Reverse Gateway
In addition, there is a less-studied reverse gateway theory that earlier regular cannabis use predicts later tobacco initiation and/or nicotine dependence in those who did not use tobacco before. One Australian study appears to have found such a correlation. However, they admittedly could not rule out the fact that cannabis is commonly (in many countries, including Australia and most of Europe) mixed with tobacco in joints to enhance burning and/or stretch supplies. Additionally, though not mentioned in the study, blunts (tobacco-leaf cigars filled with cannabis) are commonly used by some smokers in the United States as well, despite tobacco-mixed joints being relatively uncommon there. Such "traditions," as opposed to cannabis per se, could plausibly act as a "Trojan horse" for developing a nicotine habit.
Cannabis use only predicted later nicotine dependence in non-dependent tobacco smokers who also used cannabis daily, and even for them cannabis had no significant effect on the likelihood of quitting tobacco.
An American study also found a modest association between earlier cannabis use and later daily smoking (and nicotine dependence) as well, specifically among those who had smoked at least one tobacco cigarette. However, the results were not always statistically significant, and said nothing about the risk of initiating tobacco use. The smoking of blunts was also not addressed.
On the other hand, a more recent Australian twin study casts doubt on the hypothesis of a truly causal relationship between early (or any) cannabis use and later nicotine dependence. Rather, it was found to be largely due to common genetic factors, with no remaining evidence for a reverse gateway effect.
Read more about this topic: Steppingstone Theory
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