By Tim Gaynor

(Reuters) - For the first time, a clear majority of Americans say they favor legalizing marijuana, as recreational and medical use of the drug gains acceptance across the nation, a poll released on Tuesday showed.

The Gallup poll found that 58 percent of those surveyed favored marijuana legalization, up from 50 percent two years ago. By contrast, 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 pot 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 aged 30 to 49 years remained high at 62 percent. The only age group clearly against legalization were those age 65 and over, where 53 percent were opposed.

The study said the shift could be attributed to changing social mores and growing social acceptance of marijuana. The increasing use of medical marijuana as a socially acceptable way to alleviate symptoms of diseases and mitigate side effects of chemotherapy may have also contributed to acceptance.

"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 study found that support for legalization was driven by independents, 62 percent of whom now favor legalization, compared with 50 percent in November 2012. Backing was higher among Democrats, at 65 percent, compared with 35 percent for Republicans.

Advocates of legalizing marijuana say taxing and regulating the drug could be financially beneficial to states and municipalities. But opponents, including some law enforcement and substance abuse professionals, cited health risks including an increased heart rate, and respiratory and memory problems.

The poll, based on telephone interviews conducted October 3-6, 2013, had a sampling error of plus or minus 4 percentage points.

(Editing by Cynthia Johnston and Steve Orlofsky)