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
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
A customer eyes marijuana samples at a Denver dispensary. The makers of a newly unveiled vending machine are hoping to change how pot is sold in stores. Theo Stroomer/Getty Images hide caption
Marijuana plants in Seattle. Ted S. Warren/AP hide caption
Marijuana is stored in bins for trimming and packaging in preparation to be sold retail at 3D Cannabis Center, in Denver. Brennan Linsley/AP hide caption
Marijuana plants in Seattle. Ted S. Warren/AP hide caption
C. Nash smokes after possession of marijuana became legal in Washington state on Dec. 6, 2012. Ted S. Warren/AP hide caption
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
A bag of marijuana being prepared for sale sits next to a money jar at BotanaCare in Northglenn, Colorado, in this file photo taken on December 31, 2013. Rick Wilking/Reuters/Landov hide caption
Research in mice offers new clues as to why Harold and Kumar were so motivated to get to White Castle. Todd Plitt/Getty Images hide caption
Truffles are among the many foods infused with THC – the chemical in marijuana that gives you a high — already for sale in Colorado. Luke Runyon/KUNC/Harvest Public Media hide caption
A marijuana bud and cash at a shop in Denver. An owner of marijuana stores in the city says of his company's bank account, "We treat it like gold." Ed Andrieski/AP hide caption
President Obama graces the cover of Cannabis Now magazine at the HempCon medical marijuana show, May 24, 2013, in Los Angeles. Robyn Beck/AFP/Getty Images hide caption
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