Researchers conducted brain scans on eight participants who used cannabis exclusively and five participants who used both tobacco and cannabis. They found that the latter group had higher levels of a key enzyme in multiple brain regions called fatty acid amide hydrolase (FAAH), an enzyme that interacts with the endocannabinoid system, which controls levels of a neurotransmitter associated with feelings of pleasure.
Recent animal studies suggest that FAAH may affect nicotine reward systems, but it remains uncertain whether this occurs in humans. Certain genetic variants in FAAH are also associated with an increased risk of addiction and elevated anxiety levels.
Lead researcher Rachel Rabin, a professor of psychology at McGill University in Canada, said: "This is the first evidence in humans of a molecular mechanism that may be behind why cannabis and tobacco users suffer worse outcomes."
The study did not include people who smoked only tobacco, meaning that the effects of tobacco alone on the brain have not yet been analyzed. Researchers hope to conduct further studies to understand whether cannabis exacerbates these changes or poses a greater risk to mental health when combined with tobacco.
Romina Mizrahi, co-author and psychiatrist, said: "We were amazed at how strong the effect was and how clear the difference was between those who used cannabis alone and those who combined it with tobacco."
The researchers explained that most current studies are observational, which prevents establishing a causal relationship, and that they often do not take into account the amount of tobacco or cannabis used, which could influence the results. However, it seems plausible that a combination of tobacco and cannabis would have different effects than cannabis alone, and future studies will reveal more about these effects.
The study was published in the journal Drug and Alcohol Addiction Reports.
