In the United States, alcohol, cannabis, and tobacco use is common among young adults (YAs; aged 18–26). National data indicate 65% of YAs report using alcohol, 27% cannabis, and 9.5% cigarettes in the past 30 days (Schulenberg et al., 2021). YAs frequently report co-use of these substances (Cohn et al., 2018). Co-use of tobacco with alcohol or cannabis may increase risk for nicotine dependence, poor mental health, and poor smoking cessation outcomes (Cohn et al., 2018, Kahler et al., 2010, Tucker et al., 2019).

In existing literature, co-use is an umbrella term used to characterize substances used either within the past 30 days or 12 months, with few measures addressing how products are used together (e.g., within the same occasion or day) (Hindocha & McClure, 2021). Moreover, measuring co-use poses unique challenges due to the variety of product combinations, timeframes of use, and operational definitions. For example, simultaneous use often refers to the use of two substances in combination, on the same occasion, or at the same time, so that their effects overlap (e.g., “blunts”- cigar wrappers filled with cannabis, or smoking a cigarette and drinking alcohol at the same time). Further, simultaneous use can also include the use of a substance while under the influence of another (e.g., smoking a tobacco cigarette while under the influence of cannabis). Given the complexity of co-use behavior, mobile data collection methods, such as ecological momentary assessment (EMA) and daily diaries may better capture nuances of daily co-use (Lee et al., 2022, Nguyen et al., 2020, Sokolovsky et al., 2020), improving our understanding of co-use patterns.

Prior research on co-use has primarily compared days with co-use and days with single-substance use. One study found that cannabis and alcohol independently increased the odds of cigarette use on the same day (Roche et al., 2019), while another found that sexual-minority YAs were more likely to engage in same-day cigarette and cannabis co-use than their heterosexual peers (Nguyen et al., 2021). Recently, Sokolovsky et al. (2023) found that using alcohol or cannabis on a given day increased the odds of same-day tobacco use and co-use of tobacco with these substances. However, to the best of our knowledge, no study has examined the number of cigarettes smoked on days with same-occasion co-use and different-occasion co-use.

To fill this gap, we investigated the associations between co-use and daily cigarette smoking quantity. Using smartphone-based daily diaries, we examined associations between the number of cigarettes smoked and three different types of days (days with cigarette smoking only; days with co-use on the same occasion; and days with co-use but on different occasions) for co-use of cigarettes with alcohol and with cannabis.