Simultaneous alcohol and marijuana (SAM) use, often defined as use of alcohol and marijuana together so that their effects overlap, is associated with acute injury (Duckworth and Lee, 2019; Gonçalves et al., 2022; Jackson et al., 2020; Terry-McElrath et al., 2014) and adverse interpersonal consequences (Lee et al., 2020; Linden-Carmichael et al., 2020; Subbaraman and Kerr, 2015), as well as long-term increased risk of substance use disorders (Conway et al., 2020; Lee et al., 2022; Terry-McElrath et al., 2013). In a typical year, approximately one fifth to one fourth of 12^th grade students report past-year SAM, and 6-7% report frequent SAM (Lee et al., 2022). However, SAM prevalence began to decline in recent years among adolescents (Keyes et al., 2022a) while remaining stable or increasing among young adults (Terry-McElrath and Patrick, 2018).

The role of time trends in cigarette and nicotine use have potential implications for interpreting trends in SAM use. Lifetime cigarette use by the 12^th grade peaked in the late 1970s, with 75.7% of high school seniors reporting at least some lifetime cigarette use at that time (Johnston et al., 2019). Over the next 40 years, cigarette use markedly declined, and as of 2020, only 24.0% of high school seniors have ever smoked cigarettes (Johnston et al., 2019). These time trends have implications for time trends in other substance use, because adolescents who use cigarettes and nicotine are a high risk group for subsequent alcohol and marijuana use (Keyes et al., 2016, 2019; Roche et al., 2019). As cigarette prevalence has declined, more adolescents are in a lower risk group (those who do not use cigarettes) compared to a higher risk group (those who use cigarettes), thus lowering the risk of subsequent in alcohol and marijuana use through cigarette use (Miech et al., 2020, 2017).

Over the same time period, when examining the whole US adolescent population, alcohol use prevalence has substantially declined, and marijuana use has remained relatively stable (Johnston et al., 2019). Part of the mechanism underlying alcohol and marijuana use trends is the decline in cigarette use (Fleming et al., 2022, 2016; Miech et al., 2017). Specifically, a greater proportion of adolescents are at lower risk for alcohol and marijuana use because cigarette smoking has declined. Thus, this suggests that cigarette use may be suppressing what would have been less of a decline for alcohol use, and even potential increases in prevalence for marijuana. Indeed, among adolescents who have used cigarettes, marijuana use prevalence is increasing; and among adolescents who have never used cigarettes, marijuana use prevalence is also increasing (Miech et al., 2017). These observed increases are suppressed when examining the overall US adolescent population because of the shift in composition of adolescents from high risk (ever used cigarettes) to low risk (never used cigarettes) (Miech et al., 2017).

Several issues remain unresolved with respect to SAM and overall patterns of alcohol and marijuana use, however, due to changing trends in other substances. The public health successes of reduced cigarette use have been partially offset by increases in nicotine use and nicotine use disorder among adolescents through vaping devices (Miech et al., 2021,2019) since approximately 2015. Vaped nicotine use, like cigarette use, is associated with increased risk and frequency of subsequent alcohol and marijuana use (Evans-Polce et al., 2020; Park et al., 2020; Silveira et al., 2018). Thus, adolescents who vape nicotine may now again be shifting to a higher risk group for alcohol and marijuana use, and, potentially, SAM. Second, the extent to which cigarette and nicotine use may influence trends in SAM, as well as alcohol and marijuana use that is not used simultaneously, are complicated by other dynamics. Alcohol use has been declining among adolescents for decades (Jager et al., 2022; Platt et al., 2021), and recent declines in SAM use parallel declines in alcohol use (Lee et al., 2022). Thus, variation in trends examining SAM use, alcohol use without simultaneous marijuana use, and marijuana use without simultaneous alcohol use may exhibit heterogeneous trends by cigarette and vaped nicotine use. Overall, changes in more recent years when nicotine use has increased due to vaping (Miech et al., 2021), remain understudied.

In summary, SAM use remains prevalent and concerning for adolescent public health, especially in the context of underlying divergent trends in overall alcohol and marijuana use. The extent to which cigarette and nicotine use trends modify trends in SAM, as they do for marijuana and other substance use, has not been evaluated. Further, the extent to which trends in SAM use differ from trends in non-simultaneous alcohol and cannabis use remains understudied. The present paper: a) examines time trends in SAM use, and non-simultaneous use, from 2000 to 2020 among US 12^th grade students controlling for demographics; and b) tests whether trends are modified by lifetime cigarette use or nicotine vaping, both together and separately. We examine SAM in the context of other patterns of alcohol and marijuana use given their disparate trends, providing a full account of alcohol-only (adolescents who have used alcohol but never marijuana); marijuana-only (adolescents who have used marijuana but never alcohol); non-simultaneous use (adolescents who have used both alcohol and marijuana, but never simultaneously); and no use (adolescents who have never used alcohol or marijuana).

