Tobacco and cannabis are two of most widely used drugs in the world, with recent estimates demonstrating 22.5% of adults using tobacco products and 3.5% using cannabis. There is substantial overlap in their use, with approximately 90% of cannabis users also identifying as cigarette smokers. Furthermore, cigarette use increases during cannabis abstinence, and decreases following renewal of cannabis use suggesting direct substitution between the drugs.

The causal relationship between cannabis and tobacco use is complex. The ‘gateway’ hypothesis suggests cigarette smoking is a precursor (‘gateway drug’) to cannabis use, whereas the ‘reverse gateway’ hypothesis suggests cannabis use predicts tobacco smoking onset. To complicate matters, both substances are often used concurrently (‘mulling’) and sensitisation may occur, in which smoking tobacco may enhance the subjective effects of cannabis.

In a recent study published in Drug and Alcohol Dependence, the authors investigated the degree to which cigarette smoking predicted cannabis dependence both cross-sectionally...