SAN FRANCISCO (CBS SF) – As a growing number of states relax laws on the medical and recreational use of marijuana, a newly-released study estimates the number of users grew by 10 million over a 12 year span.

The study, published in The Lancet Psychiatry, analyzed the data of 596,500 adults who took part in the U.S. National Survey on Drug Use and Health from 2002 and 2014.

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Researchers found in 2014, 13.3 percent of American adults admitted using marijuana in the past year, compared to 10.4 percent in 2002. When extrapolated to the U.S. population, the study’s authors said the number of marijuana users grew from 21.9 million to 31.9 million in that time frame.

The number of heavy marijuana users (defined as five days or more per week) nearly doubled between 2002 and 2014, from 1.9 percent to 3.5 percent.

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While the number of marijuana users has increased, the researchers also found significantly fewer people finding marijuana use risky. In 2002, nearly half of Americans perceived a “great risk of harm” by smoking marijuana once or twice a week. Twelve years later, only 33 percent expressed that sentiment.

“Understanding patterns of marijuana use and dependence, and how these have changed over time is essential for policy makers who continue to consider whether and how to modify laws related to marijuana and for health-care practitioners who care for patients using marijuana,” Dr. Wilson M. Compton of the National Institutes of Health, the study’s author, said in a statement.

California was among the first states to allow marijuana for medicinal purposes, after voters passed Proposition 215 in 1996. Two dozen other states and the District of Columbia have also legalized medical marijuana.

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Proposition 64, a measure that would allow for the recreational use of marijuana, is on the November ballot.