The percentage of U.S. adults reporting they used marijuana was found to have more than doubled between 2001 and 2013, rising from just over four percent to 9.5 percent, a new study shows.
Abuse of marijuana or a dependence on it also grew during that period, most likely a result of the overall increase in the numbers of people using marijuana, researchers reported.
The rise in use mirrored a change in U.S. attitudes; while less than a third of American adults surveyed in 2002 said they would be in favor of the legalization of marijuana, a majority were for legalization by 2013, the Pew Research Center says.
Marijuana is now legal for recreational use in four U.S. states and the District of Columbia, and marijuana for medical use has been legalized in 23 states and in the district.
Increases in marijuana use in the 12 years analyzed were particularly prevalent among women, blacks and Hispanics, people middle-aged or older and in Southern U.S. states, the study authors say.
Deborah Hasin, a professor of epidemiology and psychiatry at Columbia University and the study's lead author, gives "all the changes in attitudes and changes in laws" as one reason for her interest in tracking usage trends.
Although the study confirmed an increase in usage, it could not say exactly what might be driving it, she says.
"You can speculate that Americans are increasingly viewing marijuana as a harmless substance ... or laws are changing," she suggested. "But we don't really know until you do good, empirical studies on what factors are really influencing it."
For the study, the researchers analyzed and compared two National Epidemiologic Survey on Alcohol and Related Conditions surveys, one from 2001-2002 and one from 2012-2013.
Along with the doubling of overall usage, the percentages of marijuana use disorder — defined as either dependence or abuse — rose from 1.5 percent in the first survey to 2.9 percent in the later one, they said.
That translates to one in three adult users of marijuana, or about 6.8 million Americans, having a diagnosis of marijuana use disorder, the researchers say.
"In summary, while many in the United States think prohibition of recreational marijuana should be ended, this study and others suggest caution and the need for public education about the potential harms in marijuana use, including the risk for addiction," the study authors say.
As with alcohol, a large number of individuals may use marijuana without a risk of becoming addicted, they note, but as the numbers of U.S. users grow, so likely will the group of people experiencing problems linked to marijuana use.
"This information is important to convey in a balanced manner to health care professionals, policy makers and the public," the study appearing in JAMA Psychiatry concludes.
