According to the Monitoring the Future (MTF) surveys of U.S. adolescents in grades 8, 10, and 12 from 2000 to 2019, the prevalence of lifetime and past 30-day alcohol use declined sharply [32]. There were sizable decreases in the past 30-day use of alcohol between 2010 and 2015, with 13.8%, 28.9%, and 41.2% for 8th, 10th, and 12th graders in 2010 down to 9.7%, 21.5%, and 35.3% in 2015, respectively [34]. Similar patterns were also observed in other Western countries. Reports from ESPAD indicates a downward trend in the current use of alcohol in Europe, with estimates to be about 57% in 2011, 48% in 2015, and 47% in 2019 for adolescents in ages 11 to 15, with the frequency of 15 years old pupils trying alcohol being seven times that of 11 years old pupils [13,35,36].

In Hong Kong, the surveys conducted by the Narcotics Division of Security Bureau showed that, from 2011 to 2018, the proportions of lifetime alcohol-taking in students slightly increased from 56.0% to 56.7% [37]. In Taiwan, data from the 2017 National Health Survey showed that the prevalence of alcohol among adolescents aged 12–17 slightly decreased from 25.7% to 23.8% in 2009 through 2013 and then increased up to 27.7% in 2017 [38]. Survey data collected from fifth graders through college seniors sponsored by the Social Welfare Bureau of Macau SAR showed a decreased prevalence in alcohol use of 63.16%, 56.14%, and 52.48% in 2010, 2014, and 2018, respectively [39,40,41].

The MTF surveys conducted between 2016 and 2020 showed a slight decrease in the lifetime use of cigarettes by adolescents in the U.S. Furthermore, for the same period, the prevalence of past 30-day use of cigarettes by adolescents from grades 8, 10, and 12 in the U.S. decreased from 2.6% to 2.2%, 4.9% to 3.2%, and 10.5% to 7.5%, respectively [42]. According to the ESPAD, the prevalence of current adolescent smoking in Europe is slightly higher than that in the U.S. Between 2011 and 2019, the prevalence of adolescents who were considered current smokers (smoked in the past 30 days) in Europe decreased by 10%, but this figure only accounted for adolescents between the ages of 11–15 [13]. Thus, it is likely that there would be a higher prevalence figure since the trends omitted adolescents who were older than 15 years old.

In Mainland China, according to the data collected from a series of cross-sectional National Health Service surveys conducted between the years of 2003 and 2013, smoking prevalence among adolescents aged 15–24 years increased from 8.3% in 2003 to 12.5% in 2013 [43], and the figure climbed to 18.6% in 2018 [44]. In Hong Kong, the Narcotics Division of Security Bureau found that, among all surveyed students, the proportions of lifetime tobacco-taking students somewhat decreased from 8.8% in 2011 to 7.0% in 2018 [37,45]. The Global Youth Tobacco Survey in Taiwan reported that the prevalence of smoking cigarettes among middle school students decreased from 8.0% in 2010 to 3.0% in 2019, while the prevalence among high school students decreased from 14.8% in 2009 to 8.4% in 2019 [46]. In Macau, government reports showed that the prevalence rates of smoking cigarettes among adolescents were 11.95%, 14.16%, and 10.84% in 2010, 2014, and 2018 [39,40,41].

In the U.S., the lifetime use of any illicit drug in adolescents declined gradually between 2000 and 2010 and was unvarying from 2019 and onwards; while the past 30-day use slightly decreased from 2000 to 2010 and remained stable starting from 2019 [32,34]. Marijuana is one of the many illicit substances that adolescents in Western countries frequently use because it is easily accessible and growing in popularity [11]. The past 30-day use of marijuana rose from 5.8% for 8th graders, 13.8% for 10th, and 19.4% for 12th graders in 2008, to 7%, 18%, and 22.7% in 2013 for the same grade group, respectively [47,48]. In Europe, between 1995 and 2011, the lifetime use of illicit drugs, like cocaine, ecstasy, amphetamine, and LSD, increased from 12% to 20% but started to decrease slowly after 2011 [13]. About 16% of adolescents have tried marijuana during some time in their life in the whole of Europe. The current (past-30-day) use of marijuana increased the greatest between 1995 and 1999, but the increase eased beginning in 2003 [13]. The highest prevalence of marijuana use that occurred in 2011 was 7.6% but remained relatively stable in the years after that [13]. Thus, for the past decade, the trend in illicit drug use in both the U.S and Europe displayed a downward trend in both lifetime and current use of illicit drugs among adolescents.

As discussed earlier, official annual reports issued by the National Narcotics Control Commission of China indicated that the number of registered drug users, including adolescent users, decreased dramatically between 2014 and 2019 [22,23]. According to the data collected by provincial governments in China in 2016, 8% of all registered drug users were individuals aged 25 and below [49]. Because the reporting agencies did not treat adolescents as a separate category, it is not clear how many of the young drug users were adolescents. Official statistics from Hong Kong also showed a large decrease in the number of people under the age of 21 who used illicit drugs, with the proportion of total drug users dropping from 17% in 2011 to 9% in 2020 [50]. The large decrease shown in the official statistics caused some scholars to question the reliability of the data that might have been undermined by “hidden drug use” [51] (p. 10). In Taiwan, statistics reported by the Ministry of Health and Welfare indicate that illegal drug use among adolescents decreased gradually between 2012 and 2020 after prior increases between 2007 and 2012 [52,53]. In Macau, reports from the Social Welfare Bureau showed that the prevalence of drug use among students from fifth graders to college undergraduates was 1.88%, 2.48%, and 2.92% in 2010, 2014, and 2018, respectively [39,40,41]. The data indicate a general increasing trend although the change from 2014 to 2018 was not statistically significant.

