Though the national smoking prevalence has declined, nearly 34.3 million adults in the US still smoke cigarettes¹. Moreover, use of alternative tobacco products (ATPs), such as little cigars/cigarillos (LCCs), smokeless tobacco, electronic cigarettes (e-cigarettes), and hookah, has become increasingly prevalent. Notably, adolescence and young adulthood are pivotal times for tobacco uptake. Indeed, 22.3%, 9.1% and 4.8% of young adults reported use of cigarettes, cigars, and smokeless tobacco in 2017, respectively². Additionally, in 2018, adolescent past 30-day use prevalence was 7.6% for cigarettes, 8.9% LCCs, 4.2% smokeless tobacco, 20.9% e-cigarettes, and 4.4% hookah³.

While the tobacco market has diversified, another relevant evolution in social norms and policy has occurred regarding marijuana, the most commonly used federally illicit drug^3,4. In 2017, past-month use prevalence was 9.5% in adults, marking a 35% increase in the past decade⁴. In 2017, adolescent use demonstrated the first significant increase in 7 years, with 24% reporting past-year use and 6.5% past-month use³; while 2018 data indicate 22.2% past 30-day marijuana use prevalence among 12th graders⁴. Of note, the highest use prevalence is among those aged 18–25 years (21.5%, past-month use) with the 20s being a critical age period in marijuana use trajectories⁴.

Within this rapidly changing tobacco market and the evolving social norms regarding ATP and marijuana use, there is increased occurrence and concern of multiple product use across both tobacco and marijuana^1,2, particularly for young people as earlier onset use has been associated with early onset of dependence^4-6. Different tobacco products have potentially different nicotine levels; thus, multiple tobacco product use may be associated with increased risks of nicotine dependence⁷. Moreover, tobacco and marijuana use are highly correlated⁸. Popular combinations of substances used by young adults include co-occurring cigarette and e-cigarette use, cigarette and cigar use, and hookah, cigar, and marijuana use^1,2,9.

As the landscape of tobacco and marijuana policies and social norms has evolved, trajectories of substance use have also changed. The ‘gateway’ theory, formulated originally to explain sequences in licit and illicit drug use, has been more recently applied to the range of tobacco products and marijuana^10-12. Two major questions inherent in studying ‘gateway effects’ are: 1) ‘which products are used first?’, and 2) ‘what use profiles occur subsequently?’. In relation to the former, the 2012 Legacy Young Adult Cohort study of 4201 young adults found that 73% started with cigarettes first, 11% with cigars, 5% LCCs, 3% smokeless tobacco, and 4% hookah, with insufficient precision to report on e-cigarettes¹³. Additionally, 2012 National Youth Tobacco Survey data indicated that 20.3% of middle school students and 7.2% of high school students reported using e-cigarettes as their first tobacco product¹⁴. A 2015 cross-sectional study of 3146 college students found that 38% chose cigarettes as their first product, 29% chose cigars, 6% smokeless tobacco, and 25% hookah (with no one reporting e-cigarettes as first product used)¹⁵. A 2013 study of New York undergraduate students reported that hookah was the first tobacco product used by 25.4% of ever tobacco users and 50% of never cigarette smokers¹⁶.

With regard to the second question, i.e. profiles of subsequent substance use, Suftin et al.¹⁵ found that those who initiated with cigarettes or cigar products were more likely to be current cigarette smokers compared to those who initiated with ATPs¹⁵. Other research has indicated that first using cigarettes increased the likelihood of initiating marijuana by four times¹⁷. Additionally, those who initiated with smokeless tobacco were twice more likely to initiate cigarettes and more likely to be dual or poly tobacco users¹⁵. Other studies have documented that experimenting with various products is related to increased risk for other substance use. For example, experimenting with e-cigarettes may increase risk of initiating cigarette use^18,19, and experimenting with hookah may increase the risk of using combustible tobacco products and e-cigarettes²⁰. In addition, e-cigarette or hookah use has been associated with higher risk of initiating marijuana use¹⁷. Moreover, marijuana use during adolescence and young adulthood is associated with increased risk of tobacco initiation²¹.

Cumulatively, the literature indicates some potential answers to the above questions. Specifically, it suggests that: 1) cigarettes are decreasingly the first products tried, 2) trying any single tobacco product increases the risk of trying other products, 3) profiles of subsequent use are likely different depending on which product was used first, and 4) subsequent product use risk may be specific to the first product used. However, this line of research is still limited and requires further examination.

The current study draws from a social developmental perspective²² that suggests that tobacco and marijuana use trajectories are shaped during adolescence and young adulthood by multilevel influences, including those at the individual, interpersonal, and community levels. Individual-level influences include sociodemographic and psychosocial characteristics, and other substance use. Regarding sociodemographic characteristics, data have indicated higher odds of using cigarettes first among those aged <17 years, females, Hispanics/Latinos, and those of lower socioeconomic status (SES), relative to their counterparts^15,23. Cigars and LCCs as first products used have been associated with being male and younger at initiation^15,24. Smokeless tobacco and e-cigarettes as first products used are also associated with being male and White¹⁴. Choosing hookah as first product has been related to being female, Black, and higher SES^15,16,23. Regarding marijuana, there is limited literature on prevalence and correlates of marijuana being first used; however, 2017 data indicate that 6.8% of adolescents had used marijuana before the age of 13 years²⁵. In terms of use trajectories, smoking progression is greater among men and those with lower SES²⁶. This research, although not inclusive of factors specific to trajectories of ATP and marijuana use, highlights sociodemographic differences in use trajectories to consider.

Regarding psychosocial factors, tobacco use and development of addiction have been associated with experiencing more adverse childhood events (ACEs), e.g. physical or sexual threat or abuse, parental divorce or separation^27,28; as well as having higher depressive²⁹ and ADHD symptoms²⁹. While these associations have generally been found with cigarette smoking, research regarding these mental health concerns and use of ATPs is limited.

In addition to individual-level factors, interpersonal factors, such as parental tobacco and/or marijuana use, have been shown to impact on substance use trajectories⁴. Also, community-level influences on young adult tobacco use may include whether they live in rural or urban areas^4,6, whether they pursue post-secondary education^4,6, and the type of colleges/universities attended; for example, relative to four-year colleges/universities, community or technical colleges have higher student smoking prevalence^30,31.

Thus, additional research is needed to fully characterize who is most likely to initiate use of distinct products and the subsequent use patterns relative to first product used. Thus, this study examined: 1) characteristics of young adults who initiate use of different tobacco products or marijuana; and 2) first product used relative to subsequent tobacco and marijuana use, specifically total number of products used in the lifetime, total number of products used currently, and current use of each product.