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

itoggle caption Carline Jean/MCT/Landov

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

itoggle caption Courtesy of Grenco Science

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

itoggle caption Tom Benitez/MCT /Landov

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

itoggle caption Tim Roske/AP

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

itoggle caption Ted S. Warren/AP

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

itoggle caption Menahem Kahana/AFP/Getty Images

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

itoggle caption Eric Whitney

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

itoggle caption Elaine Thompson/AP

Sen. Patrick Leahy is calling on the Justice Department to state its position on marijuana's legal status. Here, a man inspects a shirt depicting the U.S. flag made of marijuana symbols, at a medical marijuana show in Los Angeles earlier this year. Robyn Beck/AFP/Getty Images hide caption

itoggle caption Robyn Beck/AFP/Getty Images

Medical marijuana advocates demonstrate outside a San Francisco fundraiser for President Obama in February. Justin Sullivan/Getty Images hide caption

itoggle caption Justin Sullivan/Getty Images

Brian Lawson rolls a marijuana cigarette at the BC Marijuana Party Headquarters in Vancouver, British Columbia. Vancouver is in the marijuana-friendly corner of Canada. Jae C. Hong/AP hide caption

itoggle caption Jae C. Hong/AP

Even if students have a prescription for pot, marijuana remains illegal under federal law. Colleges that let students self-medicate on campus could jeopardize their federal funding. Jeff Barnard/AP hide caption

itoggle caption Jeff Barnard/AP

In July, protesters rally in San Francisco against a Drug Enforcement Agency memo they believed would lead to prosecution of individuals in compliance with California medical marijuana laws.

Justin Sullivan/Getty Images hide caption

itoggle caption Justin Sullivan/Getty Images

Jars full of medical marijuana are seen at a medical marijuana dispensary in Los Angeles. U.S. attorneys sent letters telling more than a dozen of the shops to shut down.

Kevork Djansezian/Getty Images hide caption

itoggle caption Kevork Djansezian/Getty Images

The scientists who sequenced the marijuana plant say they hope the next generation of pot will have stronger therapeutic compounds. JUAN MABROMATA/AFP/Getty Images hide caption

itoggle caption JUAN MABROMATA/AFP/Getty Images