Results
Differences in baseline characteristics (age, gender, educational level and ethnicity) are presented in Table 1. With respect to age distribution a significant difference was found between the populations in 2005 and 2009. These differences are considered in the analysis by using multivariate logistic regression.
Table 2 shows the prevalence rates for alcohol use, smoking and cannabis use among 1,854 high school students in 2005 and the prevalence rates among 2,088 high school students in 2009, adjusted for age, gender and educational level. There is a discernible decrease in alcohol consumption, smoking and cannabis use in the period 2005 - 2009. The strongest decline over time was found for alcohol use. The prevalence of last month (current) drinking decreased from 61.8 % in 2005 to 36.5 % in 2009. For last month binge drinking and last month being drunk or tipsy the prevalence decreased from 38.7 % in 2005 to 24.0 % in 2009 and from 29.7 % in 2005 to 16.5 % in 2009, respectively. Decreases were seen between 2005 and 2009 for weekly smoking and for lifetime and last month cannabis use as well.
Similar decreases between 2005 and 2009 in prevalence rates of alcohol consumption (current and binge drinking) were found among boys and girls (Table 2). The decline of being drunk or tipsy was stronger among girls than among boys (p = 0.006). Moreover, the decrease of smoking and cannabis use was found only among girls.
These trends have resulted in significant differences in prevalence rates of substance use between boys and girls in 2009 that were not found in 2005. In 2009 boys were more often than girls binge drinkers last month (OR 1.33 95 % CI 1.07–1.64) and were more often drunk or tipsy (OR 1.35 95 % CI 1.07–1.72). Moreover, in 2009 more boys than girls reported to have used cannabis at least once (OR 2.34 95 % CI 1.82–3.02) and to use cannabis weekly (OR 2.90 95 % CI 2.04–4.14). In 2009 smoking habits in boys and girls were similar, whereas in 2005 girls reported more weekly smoking than boys (OR 0.65 95 % CI 0.49–0.86).
Table 3 shows the age-related prevalence of alcohol consumption, smoking and cannabis use among high school students aged 13, 14, 15 and 16 in both surveys (2005 and 2009). For all ages a decrease in the prevalence of alcohol consumption was found between 2005 and 2009. The strongest decrease was found among 13-year-old students for last month drinking, last month binge drinking and last month being drunk or tipsy. The smallest decrease was found among 16-year-olds. In smoking habits the only significant decrease was found for 13 years olds between 2005 and 2009. On the other hand, the proportion of students who had used cannabis at least once decreased between 2005 and 2009 among students older than 13: among 14-year-olds from 15.9 % to 9.8 %, among 15 year olds from 31.6 % to 21.9 %, and among 16-year-olds from 41.9 % in 2005 to 34.3 % in 2009.
These trends have resulted in an increased age-related difference in the prevalence of alcohol consumption and smoking between 13 and 16-year-olds. Moreover, the proportion of binge drinking students (last month) among the alcohol-drinking students (last month) decreased from 61.0 % in 2005 to 55.1 % in 2009 (p = 0.012). This decrease is due to the decrease in the proportion of 15- and 16-year-old binge drinking students from 67.2 % and 78.4 % in 2005 to 57.6 % and 70.1 % in 2009, respectively.
In both educational levels the alcohol use between 2005 and 2009 decreased (Table 3). The percentage of VMBO students (lower educational level) that were current drinkers decreased from 64.2 % in 2005 to 43.2 % in 2009. Among HAVO/VWO students (higher educational level) 60.1 % were current drinkers in 2005, whereas in 2009 this percentage was 32.0 %. The prevalence of current binge drinking decreased between 2005 and 2009 among both VMBO students and HAVO/VWO students. The prevalence decreases in current binge drinking and being drunk/tipsy were larger among HAVO/VWO students than among VMBO students (p = 0.007 and p = 0.016, respectively).
A time trend in smoking habits was only observed in VMBO students. Between the surveys in 2005 and 2009 weekly smoking among VMBO students declined from 21.9 % in 2005 to 17.6 % in 2009, and daily smoking from 16.2 % to 12.6 %. In 2005, the prevalence of smoking among HAVO/VWO students was already three times lower than among VMBO students. Between 2005 and 2009 lifetime cannabis use decreased in both educational levels. Last month cannabis use only decreased among HAVO/VWO students.
In 2009 students with a lower educational level were still more at risk of using stimulants than students with a higher educational level. VMBO students were more often last month drinkers (OR 1.50 95 % CI 1.21–1.86), last month binge drinkers (OR 2.29 95 % CI 1.83–2.88), last month being drunk or tipsy (OR 1.83 95 % CI 1.42–2.35), weekly smokers (OR 2.62 95 % CI 1.96–3.50), daily smokers (OR 2.74 95 % CI 1.95–3.87), weekly (OR 1.55 95 % 1.20–2.02) and ever cannabis users (OR 1.83 95 % CI 1.30–2.59) than HAVO/VWO students.
Clustering of Substance Use
We investigated the clustering of substance use among the high school students in both the 2005 and 2009 survey ( Table 4 ). Students who did not drink alcohol in the last month hardly used any other substances. Multivariate logistic analysis with correction for gender, age and educational level showed that the prevalence rate of students who did not use substances (no recent alcohol, no weekly smoking, no cannabis use ever) increased from 36.6 % in 2005 to 60.7 % in 2009 (p = <0.001).
Multivariate logistic regression analysis with correction for gender, age and educational level, showed that in 2009 students who had recently drunk alcohol were about eight times more at risk to smoke weekly and/or to use cannabis (ever) than students who did not drink any alcohol in the last month (OR 7.59 95 % CI 5.56–10.38), in 2005 the risk was about ten times higher (OR 9.61 95 % CI 6.47–14.28). We investigated clustering of substance use in different subgroups and whether it changed over time. Table 5 shows the use of tobacco (weekly) and/or cannabis (ever) among alcohol-drinking and non-alcohol-drinking students in 2005 and in 2009, in relation to gender, age and educational level. Only among girls the clustered use of alcohol, tobacco and cannabis differed between 2005 and 2009. In the period 2005–2009 the prevalence of the use of tobacco and/or cannabis among alcohol-drinking girls had decreased from 35.3 % to 27.1 % (p = <0.001).