Whereas overall, SAM use declined among US adolescents from 2000 to 2020, its prevalence increased among adolescents who have never vaped nicotine or smoked cigarettes, and has remained largely stable among those who have ever vaped nicotine or smoked cigarettes. Additionally, the prevalence of SAM use remains orders of magnitude higher among 12^th graders who have ever vaped or smoked than among those who have not. The overall decrease in lifetime nicotine vaping or cigarette use from 2000 to 2020 explains why SAM use declined over time when not stratified by cigarette/vaping use, and why SAM use increased when stratified nicotine vaping or cigarette use from 2000 to 2020: it is because SAM use is higher among those who use cigarettes/vape, and the prevalence of cigarettes use is declining. In contrast, past-year marijuana-only (i.e., no alcohol use) is substantially increasing among 12^th grade students. Among those who have never used cigarettes or nicotine, for example, marijuana-only more than quadrupled from 0.44% in 2000-2005 to 3.02% in 2015-2020. Taken together, these results indicate that continued efforts to reduce adolescent use of cigarettes and vaped nicotine may yield health benefits that extend to use of other substances, and that new approaches may be needed to reduce adolescent marijuana use specifically.

The observed in trends in SAM, with declines in the whole adolescent population but increases among those with no history of cigarette/vaped use, occurred partially because adolescent cigarette use has dramatically declined over time; cigarette use increases the likelihood of other substance use (Keyes et al., 2016; Purcell et al., 2021). Our results are consistent with other studies showing that cigarette use among adolescents strongly predicts subsequent substance use, particularly for marijuana (Roche et al., 2019). Indeed, substantial research has documented “tri” use of alcohol, cannabis and nicotine as a substance use pattern of particular concern (Linden-Carmichael et al., 2022; Roche et al., 2019). When stratified by cigarette use, findings suggest increases in marijuana use prevalence within the stratified groups, even while the overall prevalence is relatively flat or declining (Fielding-Singh et al., 2021; Bachman et al., 2015). Stratified results focusing on vaped nicotine use suggest a relative flattening of trends from 2018 to 2020, especially among those who have vaped nicotine. These results underscore a need for focused public health efforts on reducing nicotine vaping, and a continued focus on maintaining low rates of cigarette use. Vaping prevalence among youth increased in the wake of well-funded advertising campaigns portraying vaped nicotine delivery as alluring and low risk (Marynak et al., 2019; Park et al., 2019; Vogel et al., 2021). While regulations of vaped nicotine and access among youth are becoming more restrictive, additional efforts to provide accurate information to youth on the health risks of vaped nicotine use are needed (Baker and Campbell, 2020; Graham et al., 2020; Liu et al., 2020; Noar et al., 2020, 2019), as well as continued efforts to ensure that cigarette use remains at low levels.

Marijuana-only (no concurrent or simultaneous alcohol use) is increasing both among those who have used cigarettes or vaped nicotine and those who have not. Nationally, the expansion of marijuana product availability and declining legal and social sanctions associated with use may underlie these increases in youth use (Sarvet et al., 2018b), although adolescents who live in states that have passed medical and recreational marijuana laws largely (although not uniformly across studies) do not have increased rates of marijuana use compared to those in states without these laws (Bhatia et al., 2022; Sarvet et al., 2018a). There is also an overall national increase in perceptions that marijuana poses little danger to health (Sarvet et al., 2018b), and an increase in exposure to marijuana messaging, advertising, and products (Cavazos-Rehg et al., 2016; Moreno et al., 2022). Among adolescents who have used marijuana and alcohol by the 12^th grade, marijuana is now more likely to be used before alcohol than vice versa, reversing a historical and consistent trend in substance use initiation patterns (Keyes et al., 2019). While the plurality of adolescents ages 12-17 perceived marijuana to both have high risk and be unavailable in 2002-2018 (Levy et al., 2021), and cannabis products vary substantially in health risks (Chadi et al., 2020; Spindle et al., 2019), providing timely and accurate information about marijuana to adolescents is critical as marijuana availability continues to expand in the US.