Considerable research has shown that adolescents are more likely to use psychoactive substances under the influences of certain risk factors. Family structure, parental supervision, peer association, school problems, the availability of drugs, and normative attitudes toward drug use have been frequently identified as predictors of cigarette, alcohol, and illicit drug use among adolescents [54,55].

Family structure can influence alcohol and illicit drug use in adolescents as growing up in single-parent households tends to increase the likelihood of using alcohol and drugs [56]. Relative to intact families, children growing up in single-parent families receive less parental monitoring, and inadequate parental monitoring makes the children more likely to engage in alcohol and other substance use as adolescents [57]. Likewise, adolescents residing in stepparent families are at an increased risk of becoming substance users, because these families tend to be more fragmented in terms of the bonding between the children and the stepparents [58].

Good family functioning, characterized by parental supervision and parental support, is also found to significantly impact the level of substance use in adolescents [59,60,61]. In most urban areas, like Macau, consistent involvement and support from parents in their children’s daily activities is challenging, because most of the parents have nonstandard work arrangements (e.g., shift work). Parents working with nonstandard schedules often have difficulties providing adequate parental involvement and monitoring for their children [62]. In addition, the nonstandard working schedule of parents usually impacts parent-child interaction and connectedness, which affects children’s developmental trajectory subsequently.

In addition to parents, peers can also play an important role in influencing adolescents’ substance use. Adolescence is characterized by a stage of autonomy development, and most of the connections established by adolescents during this period will be orientated towards their peers and less to their parents. Peer acceptance is a major component in adolescents’ development of autonomy because it is associated with high self-esteem and social competence [63,64]. Thus, to gain acceptance from peers, adolescents tend to mimic the behavior of their peers to acquire the identity of belonging to the peer group.

Furthermore, much research in the U.S. and Europe shows that lower school engagement or absenteeism can increase the risk of substance use in adolescents. Problematic school behaviors in adolescents are related to substance use including school absenteeism or truancy and poor academic performance [65]. School absenteeism or truancy is a robust precursor to subsequent drug use because truant youths, or absentees, are more likely to be exposed to licit and illicit substances than adolescents who are more committed to school [66,67]. The probability of smoking tends to be higher for students who have the least school engagement and face the greatest schoolwork difficulties [68]. Conversely, greater engagement in school was associated with less precocious substance use behavior, such as drinking, in most adolescents.

The availability of drugs can also systematically influence adolescents’ substance use by increasing their opportunities to obtain and use those substances. Prior research has indicated that when adolescents have easy access to alcohol, they are 3.5 times more likely to use alcohol and drink hazardously [69]. In addition, research that examined the relationship between the perceived availability of nicotine and the frequency of nicotine vaping revealed a connection between them [70].

In addition to the risk factors reviewed above, belief also plays an important role in substance use. Lower disapproval and perceived harm of drugs are correlated with a higher level of substance use [27,71,72]. Furthermore, the use of legal substances (i.e., alcohol, tobacco, and betel nuts) can increase the likelihood of illicit drug use [73,74,75,76,77]. Finally, demographic and socioeconomic variables, including sex [74,78,79], age [75,77], place of residence [78], educational levels [78], and income [80], have been reported as related to substance use.

However, most of the studies reviewed above were conducted outside the GCR. There is a lack of research on the trends in substance use among adolescents in the GCR other than government reports based on official data, such as the China Drug Situation Report, Annual Report of National Drug Abuse Monitoring, and the Central Registry of Drug Abuse Information of the Narcotics Department in Hong Kong. The survey results cited above concerning adolescent substance use in Macau were also based on unpublished reports released by the local government that are available only in Chinese. Government reports typically concentrate on the descriptions of frequencies and rates of illicit drug use, with limited attention to theoretically based risk and protective factors for adolescent substance use. Research using representative samples of adolescents in the GCR is needed to gain a better understanding the levels of adolescent drug use, its trend, and its risk and protective factors. Another issue that this study addresses is the inconsistency in the definition of adolescent in the studies conducted in the GCR, which have included samples of disparate age groups. For instance, the surveys of Student and Drugs in Macau covered youth between the ages of 10 and 22, whereas the Survey of Street Youth Drug Abuse included youths between 12 and 24. Comparison of adolescent drug use trends between the GCR and those of the other countries are difficult when different youth age groups are used. To address this problem, the current study collected data from secondary students aged 10–23. Because of the grade retention system widely implemented in Macau secondary schools, some students remained in school until their early 20s [81]. Students who were younger than 11 and older than 20, however, made up only about 0.2 percent of the samples in 2014 and 2018. The rest of the sample in either survey year consisted of students aged 11–20, an age group that is more consistent with the definition of adolescents adopted in studies conducted in other countries, especially in Europe and the U.S.