According to research derived from two major surveys, the number of adults using marijuana has more than doubled in recent years, the Washington Post reports.

From the year 2001 to 2013, the number of adults using marijuana has increased from 4.1 percent to 9.5 percent.

The findings were published Wednesday in JAMA Psychiatry.  

Researchers analyzed two rounds of the National Epidemiologic Survey on Alcohol and Related Conditions.  In the 2001-2002 round, 43,093 people participated, while 36,309 participated in the 2012-2013 round.

The researchers noted that marijuana abuse or dependence increased during that 12-year time frame. Also, the increase in marijuana use coincides with the shift in Americans' attitudes about legalizing the drug. 

In 2002, less than one third of Americans were in the favor of legalizing marijuana, while a majority of Americans favored the legalization of marijuana in 2013. From 2012 onwards, several states in the U.S. started legalizing small amounts of marijuana for recreational use. While the recreational use of marijuana is now legal in four states and the District of Columbia, medical marijuana is legal in 23 states and the District. 

Lead study author Deborah Hasin, a professor of epidemiology in psychiatry at Columbia University, said,

"You can speculate that Americans are increasingly viewing marijuana as a harmless substance... or laws are changing," she added. "But we don't really know until you do good, empirical studies on what factors are really influencing it."

Hasin said it's important to know of the risks associated with marijuana use.

"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 authors write.  "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."