The high drop-out rates and proportion of first-year students who change degree courses indicate that the transition to higher education is a critical period that should be analyzed with a view to possible intervention. The prevalence of a binge drinking pattern of alcohol consumption and co-consumption with cannabis, which reach maximal levels at this stage of life, may explain some of the difficulties associated with the transition to university. Previous data obtained by our research group [22] coincide with other studies in highlighting an increase in alcohol and cannabis use among first year university students in Spain [4,56]. The present study is one of the first of its type to assess academic performance and academic adjustment among controls, binge drinkers and alcohol and cannabis consumers in a cohort of first year university students in Spain. In addition, we tested whether academic adjustment mediates the effects of BD and cannabis consumption on academic achievement. On the basis of previously published information on the academic consequences of substance use among university students, we hypothesized that BD and the co-consumption of alcohol and cannabis will be associated with poor academic achievement. This hypothesis was partly confirmed, as the results revealed that GPA was lower in the BDCA group than in the control and BD groups, but that there was no difference between the last two groups. Although few studies have investigated the association between binge drinking and academic achievement in the university population, our study findings contradict some previous reports indicating heavy drinking and the frequency of alcohol use as negative predictors of the cumulative GPA [71,72,73,76,77]. However, our findings are consistent with longitudinal [94], cross-sectional [95,96] and prospective [55] studies reporting no effect, or a markedly attenuated effect, on academic performance among university students, after controlling for other predictors of academic failure (e.g., prior academic achievement, background variables, study habits, academic aptitude and cannabis use), or when including objective indicators of academic achievement [18,64]. Furthermore, our findings echo previous observations of an association between academic performance and alcohol dependence, but not alcohol abuse [63]. The evidence seems to indicate some weakness in academic achievement linked to BD. First, regarding problems related to the conceptualization of alcohol use, some studies examined binge drinking [64], while others examined consumption variables [55,71]. Second, different methods were used to measure the variables; thus, some studies [70,71,76,96] used objective measures (GPA), while others used subjective indicators [96]. Finally, when a number of plausible factors are controlled for, the effect of alcohol consumption upon academic performance may be negligible in healthy, young binge drinkers [94].

Thus far we have reported the effect of BD on academic achievement. Although the relationship between BD and academic achievement in university students has received some attention, the association between different types of consumption and academic performance in first-year university students has been less well studied. Only two studies have examined this association, in contrast to several studies that consistently provide evidence of academic problems associated with cannabis use in university students, such as lower GPA, lower exam and project grades, less time spent studying, decreased attendance and a higher drop-out rate [9,27,29,30]. In the present study, we expected academic performance to be poorer in the BDCA group than in the other groups. The findings show that co-consumption of alcohol and cannabis had a significant negative impact upon academic achievement. This finding was consistent with those of prior studies [18,29], which reported that the combined effect of both drugs is a significant predictor of lower GPA. Similar results have been obtained in studies of the co-consumers of tobacco, cannabis and alcohol which were studied in a university population [65] and a secondary school [28]. One potential explanation for these results is that co-consumption may be a greater risk factor for adverse academic outcomes than the risk posed by cannabis use alone [14]. Specifically, deficits in intellectual functioning, attention, memory, processing speed and visuospatial functioning were observed [14,24,61]. Such deficits can lead to lower academic motivation, poorer study skills, disrupted attendance and dropping out of university [27]. However, the body of research on the adverse consequences of the combination of binge drinking and cannabis consumption during university transition is surprisingly meager, given that peak consumption occurs at this stage.

Academic adjustment is considered a variable process that determines the subsequent path to higher education [41]. We hypothesized that BD and co-consumption of alcohol and cannabis will be associated with poor adjustment to university. We found that academic adjustment was significantly lower in the BDCA group. Again, our hypothesis was partly confirmed, as although academic adjustment was lower in binge drinkers than in controls, there was no difference between BD and BDCA groups (as determined by post-hoc analysis). One possible explanation is that drug use acts as a compensatory mechanism for coping with high levels of stress associated with the academic changes that occur during the transition from secondary to higher education, including adjustment to a highly competitive environment, the development of independent time management, new programs, new teaching practices and new institutional expectations [32,42,45]. Further research is needed to clarify the relationship between drug use and academic adjustment.

In addition, the present study examined the mechanisms that might explain the association between the co-consumption of alcohol and other drugs and academic achievement. As mentioned above, the mediation role that academic adjustment plays in the association between BDCA and academic achievement is an unexplored phenomenon in first year university students in Spain and other countries. Specifically, we expected that frequent consumption of alcohol and cannabis would make it difficult for students to adapt adequately to academic life, which in turn would contribute to poorer performance at university. The study findings confirm the proposed model, i.e., that co-consumption contributes indirectly to academic outcome, with 34.33% of the effect of alcohol/cannabis use on academic performance being mediated by academic adjustment. In other words, alcohol and cannabis co-consumers perceived feelings of dissatisfaction and disengagement from university life, such as a lower motivation to complete academic requirements and lower academic effort, leading to academic failure. Despite the difficulties in explaining academic achievement in co-consumers on the basis of academic adjustment, many previous studies include academic adjustment as a powerful predictor of university outcomes [39,42,46,51,84], or as a mediator of the relationship between drinking motives and alcohol consequences [66].

The results of this research may be useful for clarifying the relationship between alcohol and cannabis use and academic performance, and for assessing the mediator role of academic adjustment in that relationship. Thus, although the possible association between BD and academic performance remains open, the poor performance in young co-consumers of alcohol and cannabis was confirmed. In addition, without underestimating the role that other factors may play, the important mediator role of academic adjustment was confirmed. Finally, it seems particularly relevant to us to note the role played by academic adjustment, as this leads the way to designing support programs in higher education institutions that specifically focus on risk groups, such as alcohol and cannabis co-consumers. Collaboration between health professionals, educational psychologists and academic institutions is essential for implementing intervention programs.

Despite these important findings, this study has several limitations that should be considered. First, the sample was selected from one university, and should be extended to include other universities. Second, inclusion of a group of students who only consume cannabis would also represent an improvement, but this was not possible in the present study due to the low incidence of this type of drug consumption in the study sample. For reasons of experimental control, this preliminary study also did not include BDs with a family history of alcoholism or BDs with clinical-level problems, which could provide some information about the associated risk factors. In addition, application of these criteria excluded a large number of students at risk of academic failure. Third, as the study was cross-sectional, the association may not be consistent for any comparison of students in different years. Longitudinal studies would lend credibility to the relevant conclusions. Fourth, the study design did not consider the simultaneous use of cannabis and alcohol, or use within the same period. Consideration of both of these aspects would have improved the study and are of potential interest for future research. Fifth, the mediation model used did not allow us to reach any conclusions about the direction of academic adjustment. However, the cross-sectional study design does not allow the proposal of a causal order in the relationship between the variables studied.

Finally, the weight of variables such as pre-university achievement [51,97], background variables [30], social support [98] and health problems [72] should also be tested in a more complete version of the model.