Teenagers who see marijuana as a relatively low-risk drug may be more at risk for addiction than they realize, a new Yale paper says.

Co-occuring cannabis and tobacco use is significantly more likely to cause cannabis abuse and dependence than only cannabis use, according to a paper by Yale researchers accepted by the peer-reviewed journal Addiction last month. Psychiatry postdoctorate fellow Erica Peters said that she noticed consistent evidence across the literature of her conclusion and conducted the review to build momentum for the area of study. She said her study did not answer questions about the chemical pathways or the specifics of the drug interactions, but opened the door for future research.

“There’s something about tobacco use that seems to worsen marijuana use in some way,” Peters said.

She said that both simultaneous use — in the form of blunts, marijuana in a cigar wrapper — and use on separate occasions make cannabis more addictive. Co-occuring use also intensifies psychosocial problems, such as anxiety, depression and legal difficulties. The study found that adolescents were less likely to have good grades and more likely to have drunk alcohol in the past month.

Ninety percent of cannabis users also smoke tobacco, said Arpana Agrawal, an assistant professor of psychiatry at Washington University in St. Louis. By contrast, only 40 to 50 percent of tobacco users also smoke cannabis, she said.

“If you’re going to tackle one, you have to consider the other,” she said.

Agrawal said that for the first time, the Yale paper provides a “really comprehensive, systematic” understanding of cannabis dependence and abuse over the lifetime of an individual who smokes tobacco and cannabis.

The important next step is to understand the best approaches to clinical treatment for co-occuring users, Peters said. Clinicians have yet to develop a effective treatment for cannabis dependence, so understanding that tobacco affects the majority of cannabis users is important, Agrawal said.

Another of Agrawal’s papers examining nongenetic factors that motivate youth to use cannbis and marijuana was accepted by late month’s issue of Addiction. She hypothesized in the paper that the similar act of smoking may cause cannibis and tobacco use to reinforce each other.

The study reinforces the conclusions of existing literature on polysubstance use, RAND Drug Policy Research Center co-director Rosalie Pacula said in an email. Whether “cannabis and tobacco, alcohol and cocaine, alcohol and cigarettes, or cocaine and meth,” polysubstance use generally leads to more significant dependence and psychosocial problems, she said.

Yale Medical School psychiatry professor Kathleen Carroll and University of Arkansas psychiatry professor Alan Budney co-authored the paper.