Smoking marijuana just once a week can have a “significant” negative effect on the brains of teenagers and young adults, including cognitive decline, poor attention and memory, and decreased IQ, according to psychologists discussing the health implications of legalizing marijuana at the American Psychological Association’s 122nd Annual Convention.
“It needs to be emphasized that regular cannabis use, which we consider once a week, is not safe and may result in addiction and neurocognitive damage, especially in youth,” said Krista Lisdahl, Ph.D., director of the brain imaging and neuropsychology lab at the University of Wisconsin-Milwaukee.
She noted that marijuana use is increasing, pointing to a 2012 study that found that 6.5 percent of high school seniors reported smoking marijuana daily, up from 2.4 percent in 1993. Additionally, 31 percent of young adults between the ages of 18 and 25 reported using marijuana in the last month.
People who are addicted to marijuana can lose an average of six IQ points by adulthood, according to Lisdahl, who referred to a 2012 study of 1,037 participants who were followed from birth to age 38.
Brain imaging studies of regular marijuana users have shown significant changes in their brain structure, particularly among adolescents, according to Lisdahl.
“Abnormalities in the brain’s gray matter, which is associated with intelligence, have been found in 16- to 19-year-olds who increased their marijuana use in the past year,” she said.
“These findings remained even after researchers controlled for major medical conditions, prenatal drug exposure, developmental delays, and learning disabilities,” she added.
“When considering legalization, policymakers need to address ways to prevent easy access to marijuana and provide additional treatment funding for adolescent and young adult users,” she said.
She also recommends that legislators consider regulating the levels of tetrahydrocannabinol, or THC, the major psychoactive chemical in marijuana, to help reduce potential neurocognitive effects.
“Some legalized forms of marijuana have higher levels of THC than other strains,” said Alan Budney, Ph.D., of Dartmouth College, who noted that THC is responsible for most of marijuana’s psychological effects.
“Past research has shown that frequent use of high potency THC can increase the risk of future problems with depression, anxiety, and psychosis,” he added.
“Recent studies suggest that this relationship between marijuana and mental illness may be moderated by how often marijuana is used and potency of the substance,” Budney said.
“Unfortunately, much of what we know from earlier research is based on smoking marijuana with much lower doses of THC than are commonly used today.”
Additionally, people’s acceptance of legalized medical marijuana use appears to have an effect on adolescents’ perception of the drug’s risks, according to Bettina Friese, Ph.D., of the Pacific Institute for Research and Evaluation in California.
She presented results from a 2013 study of 17,482 teenagers in Montana, which found marijuana use among teenagers was higher in counties where larger numbers of people voted to legalize medical marijuana in 2004. In addition, teens in counties with more votes for the legalization of medical marijuana perceived marijuana use to be less risky.
“These findings suggest that a more accepting attitude toward medical marijuana may have a greater effect on marijuana use among teens than the actual number of medical marijuana licenses available,” she noted.