The number of American adults using marijuana more than doubled in recent years, according to new research culling data from two massive surveys.

In 2001 just 4.1 percent of adults said they used marijuana. That increased to 9.5 percent by 2013. The findings were published yesterday in JAMA Psychiatry.

Researchers also found marijuana abuse or dependence increased during that 12-year period, probably because the overall number of adults using increased so much.

Increased marijuana use came during roughly the same timeframe that Americans' attitudes about legalizing the drug shifted; less than one-third of Americans were in favor of legalizing marijuana in 2002, while by 2013 a majority favored legalization, according to the Pew Research Center.

Starting in 2012, states began legalizing small amounts of marijuana for adult recreational use; now it's legal in four states and the District of Columbia. And medical marijuana is legal now in 23 states and in D.C., according to the National Conference of State Legislatures.

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Lead study author Deborah Hasin, a professor of epidemiology in psychiatry at Columbia University, said she's been particularly interested in tracking usage trends "given all the changes in attitudes and changes in laws."

But it's unclear what's behind such a dramatic increase in marijuana use. "We showed that it happened," Hasin said. "Now the thing that really needs to be researched is the why."

-- The Washington Post