Table 1.

Summary of studies examining the relationship between cannabis and tobacco use

Author (Year)	Study Population	Design	Key Findings
Filbey et al., (2015)	▪36 MJ, 19 Tob, 19 MJ+Tob, 16 HC ▪Age range: 18–50	▪Cognitive/MRI	▪Smaller hippocampal volumes in all MJ using groups. ▪Inverse relationship between memory and hippocampal volume was noted in comorbid users whereas in HC’s a positive correlation was observed.
Wetherill et al., (2015)	▪19 MJ, 24 Tob, 23 MJ+Tob, 21 HC ▪Age range: 20–57	▪Resting state fMRI	▪Comorbid users demonstrated reduced connectivity between the PCC and temporal cortex. MJ and Tob were independently associated with differential connectivity patterns between the PCC, mPFC and cerebellar and parahippocampal regions.
Wetherill et al., (2015)	▪19 MJ, 21 Tob, 21 MJ+Tob, 21 HC ▪Mean age range: 28–34	▪Structural MRI	▪Comorbid users demonstrated larger gray matter volume in the left putamen. Smaller thalamus gray matter volume was observed in all MJ groups and smaller gray matter in the left cerebellum was observed in all Tob groups.
Leroy et al., (2011)	▪14 Tob, 13 MJ+Tob, 11 HC ▪Mean age range: 25.9–30.2	▪PET – HRRT/MRI	▪DAT binding and availability was significantly reduced in comorbid and Tob users in striatal regions. DAT availability was also reduced in extrastriatal regions in both substance-using groups.
Karoly et al., (2015)	▪14 MJ, 34 Tob, 17 MJ+Tob, 12 Alcohol, 17 MJ+Tob +Alcohol, 38 HC ▪Age range: 14–18	▪Task-dependent fMRI: MID	▪The Tob using group demonstrated decreased activation to reward anticipation in the bilateral NAcc compared to all other substance using groups except MJ.
Crane et (2015)	▪Baseline: n = 1108; Age range: 15–16 ▪At 6 years: n= 1064; Age range: 21–22	▪Longitudinal study	▪Higher frequency of cigarette use was related al., to increased frequency of MJ use, which was significantly correlated, with increased symptoms of depression, especially in males.
Schuster et al., (2015)	▪64 MJ ▪Mean age: 20.81	▪Cognitive	▪Higher levels of past year MJ use were associated with poorer episodic memory among sporadic Tob users but not among consistent Tob users.
Jansma et al., (2013)	▪10 NAD, 11 HC ▪Mean age range: 21.2–25.6	▪Pharmacological fMRI Task – MID	▪THC administration was found to significantly reduce activity in the nucleus accumbens and caudate putamen during the reward anticipation phase in NAD subjects.
Rubinstein et al., (2014)	▪165 participants ▪Age range: 13–17	▪General linear model analysis of MJ use and NAD	▪Frequency of MJ use was positively correlated with nicotine addiction across all measures of dependence
Bonn-Miller et al., (2011)	▪39 Tob, 34 MJ, 82 MJ+Tob, 67 HC ▪Mean age: 22.43	▪Clinical	▪The tobacco-only group reported significantly higher negative affectivity with regard to anxiety and depressive symptoms compared to the other 3 groups. ▪The combined user group also reported greater anxiety symptoms than the MJ only group and non-users.
Dube et al., (2015)	▪271 male, 319 female ▪Age range: 17–24	▪Clinical	▪A “U” shaped association was observed between cannabis use and change in adiposity in male and female non-smokers. ▪An inverted “U” shape association between cannabis use and change in adiposity was observed in male smokers.
Gage et al., (2014)	▪1756 participants ▪Age: ~18	▪Clinical	▪Both MJ and Tob use were similarly associated with psychotic experiences although a majority of MJ users also reported Tob use.

MJ = Marijuana/Cannabis; Tob = Tobacco/Nicotine; HC = Healthy Control; MRI – Magnetic Resonance Imaging; fMRI – functional MRI; MID – Monetary Incentive Delay; THC – Δ⁹-tetrahydrocannabinol; PET-HRRT – Positron Emission Tomography – High Resolution Research Tomograph; DAT – Dopamine transporter; NAD – Nicotine addiction; PCC – posterior cingulate cortex; mPFC – medial prefrontal cortex; NAcc – Nucleus accumbens