Marijuana is illegal for any reason under federal law, but states have boldly experimented with allowing its use anyway, starting with California 20 years ago.
Some states have made the drug legal for medical purposes; others have removed jail sentences for carrying small amounts; and some let adults 21 and older use it for any reason.
Here's a look at where the states are on pot as well as legalization developments in other countries:
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LEGAL FOR MEDICAL USE
Eight states allow people with certain medical conditions to use marijuana, according to the Marijuana Policy Project, a pro-legalization group that tracks state pot laws.
Arizona, Illinois, Michigan, Montana, New Jersey, New Mexico, Pennsylvania and Vermont each have their own lists of ailments for which sufferers can use the drug with a doctor's recommendation.
The drug cannot legally be prescribed in any state, because it has no accepted medical use under federal drug law. But some doctors are willing to recommend it under certain conditions.
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LEGAL IF IT DOESN'T GET YOU HIGH
At least 17 states, many in the South, have passed laws opening the door to marijuana use as long as the drug is extremely low in THC, the intoxicating ingredient. The laws have emerged in the last three years following publicity about children with severe seizures benefiting from oils derived from marijuana.
Marijuana legalization activists often disregard these laws for being loaded with so many caveats that the drug isn't being used. The laws, which still violate U.S. law, exist in Alabama, Florida, Georgia, Idaho, Iowa, Kentucky, Mississippi, Missouri, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, Utah, Virginia, Wisconsin and Wyoming.
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NOT LEGAL BUT WON'T PUT YOU IN JAIL
Five states have removed the potential for jail time for those caught with small amounts of the drug. That means pot isn't legal for recreational use, but people smoking it to get high can't be put behind bars. Those states are Mississippi, Missouri, Nebraska, North Carolina and Ohio.
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COMBINATION OF THE ABOVE
A few states both have approved marijuana use by sick people and removed jail sentences for recreational users. One is California, whose voters passed the nation's first medical marijuana law in 1996. Others are Connecticut, Delaware, Maine, Maryland, Massachusetts, Minnesota, Nevada and Rhode Island.
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LEGAL FOR ADULTS OVER 21
Four states and Washington, D.C., allow marijuana possession in small amounts by adults over 21 for any reason. They are Alaska, Colorado, Oregon, Washington state and the nation's capital.
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INTERNATIONAL POT LAWS
Cannabis possession is illegal in most countries under a 1925 treaty called the International Opium Convention. But just like the U.S., some nations either flout the treaty or don't enforce it.
Legalization supporters consider pot possession either legal or tolerated in Argentina, Bangladesh, Cambodia, Canada, Chile, Colombia, the Czech Republic, India, Jamaica, Jordan, Mexico, Portugal, Spain, Uruguay, Germany and the Netherlands.
Each country has many caveats. Some consider the drug just as illegal as heroin but don't enforce the ban. Others, like Uruguay and the Netherlands, allow its recreational use.