The estimated prevalence of adults who used marijuana in the past year more than doubled in the United States between 2001 and 2013 to 9.5 percent, according to an article published online by JAMA Psychiatry.

Laws and attitudes about marijuana are changing, with 23 states having medical marijuana laws and four of these states having also legalized marijuana for recreational use.

Bridget F. Grant, Ph.D., of the National Institute on Alcohol Abuse and Alcoholism, Rockville, Md., and coauthors used nationally representative data on past-year prevalence rates of marijuana use, marijuana use disorder and marijuana use disorder among marijuana users in the United States. Data came from the 2001-2002 National Epidemiologic Survey on Alcohol and Related Conditions (NESARC) and the 2012-2013 National Epidemiologic Survey on Alcohol and Related Conditions-III (NESARC-III).

The prevalence of past-year marijuana use climbed to 9.5 percent of adults in 2012-2013 from 4.1 percent in 2001-2002, with increases particularly notable among women and individuals who were black, Hispanic, living in the South, middle-aged or older, the authors report.

The prevalence of a diagnosis of a past-year marijuana use disorder (abuse or dependence) also increased to 2.9 percent in 2012-2013 from 1.5 percent in 2001-2002, which means nearly 3 of every 10 Americans who used marijuana in the past year had a diagnosis of a marijuana use disorder (approximately 6.8 million Americans). Groups with notable increases included individuals ages 45 to 64 and those individuals who were black or Hispanic, with the lowest incomes or living in the South.

Among marijuana users, the prevalence of marijuana use disorder decreased to 30.6 percent in 2012-2013 from 35.6 percent in 2001-2002. Because there was no increase in the risk for marijuana use disorder found among users, in fact there was a decrease, the increase in prevalence of marijuana use disorders can be attributed to the increase in marijuana users between the two surveys, the authors note.

"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. As is the case with alcohol, many individuals can use marijuana without becoming addicted. However, the clear risk for marijuana use disorders among users (approximately 30 percent) suggests that as the number of U.S. users grows, so will the numbers of those experiencing problems related to such use. This information is important to convey in a balanced manner to health care professionals, policy makers and the public," the study concludes.