Using marijuana may lead in short order to using even more marijuana as well as abusing other drugs and alcohol, suggests a new analysis of U.S. data.
Based on national surveys three years apart, researchers found that adults who reported using pot in the first survey were two to nine times more likely to have a substance abuse problem by the time of the second survey.
These risks should be considered not only by patients and doctors, but also by policymakers in states where marijuana may be up for legalization for recreational or medical use, experts say.
"Patients who may be considering using cannabis should know that by using cannabis they are approximately doubling their risk of developing a drug use disorder over the next few years," said senior study author Dr. Mark Olfson of Columbia University Medical Center and the New York State Psychiatric Institute in New York City.
"Patients who already use cannabis should be aware that increasing their use may further increase their risk of developing a substance use disorder, while reducing or stopping their cannabis use is likely to reduce that risk," he told Reuters Health by email.
Olfson and colleagues analyzed survey responses from a nationally representative sample of U.S. adults interviewed in 2001-2002 and again in 2004-2005. More than 34,650 people responded, almost evenly split among men and women, with a mean age of about 45.
In the first survey, 1,279 individuals reported using cannabis. Three years later, that was linked to a nearly three times higher rate of abusing alcohol compared to people who didn't use cannabis in the first survey. The risk of abusing other drugs or being dependent on tobacco was twice as high, and the risk of having a cannabis abuse disorder by the second survey was nine times higher.
Contrary to some past studies, the researchers did not find any link between cannabis use and the development of mood disorders like depression or anxiety or with any other mental illnesses.
The study team points out that these associations do not prove cannabis use causes other substance abuse problems.
Still, they write in JAMA Psychiatry, there could be overlap in brain "circuitry" that drives drug use and dependence. Shared mechanisms between marijuana and other substances "may contribute to the association of cannabis use with (substance use disorders) but not with most other disorders examined," they explain.
"Use of cannabis can also lead to behavioral disinhibition, which increases the likelihood of use of other substances and the risk of abuse or dependence on those substances," the authors add.
"Policymakers who may have to vote on legalization of marijuana should consider potential adverse effects of marijuana use on the risks of developing other drug and alcohol abuse problems," Olfson said. "In states with marijuana laws that permit recreational marijuana use, regulators and public health officials should develop means of monitoring and communicating this risk."
"By studying the effects of existing state-to-state variation in marijuana laws in relation to key outcomes, such as cannabis-related traffic injuries and fatalities, emergency department mentions, poison control calls, and admissions to addiction facilities, it might be possible to increase our understanding of the public health effects of marijuana legislation," he added.
Dr. Nora Volkow, director of the National Institute on Drug Abuse, told Reuters Health by email, "These results delineate more clearly the outlines of the possible adverse psychiatric outcomes associated with cannabis use. Patients, doctors, advocates, and policymakers should understand that cannabis use, even for medicinal purposes, is associated with a clear risk of developing cannabis and/or other substance use disorders. This knowledge should be incorporated into clinical care and policy planning."
The message coming out of this and similar studies, Volkow said, is that marijuana is not a benign drug.
"The potential for chronic cannabis exposure to induce or exacerbate various mental illnesses is an area of continuous debate," she added.
"This particular study is a step forward in that we can now state with a higher degree of confidence that cannabis use is associated with an increased prevalence and incidence of substance use disorders but less likely to be linked with any mood disorder or anxiety disorder," Volkow said.