside New York City Hall, a policeman watches a protest against racial disparities in marijuana arrests. The majority of those arrested are black or Latino, even though those groups are not more likely to smoke pot. Andrew Burton/Getty Images 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

itoggle caption Ted S. Warren/AP

The teenage years are the last golden opportunity to build a healthy brain, researchers say. So smoking pot might not be so smart. Tomas Rodriguez/Corbis hide caption

itoggle caption Tomas Rodriguez/Corbis

The legalization of marijuana could dry up a revenue stream for police, according to reports. Here, two men share a water pipe underneath the Space Needle shortly after a law legalizing the recreational use of marijuana took effect in Seattle in 2012. Stephen Brashear/Getty Images hide caption

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New York Gov. Andrew Cuomo is reportedly considering allowing the use of marijuana for medicinal purposes. Tim Roske/AP hide caption

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A man uses the Nasal Ranger to detect smells in the southern U.S., in this photo provided by St. Croix Sensory. In Denver, the device is being used to monitor complaints of strong marijuana smells. Courtesy of Nasal Ranger hide caption

itoggle caption Courtesy of Nasal Ranger

An ATM sits next to a rack of marijuana clone plants that are used to grow medical marijuana on Wednesday at The Joint, a medical marijuana cooperative in Seattle. Last week Washington became the second U.S. state to adopt rules for the recreational sale of marijuana. Ted S. Warren/AP hide caption

itoggle caption Ted S. Warren/AP

A man smokes marijuana outside Uruguay's parliament in Montevideo on Wednesday, where lawmakers in the lower house debated and passed a bill that would legalize marijuana and regulate its production and distribution. Matilde Campodonico/AP hide caption

itoggle caption Matilde Campodonico/AP