As states increase access to cannabis, concern has developed regarding how increases in cannabis use may affect cigarette smoking. Most of this attention is focused on cannabis increasing smoking rates, where a recent study puts these fears to rest. However, much less attention is given to the health risks involved in dual cannabis and tobacco use, where unique harms may exist.
Individuals in the US who use cannabis are three times more likely to be cigarette smokers than those who do not use cannabis. However, this statistic is an example of the “gateway drug fallacy”.
If you know a person uses cannabis, you know they are willing to use psychotropic drugs. This willingness is a necessary but not sufficient condition for any substance use, and knowing that an individual possesses this willingness increases the chances they use any substance. By way of comparison, coffee drinkers are up to seven times more likely to be smokers than non-drinkers.
Assessing whether cannabis directly causes smoking requires more sophisticated research methods than simple cross-tabulation estimates and a recent study does just that. Using a classic econometric technique known as a difference-in-difference estimation, researchers analysed four well-known nationally-representative datasets comparing changes in the rate of smoking in states with a recreational cannabis law (RCL) against the change that occurred in states that did not.
They subtract the difference that occurred in states that did not have an RCL from the difference that occurred in states that did, thereby eliminating any decline in smoking that would have occurred anyway and leaving an estimate of the change due to the policy. They found that immediately after the enactment of RCL, there was no change in smoking, while after three or more years there was “a statistically significant 0.9 to 1.9 percentage- point decline in cigarette use”.
What this means is that after controlling for changes in smoking rates that were already going to happen, and adjusting for demographic/geographic variables that may differ in areas that legalised cannabis versus areas that did not, smoking was noted to have eventually gone down in areas that legalised cannabis relative to those that did not. This would be impossible if cannabis use caused tobacco smoking.
While it is an overstatement to understand the modest dip in smoking rates as evidence that cannabis use may help smokers quit, the breadth of the study is convincing that greater access to legalise cannabis will not cause smoking rates to increase.
Cannabis and cigarettes together: a unique risk?
However, while it proves cannabis does not cause cigarette use, the above research still shows that they are often used together. Does such co-use present any unique risk?
Despite its importance, research into this question is lacking. Two studies from 2020 found that co-users had higher levels of biomarkers measuring toxicant exposure compared to tobacco use alone, despite similar levels of cigarette use. While cannabis smoking has not been definitively linked to an increase in cancer risk, cigarette smoking has in a manner that is dose-dependent – that is, more exposure means more risk.
If smoking and cannabis use pose a unique risk with respect to carcinogenic exposure, dual users of tobacco and cannabis might be wise to consider reduced-risk products such as e-cigarettes that have been shown to reduce exposure to harmful compounds. Sadly, research into the unique harms of tobacco and cannabis use will often treat cigarette use and vaping as posing a similar level of risk.
– Clayton Hale CannIntelligence science correspondent
Photo: Alejo Turola