Researchers at McGill University have discovered that combining cannabis and tobacco use alters brain chemistry, potentially increasing anxiety and complicating quitting efforts. PET scans of young adults showed elevated levels of an enzyme that breaks down the mood-regulating 'bliss molecule' anandamide in co-users. The findings highlight a molecular mechanism behind worse outcomes for those using both substances.
A team from McGill University's Douglas Research Centre analyzed brain activity in 13 young adults to explore the effects of cannabis and tobacco co-use. Eight participants used only cannabis, averaging slightly more than one gram per day, while five also smoked cigarettes daily, ranging from one to 12 per day. The study, published in Drug and Alcohol Dependence Reports in 2025, used PET scans to measure levels of FAAH, an enzyme that degrades anandamide, known as the brain's 'bliss molecule' for its role in mood and stress regulation.
Co-users exhibited higher FAAH levels, leading to reduced anandamide, a pattern linked to anxiety, depression, and higher relapse rates in cannabis cessation. 'This is the first evidence in humans of a molecular mechanism that may underlie why people who use both cannabis and tobacco experience worse outcomes,' said lead author Rachel Rabin, an associate professor in McGill's Department of Psychiatry. The results suggest this imbalance explains why co-users face greater challenges quitting compared to cannabis-only users.
In Canada, about one in 20 past-year cannabis users is at risk for cannabis use disorder, rising to one in three among frequent users. Despite declining tobacco use, most cannabis users still co-use tobacco, a gap previous research overlooked by studying substances separately. 'What surprised us was how strong the effect was, and how different it was from those who only used cannabis, compared to those who used both tobacco and cannabis,' noted co-author Romina Mizrahi, professor of psychiatry and director of the McGill Research Center for Cannabis.
The small study's data originated from an unrelated project, lacking a tobacco-only group, so changes might stem from tobacco alone or an interaction. Funded by the National Institute of Mental Health, the team is now recruiting cigarette smokers and nicotine vapers without cannabis for follow-up to clarify tobacco's role. Current treatments for cannabis use disorder remain limited to behavioral therapies like counseling, underscoring the need for targeted medications.