By Andrew M. Seaman

(Reuters Health) - As attitudes and laws in the U.S. have become more tolerant of marijuana, the proportion of adults using and abusing the substance at least doubled between 2001 and 2013, according to a new study.

Although marijuana dependence and abuse was found to be on the rise, that is largely due to the overall increase in new users, researchers note, while existing marijuana users experienced a 15 percent decline in pot-related disorders.

About 4 percent of adults between 2001 and 2002 reported having used marijuana in the past year, compared to about 10 percent between 2012 and 2013. Similarly, 1.5 percent had abuse or dependence problems - so-called marijuana use disorder - at the start of the 21st Century, compared to about 3 percent from 2012 to 2013.

"What was quite clear is the prevalence of use among adults had more than doubled," said Deborah Hasin, the study's lead author from Columbia University in New York.

Twenty-three U.S. states allow for medical marijuana use, and four also allow recreational use, the researchers write in JAMA Psychiatry. More Americans also favor marijuana legalization than before, and fewer see the substance as risky.

Yet, the researchers write, little was known about how the prevalence of marijuana use and disorders changed during the first decade of the century.

For the new study, they compared data from face-to-face interviews with over 43,000 U.S. adults between 2001 and 2002 to data from over 36,000 people collected between 2012 and 2013.

They found that reports of having used marijuana in the past year more than doubled between the two time periods with especially large increases among women, blacks, Hispanics, southerners and middle-aged and older people.

The data also included information on whether participants were experiencing abuse or dependence.

Between the two time periods, the researchers found that such disorders were about twice as common during the 2012 and 2013 interviews. About three of every 10 marijuana users - about 7 million Americans - were diagnosed with a marijuana use disorder in the latter survey, they write.

Past studies have tied marijuana use to mental health issues, poor quality of life, use of other drugs and withdrawal after heavy use, they add.

Earlier this year, the American Academy of Pediatrics restated their opposition to marijuana legalization based on its well-documented negative effects on the developing brains of children and adolescents, such as impaired memory and problem solving (see Reuters Health story of January 26, 2015 here: http://reut.rs/1GiiDI9).