Researchers are changing how they study marijuana’s effects on the brain. Historically, many studies excluded tobacco users, but work from the Center for BrainHealth at The University of Texas at Dallas shows that excluding tobacco use can miss important differences: people who use both marijuana and tobacco have distinct brain and memory patterns compared with those who use marijuana alone.
In a study published in Behavioural Brain Research, investigators report that marijuana use is associated with smaller hippocampal volume. The hippocampus, a critical structure for learning and memory, was significantly smaller in both groups that used marijuana (marijuana-only and marijuana-plus-tobacco) compared with nonusing controls and people who used only tobacco. Yet the relationship between hippocampal size and memory performance differed across groups, suggesting complex interactions between marijuana and nicotine on brain structure and cognitive function.
Among nonusers, hippocampal size followed the expected pattern: smaller hippocampal volumes correlated with poorer memory performance. For people who used both marijuana and tobacco, however, the relationship reversed: smaller hippocampal volumes were associated with better memory scores. That surprising inversion was not observed in participants who used only marijuana or only tobacco. Within the combined-use group, the number of cigarettes smoked per day appeared related to the extent of hippocampal shrinkage—the more cigarettes smoked daily, the smaller the hippocampal volume—but this was accompanied by relatively better memory performance in that subgroup.
“Approximately 70% of individuals who use marijuana also use tobacco,” said Francesca Filbey, Ph.D., the study’s lead investigator and Director of Cognitive Neuroscience of Addictive Behaviors at the Center for BrainHealth. She emphasized that because most prior research has not accounted for tobacco use, findings about marijuana’s effects on the brain may not apply to the majority of people who consume marijuana in real-world settings. This study is among the first to separate the unique and combined impacts of marijuana and nicotine on brain anatomy and memory.
The research team used magnetic resonance imaging (MRI) to measure hippocampal volume and administered standardized neuropsychological tests to assess memory. Participants completed a detailed substance use history and cognitive testing approximately three days before undergoing MRI scanning. Four groups were compared: nonusers (no marijuana or tobacco in the past three months), chronic marijuana users (at least four marijuana uses per week), frequent nicotine users (10 or more uses per day), and combined users (at least four marijuana uses per week plus 10 or more nicotine uses per day).
Results controlled for total brain size and recent alcohol use. Both the marijuana-only and marijuana-plus-nicotine groups showed smaller total bilateral hippocampal volumes than controls and nicotine-only users. Although there were no clear group differences in immediate versus delayed story recall on the memory test, the study highlighted altered brain–behavior relationships: controls tended to show larger hippocampal volume associated with better memory, while the combined-use group exhibited an opposite trend.
Filbey noted that the interaction between marijuana and nicotine is unlikely to be a simple additive effect. Different neurobiological mechanisms appear to be at play when both substances are used together, producing complex and sometimes counterintuitive patterns in structure–function relationships. She urged future studies to explicitly consider combined substance use—common in forms such as blunts (marijuana wrapped in tobacco leaf) and spliffs (marijuana rolled with tobacco)—to better understand real-world impacts on memory and behavior.
Funding: This study was supported by grants from the National Institute on Drug Abuse / National Institutes of Health (K01 DA021632, FMF).
Source: Emily Bywaters, UT Dallas Center for BrainHealth.
Image source: Public domain image used for illustration.
Original research: “Combined effects of marijuana and nicotine on memory performance and hippocampal volume” by Francesca M. Filbey, Tim McQueeny, Shrinath Kadamangudi, Collette Bice, and Ariel Ketcherside, published in Behavioural Brain Research (published online May 25, 2015). DOI: 10.1016/j.bbr.2015.07.029.
Abstract
Combined effects of marijuana and nicotine on memory performance and hippocampal volume
The combined use of marijuana (MJ) and tobacco is common. Prior research has linked individual use of either substance to structural brain changes and altered cognitive function, particularly reductions in hippocampal volume and changes in memory. This study examined both individual and combined effects of MJ and nicotine on hippocampal volume and memory in four adult groups drawn from larger studies: MJ-only (n = 36), nicotine-only (n = 19), combined MJ + nicotine (n = 19), and non-using healthy controls (n = 16). After controlling for total brain size and recent alcohol use, MJ and MJ + nicotine groups showed smaller total hippocampal volumes than nicotine-only users and controls. Immediate and delayed story recall did not differ significantly across groups. Controls displayed a trend in which larger hippocampal volumes correlated with better memory scores; in contrast, combined MJ + nicotine users showed a unique inversion, where smaller hippocampal volumes were associated with better memory. These findings point to abnormalities in brain–behavior relationships related to memory when marijuana and nicotine are used together, underscoring the need for further research into complex interactions between these substances.