Moreover, among the trends assessed, marijuana-use only increased in the overall adolescent population while SAM use, alcohol and marijuana use not simultaneously, and alcohol-use only declined. This suggests that trends in alcohol and marijuana co-occurrence, whether use simultaneously or concurrently, are following overall trends in alcohol use rather than marijuana use. Explanations of the reasons adolescent alcohol and marijuana co-use followed alcohol trends rather than marijuana trends remain speculative at this point. Certainly, alcohol use remains significantly more prevalent than cannabis use in the overall adolescent population, thus declines in use have a greater population-level impact and affect more youth than increases in cannabis use. Given the documented correlation between alcohol and marijuana use (Jackson et al., 2008), we speculate that continued declines in alcohol use, may also reduce overall marijuana use for a substantial portion of adolescents. However, the extent to which offsetting increases in marijuana-use only will disrupt this pattern remain to be seen.

The links between cigarette use and nicotine vaping with subsequent marijuana use remain to be disentangled. Mechanisms may involve increased opportunities to use marijuana after cigarette or nicotine onset given exposure to smoking peer groups (Van Etten and Anthony, 1999; Wagner et al., 2002), as well as lowering inhibitions to trying intoxicating substances. Further, adolescents who vape nicotine are at particularly high risk of vaping marijuana products. Marijuana vaping has also increased, and there is substantial overlap in marijuana and nicotine vaping among adolescents (Dai, 2020; Farsalinos et al., 2021; Keyes et al., 2022b; Patrick et al., 2020; Trivers et al., 2018). Indeed, similar modes of administration of nicotine and marijuana may drive SAM use among adolescents who have vaped nicotine use. Simultaneous or overlapping substance use amplifies the odds for other substance use. In a 2017 sample in the UK, 93% of adolescents who used e-cigarettes reported past-month tobacco, alcohol, or marijuana use, and 48% reported use of all three (Staff et al., 2020). As polysubstance use grows, bolstered by trends in vaping and especially in young adult cohorts (Terry-McElrath and Patrick, 2018), adolescents will have more opportunities for SAM use. Interventions to reduce simultaneous use of marijuana and alcohol will need to account for vaping and its co-occurring impact on other substance use. Vaping smoking interventions that utilize text-based motivations and information are promising avenues for prevention and intervention efforts (Baker and Campbell, 2020; Graham et al., 2020; Noar et al., 2019).

Finally, we note that demographic predictors of substance use patterns were observed and are largely consistent with prior literature. Black and other race students were underrepresented in SAM use and alcohol-only (Patrick et al., 2018), but were more likely to use marijuana-only (Wu et al., 2016), and students with more highly educated parents were more likely to use substances (Patrick et al., 2012). However, emergent racial and ethnic disparities in SAM use by college age (Hai et al., 2022) suggest that continued surveillance across broader developmental periods is necessary. Notably, female adolescents are emerging as a high-risk group for alcohol use, consistent with other data sources (Cheng and Anthony, 2018). This emergent sex difference is primarily due to faster declines in alcohol use among males compared with females. While reasons that alcohol use is not declining at the same rate among female adolescents remain largely speculative, available evidence suggests that changes in social expectations by gender and sex roles are shifting in recent decades for both adolescent and adults, which may partially mediate the converging sex differences in drinking (Keyes et al., 2021).

Data limitations included that the MTF only includes school-attending adolescents, thus may not generalize to students who dropped out or are otherwise not attending school. Adolescents who are school absentees are more likely to engage in substance use (Bray et al., 2000; Roebuck et al., 2004); adolescents not captured in MTF may have higher substance use than non-participants. Self-reported data may have misclassification, and are not validated with drug testing. However, MTF rates of substance use, capture in school, are higher than other national estimates where students are interviewed at home (Gfroerer et al., 1997; Kann et al., 2002), suggesting that school-based assessment is an important context for drug use assessment. MTF did not measure vaped product use before 2017. Other national surveys that have estimated prevalence of vaped product use since approximately 2014 indicate that approximately 10-15% of high school seniors reported e-cigarette use from 2014-2016 (Sun et al., 2021; CDC, 2015). Our findings may underestimate trends in vaped product use, to the extent that some adolescents in those years did not use cigarettes as well. However, we expect this misclassification to be minimal given the overlap between e-cigarette and combustible cigarette use, and the lower average prevalence of e-cigarette use in 2014-2016 compared with later years.

In sum, overall decreases in adolescent simultaneous alcohol and marijuana use are primarily driven by declines in cigarette use. Indeed, among adolescents who have both used and not used cigarettes, prevalence of simultaneous use has increased. Marijuana use, especially marijuana use without any concurrent or simultaneous cigarette or vaped nicotine use, is increasing among US adolescents; as product availability continues to increase through the US, concerted prevention efforts are warranted. Future changes may be further potentiated by increases in nicotine vaping. As prevalence of nicotine vaping has been dynamic across recent history, its potential impact on the prevalence of substance use should be monitored.