In the present study, we investigated whether the 12-month prevalence of PN among university students was higher during the COVID-19-pandemic compared to the prevalence before the pandemic. Therefore, three waves of survey in the summer terms of the respective years were conducted, one before the pandemic (2019), one during the first German lockdown (2020), and one after the second German lockdown (2021), when the infection case rates were continuously decreasing in Germany and lockdown measures were loosened. The sample sizes of the three surveys decreased from 2019 to 2020 and from 2020 to 2021. As we used the same methodological approach for recruiting university studens in all three surveys by contacting all students of the University of Mainz per E-Mail via a central mailing list (41), we do not think that the decrease in sample size had methodological reasons. However, we noticed (although empirical data are lacking for this hypothesis) an increase in “tiredness” of being online most time of the day for example for working, studying, and social interactions. Consequently, we hypothesize that the university students lost their motivation to participate in one more voluntary online survey during the pandemic what may be a reason for the decrease in sample size.

Contrary to our expectation, the prevalence of PN was relatively constant in 2019 and 2020 but decreased significantly in 2021. At all three time points, cannabis was the most commonly used substance for the purpose of PN, which made up around two-thirds to three-fourths of the total prevalence of PN at all measurement points. Consequently, the results suggest that the prevalence of PN was highly intertwined with the prevalence of cannabis use for PN.

The relatively constant or slightly increasing numbers from 2019 and 2020 are in line with the recently published drug survey 2021 of the federal government, indicating that the prevalence of the use of cannabis among young adults is continuously increasing since the last years (42). Furthermore, as stated in the world drug report 2021 of the United Nations, cannabis use patterns had remained relatively stable during the first lockdown period in the European Union, with nearly half of the participants reporting no change in their cannabis use, compared with the pre-lockdown period. In addition, as described in the second booklet on the global overview of drug demand and drug supply of the world drug report 2021, supply chains for Cannabis and also for other psychoactive substances were not affected by the pandemic. In the fifth booklet of this report on the impact of COVID-19 on drugs, it is further stated that COVID-19 may have accelerated the pre-existing trends toward increased use and availability of cannabis in some high-income countries as some people have turned to the drug to alleviate stress or manage boredom brought on by stay-at-home orders (43). However, these reports refer to the prevalence for the use of cannabis per se and not for the specific purpose of PN, as we did in our study. Therefore, the comparability of numbers has to be seen with caution.

The decrease in the prevalence of PN of around three percentage points in 2021 compared to the previous years was a surprising finding. It may be mainly due to the decrease in the prevalence of cannabis for the purpose of PN, which was also around three percentage points. In contrast, the prevalence for the other surveyed substances for PN remained the same. However, as literature regarding the prevalence of PN during the COVID-19-pandemic is rare, any discussion of this finding will be mostly hypothetical. One reasoning could be that potential demands (e.g., mental, social, and study-related) that were present during the lockdown and university closure period may have decreased after the end of the lockdown when infection case rates declined and restrictions were continuously loosened. In contrast to this reasoning, a study among college students performed at seven colleges in the United States showed that depressive symptoms and anger were modestly higher post-college closure compared to pre-college closure period, whereas no differences were observed in anxiety symptoms or insomnia and variables of cannabis use. However, the data were subject to both self-report and self-selection bias (44). One theoretical approach to explain the decrease of the prevalence of PN in 2021 could be Kahnemanns's Prospect Theory (45). According to this approach, the slightly higher prevalence of PN at the beginning of the pandemic (2020) could be explained by the situation and circumstances that students were confronted with, which were characterized by many uncertainties such as loss of personal contacts with peers and faculty, research, practical work, and exchange programs, profound changes regarding their financial and housing situation as well as the abrupt switch to online learning. These may have increased tendencies toward risk behaviors like PN. In contrast, the decreased prevalence of PN in 2021 reflects that the experiences after 1 year of studying under the conditions of the pandemic may have given a certain kind of security to the students that studies can be handled and even solutions like online-exams may cause less stress and therefore less risk behviors like PN.

For a more in-depth interpretation of the present results, especially the decrease in the prevalence of PN in 2021 compared to the previous years, more studies are needed addressing the prevalence of PN among university students and potential explanatory variables of PN during the COVID-19-pandemic. Despite the necessity of further research, the fairly high prevalence of PN of around 8% is still an important finding that demonstrates that there is still an urgent need for prevention initiatives among university students to combat the use of PN. To be able to plan evidence-based and effective PN-prevention initiatives for university students, it is important to understand the conditions and factors predicting PN among this target group. In this context, using a stepwise binary logistic regression model, Heller et al. (24) showed that specific varibles of health behavior predicted the use of PN among university students indicating that initiatives strengthening health behavior may prevent PN. This is in line with other research indicating that strengthening health-related key skills and ressources (in the sense of positive coping strategies) leads to a decrease in the prevalence of PN. For example, Bagusat et al. (26) concluded that tailored resilience interventions that improve the ability to adapt to and recover from stressors prevent the use of PN. Consequently, we recommend that initiatives aiming to prevent PN among university students have to be multifactorial taking the specific conditons of studying into account and have to focus on strengthening competences with regard to health behavior, mental health literacy and non-pharmacological ressources and strategies (24, 46, 47). Especially during the COVID-19-pandemic and in times of distance-teaching, online programs are of particular relevance. To name just some concrete examples of evidence-based online intitiatives for university students, at the university of Mainz, Germany, KEN-Online, and STUDYCoach are programs where students learn to deal with, for example, their emotions, stress, or symptoms of depression or anxiety. Another approach which aims to transport the topics physical activity, sedentary behavior and digital detox into (online) lectures is the program called Health Express. Here, long lectures are interrupted by short video clips which address a specific health-related topic and which were specifically developed for the target group university students under participation of university students. Moving from sitting into standing position is obligatory at the beginning of all video clips (48).

As potential limitation, it has to be mentioned that no causal inference can be drawn from cross-sectional data, as performed in the present study.