Review finds mixed evidence on cannabis and tobacco co-use effects

RATIONALE: Cannabis and tobacco are widely used substances with addiction potential, which have opposing effects on cognition and subjective experience and are often co-used in a single occasion. OBJECTIVES: The purpose of this systematic review was to synthesize evidence of the acute effects of same-occasion co-use of cannabis and tobacco. METHODS: Databases Embase, Medline, CINAHL, PsycINFO, and Web of Science were searched from inception to May 2025. Inclusion criteria were broad. Included studies were peer-reviewed papers, theses, grey literature and abstracts. Designs included randomized controlled trials, laboratory studies, epidemiological studies, qualitative studies, case reports, before-and-after studies, cohort, case-control, cross-sectional, longitudinal, secondary data analysis and naturalistic studies. Papers were screened and selected by two authors and studies were evaluated for quality with the SIGN methodology. RESULTS: In total, 18 studies were included in the present synthesis, and some contained information on multiple outcomes. Five studies examined effects of same-occasion co-use of cannabis and tobacco on cognition; 12 studies evaluated same-occasion co-use on subjective experience; and seven studies evaluated effects on physiological measures. With respect to cognition, three studies found that cannabis and tobacco each attenuated the effects of the other, but the findings were not always consistent. For subjective experience, eight studies reported that the effects of same-occasion co-use of cannabis and tobacco did not differ from the effects of either drug on its own. Two studies of subjective experience found that tobacco dulled the effects of cannabis and two found that tobacco increased the stimulant properties of cannabis. CONCLUSIONS: In sum, the evidence for additive effects of cannabis and tobacco is mixed, with some studies reporting that cannabis and tobacco attenuate the effects of the other on cognition. There are inconclusive findings with respect to the combined effects of cannabis and tobacco on subjective effects and physiological outcomes. However, studies are limited and more large-scale studies that investigate the full range of diverse effects of cannabis and tobacco are needed.