In 2022, nearly 145 million Americans lived in a state with some form of legalized recreational or medical cannabis use, accounting for 45% of the U.S. population. Legal cannabis markets may be particularly relevant to young adults (ages 18–24) as they have the highest proportion of past-year and past 30-day cannabis use (23.2%) compared to youth (6.3%) and adults ages 25 and older (10.4%) and cannabis use continues to rise in this age group (Substance Abuse and Mental Health Services Administration, 2021). Cannabis use is linked to a host of mental and physical health consequences, including alcohol and tobacco dependence, illicit drug use, mental illness, motor vehicle accidents, and cognitive impairment (National Academies of Sciences, Engineering, and Medicine, 2017). Young adulthood is a critical developmental period where problematic substance use escalates (White et al., 2005, White et al., 2006), and risks of cannabis use on brain development are high (Filipek et al., 1994, Gennatas et al., 2017).

The rapidly changing policies surrounding cannabis’s legal use is correlated with decreased perceptions of cannabis-related harm (Chiu et al., 2021, Levy et al., 2021) and have been purported to influence curiosity and intentions to use (Cohn et al., 2017) and the development of problem cannabis use (Budney and Borodovsky, 2017). Lower perceived harm and reduced stigma about cannabis (Pew Research Center, 2019), coupled with greater cannabis availability, may lead to increased rates of cannabis initiation and could cause cannabis to emerge as the new “gateway” drug. For example, 13% of US young adults said they would use cannabis more often if it were legalized, and 9% of young adult non-cannabis users say they would do so (Cohn et al., 2017). Other studies have found that youth and young adult exposure to cannabis marketing, a by-product of legalized cannabis, is consistently associated with current cannabis use (Trangenstein et al., 2019, Trangenstein et al., 2021, Whitehill et al., 2020), problematic use (Trangenstein et al., 2021), lower perceptions of harm (Firth et al., 2022), and intentions to use (D’Amico et al., 2018).

Previous work examining sequencing of alcohol, tobacco, and cannabis initiation has consistently identified alcohol as a robust catalyst to later substance use (Barry et al., 2016, Kandel et al., 1992, Kirby and Barry, 2012, White et al., 2005, White et al., 2006). However, attention has turned to understanding whether patterns of initiation have changed, given changes in legal cannabis use. This is commonly referred to as the “reverse gateway” hypothesis, in which cannabis use leads to the onset of other substances (Patton et al., 2005). Indeed, a number of studies show reciprocal and unidirectional associations linking cannabis initiation and use behavior (e.g., frequency of use, any use versus no use) to onset of non-cannabis substance use, or to increasing use of other substances (Badiani et al., 2015, Patton et al., 2005, Swift et al., 2012). Villanti et al. (2015) found that the odds of hookah tobacco initiation in a national sample of young adults were predicted by “everyday/somedays” cannabis use that was reported in the 6-month prior to initiation. Similarly, using a national sample of young adults, Cohn, Johnson, Fryer et al., 2018 found that baseline use of cannabis predicted subsequent initiation of little cigar/cigarillo use. Fairman et al. (2018) examined ten years of data from the National Survey of Drug Use and Health (NSDUH) to determine the impact of using cannabis before other substances on heavy cannabis use, cannabis use disorder, and nicotine dependence in individuals ages 12–21. The percent of respondents using cannabis first, before any other substance, increased significantly over time, from 4% in 2004 to 8% in 2014. Furthermore, those who used cannabis first, before other substances, were significantly more likely to report heavy cannabis use and a cannabis use disorder. However, another study using NSDUH data found that the average age of onset for cannabis use has shifted upward to 19 years old (Lipari and Ahrnsbrak, 2017), suggesting that cannabis policies may not be reducing the age of onset for cannabis use.

No studies to our knowledge have examined whether using cannabis before alcohol and tobacco compared to using cannabis at the same age as alcohol or tobacco confers greater risk of reporting current poly-substance use and other drug use. This study used data from U.S. young adults to examine the prevalence of initiating cannabis before initiating alcohol and tobacco and the degree to which initiating cannabis first, before alcohol and tobacco use (exclusive initiation), predicts subsequent past 30-day alcohol, tobacco, cannabis use, and poly-substance use. Lastly, we examined the degree to which initiating cannabis at the same age as alcohol or tobacco, or after initiating alcohol and alcohol, predicted these same substance use outcomes.