| Summary: | Cannabis and tobacco are two of the most commonly used drugs in the world and their use often co-occurs. Cigarette smoking continues to be a global epidemic where novel drugs for smoking cessation are necessary. In chapter 1, I review the literature concerning cannabis, tobacco and co-used cannabis and tobacco, in relation to their prevalence and effects on cognition, addiction and psychosis. In chapter 2, I provide a 'worldwide' overview of routes of administration (ROA) of cannabis with and without tobacco (n=33, 687). Tobacco-based ROAs were most common in Europe (77.2-90.9%) and Australasia (20.7-51.6%) but uncommon in the Americas (4.4-16.0%). Tobacco-based ROAs were associated with reduced motivation to quit tobacco. In chapter 3, I describe the first investigation of the individual and interactive effects of cannabis and tobacco in a randomized, placebo-controlled, double-blind crossover design (n=24). I found tobacco may offset effects of cannabis on delayed recall, had no effect of cannabis-induced psychotomimetic or subjective effects and was more harmful for cardiovascular outcomes. In chapter 4, in the same sample, I found tobacco did not influence the rewarding effects of cannabis. In chapter 5, in the same sample, I developed an innovative "roll a joint" paradigm to assess quantity of both drugs. I found self-reported quantity was accurate for tobacco but overestimates cannabis exposure. In chapter 6 and 7, in a sample of overnight-abstinent dependent cigarette smokers (n=30), I investigated if cannabidiol (CBD) can reduce nicotine withdrawal. Results showed CBD reduced attentional bias and pleasantness ratings but increased errors on the go/no-go, compared to placebo. There were no effects on verbal episodic, working memory or delay discounting. Finally, in chapter 8, I summarise and integrate my findings into the literature, discuss implications, consider limitations and suggest future research on the interaction between cannabinoids and tobacco.
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