A new study, published in JAMA Psychiatry, showed cannabis use in the U.S. has more than doubled.

The study, by scientists from the National Institute on Alcohol Abuse and Alcoholism, compared cannabis use in 2001–2002 at 4.1 per cent to 9.5 per cent in 2012–2013, a significant increase that is coupled with shifting laws and attitudes.

“I’m not too surprised by the results,” said UCLA Addiction Medicine Clinic director Dr. Larissa Mooney, who was not involved with the research. “There’s an increase in prevalence of marijuana use because it’s become more and more available.”

In the U.S., 23 states have legal medical cannabis, with four states legalizing cannabis for recreational use.

Mooney said legalization is a large factor in the substance’s increased use, and the change in perception that it is less dangerous.

“A greater number of people view marijuana as less risky,” Mooney said. “There are less perceived harms.”

With cannabis offering medical benefits, Mooney said a balanced view needs to be taken that also looks at the potential risks of use.

“Some people use it recreationally and don’t become addicted, but for a subset of users, marijuana can be addictive,” Mooney said. “There are many factors that contribute to someone’s vulnerability. A family history of addiction, genetics, social factors have a role. Having a co-occurring psychiatric disorder or a history of trauma or abuse.”