Secondhand exposure to cannabis smoke appears linked to signs of cannabis dependence, a researcher said at a Drug Enforcement Agency (DEA) Museum forum on marijuana.

The researcher, Adriaan W. Bruijnzeel, PhD, said at the forum that his team’s work with rats raises the prospect of a link between chronic passive exposure and addiction.

Dr. Bruijnzeel led a study in 20 rats – 10 controls and 10 that were exposed to an hour of cannabis smoke exposure 5 days a week for 8 weeks. The investigators reported that “exposure to cannabis smoke leads to clinically relevant Delta-9-tetrahydrocannabinol levels and development of dependence as assessed by somatic withdrawal signs” (PLoS One. 2016 Apr 11. doi: 10.1371/journal.pone.0153327 ).

“Immediately after the smoke exposure, you can detect high levels of THC [tetrahydrocannabinol], and then there’s a very quick drop,” Dr. Bruijnzeel , of the department of psychiatry at the University of Florida, Gainesville, said at the Oct. 5 forum at the DEA Museum in Arlington, Va. “Negative mood state associated with the cessation of drug intake helps to maintain the drug addiction.”

In an interview, Mark S. Gold, MD , an expert in addiction who serves as the chair of RiverMend Health’s scientific advisory boards, said Dr. Bruijnzeel’s findings offer lessons for clinicians, particularly for those who treat children.

“Like tobacco smoke, marijuana smoke is an environmental toxin that causes brain changes and addiction,” said Dr. Gold, the 17th Distinguished Alumni Professor at the University of Florida who is also with Washington University in St. Louis. “Second and thirdhand effects of cannabis are important new risks to consider when evaluating the children of marijuana smokers.”

Meanwhile, presenters at the forum, which was webcast , also described marijuana misuse as a threat to general mental health and the adolescent brain, as well as a danger to drivers.

Can preventing cannabis use reduce mental illness? “I think the answer is yes, but the pathway is likely to be fairly complicated and not as straightforward as causation,” said Arpana Agrawal, PhD , a professor of psychiatry at Washington University in St. Louis. “Without a doubt, reduction of cannabis, particularly heavy and persistent use, will likely assist in recovery from psychiatric illness.”

In regard to marijuana’s link to psychosis, in particular, the picture is complex, she said. “There does not appear to be much evidence for a straightforward causal model,” she said. “There’s some evidence of risk in genetically vulnerable individuals, there’s overwhelming support for shared biology; factors other than shared biology are likely to be important. And there’s also some support for increased correlation in the context of high potency use.”

Also, evidence suggests that cannabis is a “reverse gateway” drug among U.S. youth, as well as young people in Australia. “The idea is that youth actually initiate their substance use trajectories with marijuana and then work their way back to nicotine,” Dr. Agrawal said.

A stark warning about marijuana use came from Bertha K. Madras, PhD , professor of psychobiology in the department of psychiatry at Harvard Medical School, Boston; a former deputy director of demand reduction at the Office of National Drug Control Policy ; and a member of the President’s Commission on Combating Drug Addiction and the Opioid Crisis , chaired by Gov. Chris Christie (R-N.J.). “We’re not waging a war on drugs,” she said. “We are, in fact, defending our brain, which is the repository of our humanity. And supply reduction, which is what the DEA focuses on to some extent, is a form of prevention. ”

She encouraged adopting a prevention message to discourage all drug use by youth.

“The initiation [of marijuana use] in midteens is associated with much higher prevalence than initiation in adulthood, and daily use is clearly a very high risk factor for cannabis use disorder,” said Dr. Madras, also chair of the division of neurochemistry at Harvard.

Robert L. DuPont, MD , who moderated the forum, said heavy, chronic cannabis users show impairment of psychomotor skills linked to driving for as long as 3 weeks after last use. He also pointed to 2010-2014 data from Washington state that showed increases in the percentages of drivers involved in fatal crashes who tested positive for THC, mainly in addition to alcohol and/or other drugs.

There’s hope that a test will be developed to determine the cannabis tissue level that causes the equivalent impairment as the 0.08 g/dL blood alcohol content standard used to define drunken driving because there is no consistent relationship between THC levels and impairment, said Dr. DuPont, who is the first director of the National Institute on Drug Abuse, the second drug czar, and the president of the Institute for Behavior and Health in Rockville, Md.

Still, several states make driving illegal for drivers with any level of THC and/or THC metabolites, according to the Governors Highway Safety Association. THC can stay in the body for days after marijuana use, while metabolites can remain for weeks, according to the Marijuana Policy Project . An additional 16 states outlaw driving with specific THC levels.

For his part, Dr. DuPont said that, although addressing the opioid epidemic is a top national priority, the legalization of marijuana may be the more enduring threat to the nation’s public health; legalization would make it the third legal drug, joining alcohol and tobacco, which are the two leading causes of illness and death in the country.

“This is the third time in the last 45 years that drugs are front and center” in the United States, Dr. DuPont said. “The first one was heroin addiction related to crime in the early 1970s; the second was the crack epidemic in the late 1980s. In many ways, I think [marijuana is] the more important issue for us.”

Dr. Brijnzeel had no disclosures. Dr. Gold serves as chairman of the scientific advisory boards for RiverMend Health. Dr. Agrawal disclosed NIDA grants. Dr. Madras reported serving on the RiverMend advisory board and working with several organizations, such as the U.S. Department of Justice and the American Bar Association. Dr. DuPont also serves on the RiverMend advisory board.


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