Marijuana users had smaller hippocampal volumes, and combined marijuana-nicotine users showed an inverted brain-behavior pattern where smaller hippocampus was associated with better memory performance.
Read this if you use both marijuana and tobacco and are curious about the combined brain effects.
Inverted brain-behavior relationship unique to combined MJ+nicotine use
What the researchers found
Researchers compared hippocampal brain volumes and memory performance across four groups: marijuana-only users (n=36), nicotine-only users (n=19), combined marijuana+nicotine users (n=19), and non-using controls (n=16).
Marijuana and combined marijuana+nicotine groups had smaller total hippocampal volumes compared to nicotine-only and control groups. No significant group differences were found in immediate or delayed story recall performance.
The most intriguing finding was a brain-behavior interaction: in controls, larger hippocampal volume correlated with better memory as expected. But in combined marijuana+nicotine users, this relationship was inverted: smaller hippocampal volume was associated with better memory scores. This unique inversion suggests abnormal brain-behavior relationships in combined users that may reflect compensatory neural reorganization.
Why it matters
The inverted brain-behavior relationship in combined marijuana+nicotine users suggests that the brain may reorganize memory processes when both substances are used together. Understanding this interaction is important because combined use is extremely common.
The numbers in context
4 groups: MJ-only (n=36), Nic-only (n=19), MJ+Nic (n=19), controls (n=16). MJ and MJ+Nic groups had smaller hippocampal volumes. No memory performance differences between groups. Inverted volume-memory relationship in MJ+Nic group.
How the study worked
Cross-sectional study comparing hippocampal volumes (MRI) and memory performance (WMS-III logical memory) across four groups drawn from two larger studies. Total bilateral hippocampal volumes were compared controlling for total brain size and recent alcohol use.
What this study cannot tell us
Small sample sizes, especially for nicotine-only (19) and controls (16). Cross-sectional design cannot determine causation or temporal direction. Groups were drawn from two separate studies, introducing potential batch effects. The inverted relationship may be a statistical artifact in a small sample.
How to read the evidence
Small cross-sectional study combining data from two prior studies. Novel finding but requires replication.
When this study was published
Published in 2015. Research on combined cannabis-tobacco brain effects has continued.
The bigger picture
Most cannabis research studies marijuana in isolation, but the majority of cannabis users also use tobacco. This study shows that the combined effect on the brain is not simply additive but produces unique neurobiological patterns that researchers are only beginning to understand.
Questions still open
- What neural mechanisms explain the inverted volume-memory relationship in combined users? Does the interaction change with duration of combined use? Would other memory measures show similar patterns?
Common questions
Does marijuana shrink the hippocampus?
Is using marijuana and tobacco together worse for the brain?
Read the original research
Combined effects of marijuana and nicotine on memory performance and hippocampal volume.
Behavioural brain research, 293, 46-53
Citation
Filbey, Francesca M; McQueeny, Tim; Kadamangudi, Shrinath; Bice, Collette; Ketcherside, Ariel. (2015). Combined effects of marijuana and nicotine on memory performance and hippocampal volume.. Behavioural brain research, 293, 46-53. https://doi.org/10.1016/j.bbr.2015.07.029
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