Given widespread co-use of T/C among young adults, and recent changes in T/C product and regulatory landscapes, it is important to understand how T/C use during adolescence predicts future product use and types of T/C co-use in young adulthood. This is one of the first longitudinal studies to examine classes of adolescent and young adult T/C use based upon longitudinal patterns of use of both types of drugs, modeled simultaneously in a parallel process growth mixture model. We identified 4 distinct T/C trajectory classes, characterized by: (1) consistently low or no engagement in T/C use over time; (2) sharply decreasing probability of tobacco use and moderate decreasing probability of cannabis use; (3) progression to moderate-high probability of concurrent T/C co-use; and (4) rapid progression to stable, high probability of persistent concurrent T/C co-use. The current study adds to the existing literature on T/C co-use by examining prospective associations between these T/C co-use trajectory classes and the number and types of products (vaping; combustible) used as well as providing information on more nuanced patterns of T/C co-use (eg, same-occasion sequential co-use and co-administration) in young adulthood.

Consistent with hypotheses, those in the Early Concurrent T/C Co-use class were more likely to use combustible products, engage in poly-product use for tobacco and cannabis, and report concurrent and same-occasion co-use (ie, sequential use; co-administration) of T/C in young adulthood compared to individuals in other classes. Numerous studies suggest that young people who initiate tobacco use or cannabis use at earlier ages may be at greater risk for developing dependence on these drugs and progressing to heavier use,^{40bibr41-1179173X20949271bibr42-1179173X20949271bibr43-1179173X20949271-44} and recent work by our team suggests that more rapid escalation of past-year T/C co-use is associated with a range of poorer outcomes in young adulthood.⁴⁵ The current study expands upon prior work by showing that young people who report co-use of T/C products earlier in life and who rapidly progress toward persistent past-month co-use of both drugs in adolescence are more likely to go on to use more different types of tobacco/nicotine and cannabis products, and more likely to show riskier patterns of T/C co-use (eg, co-administering T/C in the same use episode) in young adulthood compared to their peers who show different T/C use profiles in adolescence. One possibility is that persistent engagement in concurrent T/C co-use during adolescence may increase exposure to opportunities to use T/C in combination, reinforcing this pattern of use over time.⁴⁵ In addition, concurrent T/C co-use may increase awareness and exposure to more types of products, which may in turn increase likelihood of poly-product use. These findings suggest that, rather than targeting a single substance or type of product (eg, cigarettes), adolescent tobacco (and cannabis) prevention programs should include content on co-use of T/C and its potential consequences, as well as the full breadth of product types now available that may be used to support same occasion co-use, especially given that some products (eg, combustible products) may carry different health risks compared to others.

We also observed other important class distinctions and similarities on outcomes. The Early Concurrent T/C Co-use group evidenced the highest likelihood of combustible cannabis product use and combustible T/C co-use in young adulthood compared to other groups. For example, individuals in the Early Concurrent T/C Co-use group had a 53% probability of endorsing past-month co-administration (ie, mixing both tobacco and cannabis together) of combustible T/C. This is consistent with prior work suggesting that combustible T/C remains the dominant form of T/C co-administration among young people.^29,46 This is particularly concerning given the well-established, serious health consequences associated with combustible product use.³⁰ Moreover, combustible product use may be associated with greater dependence, which could make cessation more difficult for individuals who use these types of products.⁴⁷ As the tobacco and cannabis markets become increasingly diverse, this indicates a need to emphasize the established risks of combustible products (ie, at the far end of the risk continuum³²) compared to other types of available products in prevention and intervention efforts to reduce harms to public health.

We also observed notable group differences with respect to T/C vaping product use. For example, although individuals in the Early Concurrent T/C Co-use and Late Concurrent T/C Co-use groups showed similar rates of cannabis vaping in young adulthood, the Early Concurrent T/C Co-use group had remarkably high rates of concurrent T/C vaping co-use and sequential vaping co-use compared to all other classes. Specifically, individuals in this group had a 95% probability of using both T/C vaping products in the past month, approximately 30% probability of reporting sequential co-use of T/C vaping products (ie, 1 product right after the other), and over 10% probability of reporting co-administration of T/C vaping products. High rates of T/C vaping co-use among some young adults are concerning due to limited data on long-term consequences of use and co-use of these relatively novel products. Although extant evidence suggests that nicotine vaping (ie, ENDS use) exposes individuals to fewer known toxicants compared to combustible tobacco products,³¹ and is likely to be considerably less harmful to health relative to combustible product use,³² findings from studies of ENDS cannot be generalized to cannabis vaping products, for which the research base is quite limited. Moreover, in the context of other findings showing high probability of combustible T/C co-use, poly-tobacco use, and poly-cannabis use in the Early Concurrent T/C Co-use group, these individuals may risk particularly high exposure to nicotine, THC, and various other compounds that could increase risk of dependence and other negative health consequences.^{13bibr14-1179173X20949271-15,19} Cannabis vaping products may deliver particularly high concentrations of THC, which could place individuals who use these products at risk for negative consequences including cannabis use disorder.^23,48 Moreover, recent reports indicate that some cannabis vaping products may lead to serious health problems: as of 25 February 2020, the Centers for Disease Control and Prevention documented 2807 cases of serious lung injury associated with use of vaping products. Approximately 3-quarters of cases involved individuals under age 35, and recent use of cannabis vaping products was reported by a majority (82%) of hospitalized patients.⁴⁹ As cannabis vaping products become increasingly accessible, efforts to protect public health may benefit from clearly communicating differences between nicotine and cannabis vaping products, particularly “black market” cannabis vaping products⁵⁰ that are not subject to stringent regulatory oversight and may carry differential health risks compared to ENDS. Such efforts may be particularly important given data suggesting that perceived risk of cannabis use among young people may be decreasing in line with changes in cannabis legalization.⁵¹

Findings should be considered in context of limitations. First, data were self-reported. Although we have no indication that individuals misrepresented their T/C use, data were not biochemically validated. In addition, although a strength of the study was its inclusion of ENDS and smokeless tobacco products (ie, in addition to cigarette smoking) in characterizing trajectories of tobacco/nicotine use, data on past-month tobacco/nicotine use were dichotomized, which precluded assessment of trajectory groups in relation to frequency of past-month tobacco use. Similarly, although our use of longitudinal data to characterize progression toward stable, concurrent T/C co-use is a considerable strength, our use of dichotomous indicators for any past-month T/C use may have captured those who were simply experimenting with products at a given time point. Future work is needed to examine potential differences in young adult outcomes in relation to frequency of tobacco/nicotine and cannabis use during adolescence (eg, progression to daily T/C use). Although the sample is racially/ethnically diverse and models adjusted for race/ethnicity (and other demographic characteristics), a limitation of our data is that the proportion of black participants in the sample is low relative to the general population. Finally, the sample was based predominantly in California, and findings may not generalize to other samples in different tobacco/nicotine and cannabis regulatory environments. However, as more states consider adopting policy measures similar to those in California (eg, legalizing recreational cannabis), these findings may represent an important example of young adult T/C use patterns under these regulatory conditions.