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The poll found that 58 percent of those surveyed favored marijuana legalization, up from 50 percent two years ago.
When Gallup first asked the question in 1969, only 12 percent favored allowing the drug.
Washington state and Colorado became the first states to legalize marijuana for recreational adult use by approving separate ballot measures in November 2012. Some 20 states and the District of Columbia allow the drug to be used for medical purposes.
The poll, which drew on a random sample of 1,028 adults living in all 50 states and the District of Columbia, found support for legalization was strongest among 18- to 29-year-old adults, 67 percent of whom were in favor.
Backing for legalization among Americans ages 30 to 49 remained high at 62 percent. The only age group clearly against legalization were those age 65 and over, where 53 percent were opposed.
"Whatever the reasons for Americans' greater acceptance of marijuana, it is likely that this momentum will spur further legalization efforts across the United States," the study said.
The poll, based on telephone interviews conducted Oct. 3 through 6, had a sampling error of plus or minus 4 percentage points.
Al Jazeera and Reuters
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