This study investigated the SPU of 3 pairings of widely used substances: alcohol, tobacco and cannabis. It aimed to explore SPU patterns and the relationships between different kinds of SPU and severity of substance dependence.
First of all, SPU seems to be a usual drug use pattern among young adults, especially for combinations of alcohol with another substance. Cross-SPU and partial forms of SPU were more frequent than non/occasional SPU for all 3 combinations of substance pairings. As reported in previous studies, frequent SPU seemed to be the most common pattern of polydrug use [6
]. Furthermore, different kinds of SPU were distinguished with regard to each subsample pairing. Alcohol and tobacco were often cues triggering cannabis use (35.7% and 38.8%), whereas alcohol and tobacco accompany each other (cross-SPU) for almost half of the alcohol and tobacco users. Cannabis as a cue triggering alcohol or tobacco use was a less common pattern (12.8% and 6.0% of participants, respectively). This result seemed consistent with the pathway that goes from licit drugs (alcohol, tobacco) to cannabis use [28
The polydrug pairing users showed different numbers of symptoms of substance dependence according to their patterns of simultaneous use. First, cross-SPU participants showed a higher number of symptoms of substance dependence than other forms (non/occasional SPU or partial SPU), except in 4 cases out of 18. That is, participants who frequently used substances simultaneously were more dependent on them than participants who reported other forms of SPU, and this was independent of the level of use of each substance, as we adjusted for hazardous use. This result was consistent with our first hypothesis, a), which supposed that the severity of polydrug dependence is most common with SPU rather than with non/occasional SPU. This result replicated those reported for alcohol and cannabis co-use [13
], tobacco and cannabis co-use [16
], and alcohol and tobacco co-use [18
], even these results did not specifically focused on SPU.
However, frequent SPU was not necessarily synonymous with an increased number of symptoms of substance dependence compared to more occasional SPU. When the participants started to use one substance while using another (partial forms of SPU), the number of symptoms of substance dependence was higher for the substance that was triggered by the cue, as reported in the cue-reactivity model [20
]. For example, the number of symptoms of nicotine dependence was higher among users who started to smoke while drinking (alcohol cue triggered tobacco use), than for users who never or occasionally used these 2 substances at the same time, but the number of symptoms of alcohol dependence in this group did not differ from the number of symptoms of occasional/non SPU users. Thus, severity of substance dependence was not only related to the frequent versus non/occasional SPU, but also to the kind of SPU. People who light a cigarette while drinking (alcohol cue triggered tobacco use) may be more addicted to nicotine than people who poured themselves a drink while smoking (smoking cue triggered alcohol use). When the use of one substance was triggered by the use of a first one, the need to start using the second one seemed related to the severity of dependence to this substance. It is possible that the more participants were dependent on a substance, the more they felt the craving to start using it while using a cue substance. These results were highlighted among alcoholic smokers (i.e. alcohol-dependent users), for whom smoking cues trigger cravings to drink alcohol [22
]. Data are scarce for the other substances. Previous studies did not focus on substance dependence and substance use as a cue, but on craving and other cues such as visual ones (e.g. pictures of glasses of beer or cigarette packs). For example, for alcohol cue and cravings to smoke, Carpenter et al. [33
] showed that daily smokers reported a higher response to alcohol visual cue rather than occasional smokers. Polydrug use has been well studied for alcohol and tobacco use [34
], and we know that cigarette smoking increases during alcohol self-administration and vice versa [35
]. For cannabis and alcohol, a study reported higher craving to use cannabis among cannabis users when they were presented a visual stimulus related to alcohol (not controlling for baseline craving) [36
]. For cannabis use as a cue for nicotine dependence, we only know that cannabis use is a predictor of nicotine dependence, according to the reverse gateway theory [37
]. Indeed, studies with cannabis cue are very rare [38
]. This study filled in the gap for substance use as a cue for polydrug dependence for the couple of substance alcohol/cannabis and tobacco/cannabis. Therefore, the specific type of partial SPU may be an indicator of severity of substance dependence, and further investigations will be necessary to explore this assumption. This result was consistent for all 3 polydrug use pairings and was related to the severity of dependence, except for the number of symptoms of alcohol dependence: When alcohol/tobacco users drank while smoking (smoking cue triggered alcohol use), they did not have more symptoms of alcohol dependence than users who smoked while drinking (alcohol cue triggered tobacco use). However, they did have more symptoms of alcohol dependence than non/occasional simultaneous users. These results were consistent with our second hypothesis, b), which suggested that the severity of substance dependence is more strongly associated with the triggered substance than by the cue substance.
This study has some limitations. The first was that we assessed frequent SPU versus non/occasional SPU instead of presence of SPU versus absence of SPU. Indeed, it was difficult to assess presence versus absence, because SPU was the most common pattern among polydrug users (67.6%–88.4% of frequent SPU) and total absence of SPU was rare. Thus, this assessment was not able to show the “pure” effect of presence of SPU versus absence of SPU. The second limitation was the study’s cross-sectional design. It was impossible to show whether more frequent SPU leads to an increased severity of substance dependence, or whether more frequent SPU was a consequence of the severity of substance dependence. Further longitudinal studies are needed to verify the hypothesis of SPU’s harmful effects (i.e. SPU worsened substance dependence). A third limitation was that no women could be included. This study is largely representative of men, but further investigations will be needed to know if women show the same patterns of SPU and substance dependence. Indeed, previous studies showed that polydrug use was higher among men than women (see for example Font-Mayolas et al. [39
]). It is also the case for SPU [40
]. Concerning cue reactivity, results are inconsistent. Some studies reported that women seemed more sensible to cues [33
], others reported that men were [41
], whereas others did not find any difference [42
]. More investigations are needed on this topic. Finally, a last shortcoming was that at age 20, the participants were more likely to still engage in experimental substance use rather than dependent substance use. For example, withdrawal seems rare among youth. On the opposite, young people are likely to experiment tolerance and hazardous use [43
]. To avoid misinterpretations and use of an inaccurate diagnose [44
], we used the whole continuum of severity instead of a threshold. Indeed, studies reported that the total information provided by the dependence criteria was comparable among younger and older adults [45
This study provided insight into the patterns of simultaneous polydrug use. Two results highlighted the necessity of taking SPU into account. First, frequent SPU was the most common pattern for young polydrug users and few of them never or occasionally used substance pairings at the same time. Second, a frequent SPU was associated with increased severity of substance dependence compared to non/occasional SPU. For both these reasons, SPU should be a serious concern for drug use prevention and intervention purposes. Moreover, the kind of partial SPU (i.e. one substance is a trigger for a second one) should be considered, as the level of use of a substance, triggered by a cue, reveals the severity of dependence to that substance.