9. Discussion of Trends in Substance Use among Youths and Young Adults

This report presents findings from the 2007 National Survey on Drug Use and Health (NSDUH). Conducted since 1971 and previously named the National Household Survey on Drug Abuse (NHSDA), the survey underwent several methodological improvements in 2002 that have affected prevalence estimates. As a result, the 2002 through 2007 estimates are not comparable with estimates from 2001 and earlier surveys. Therefore, the primary focus of the report is on comparisons of measures of substance use and mental health problems across subgroups of the U.S. population in 2007 and changes between 2006 and 2007, as well as between 2002 and 2007. This chapter provides an additional discussion of the findings concerning a topic of great interest—trends in substance use among youths and young adults.

An important step in the analysis and interpretation of NSDUH or any other survey data is to compare the results with those from other data sources. This can be difficult sometimes because the other surveys typically have different purposes, definitions, and designs. Research has established that surveys of substance use and other sensitive topics often produce inconsistent results because of different methods used. Thus, it is important to understand that conflicting results often reflect differing methodologies, not incorrect results. Despite this limitation, comparisons can be very useful. Consistency across surveys can confirm or support conclusions about trends and patterns of use, and inconsistent results can point to areas for further study. Further discussion of this issue is included in Appendix D, along with descriptions of methods and results from other sources of substance use and mental health data.

Unfortunately, few additional data sources are available at this time to compare with NSDUH results. One established source is Monitoring the Future (MTF), a study sponsored by the National Institute on Drug Abuse (NIDA). MTF surveys students in the 8th, 10th, and 12th grades in classrooms during the spring of each year, and it also collects data by mail from a subsample of adults who had participated earlier in the study as 12th graders (Johnston, O'Malley, Bachman, & Schulenberg, 2007c, 2008a, 2008b). Historically, NSDUH rates of substance use among youths have been lower than those of MTF, and occasionally the two surveys have shown different trends over a short time period. Nevertheless, the two sources have shown very similar long-term trends in prevalence. NSDUH and MTF rates of substance use generally have been similar among young adults, and the two sources also have shown similar trends.

A comparison of NSDUH and MTF estimates for 2002 to 2007 is shown in Tables 9.1 and 9.2 at the end of this chapter for several substances that are defined similarly in the two surveys. MTF data on 8th and 10th graders combined give the closest match on age to estimates for NSDUH youths aged 12 to 17, while MTF follow-up data on persons aged 19 to 24 provide the closest match on age to estimates for NSDUH young adults aged 18 to 25. The NSDUH results are remarkably consistent with MTF trends for both youths and young adults, as discussed below.

Both surveys generally show decreases between 2002 and 2007 in the percentages of youths who used marijuana, Ecstasy, LSD, alcohol, and cigarettes in the lifetime, past year, and past month (Table 9.1). One exception was for LSD in the past month for MTF, and information on cigarettes in the past year was not available for MTF. Estimates from both surveys indicate a decline in cocaine use between 2002 and 2007, although the trend was statistically significant in NSDUH data but not in MTF data. Both surveys show no decrease in the rates of past year and past month inhalant use among youths between 2002 and 2007, although only NSDUH shows a significant decrease in lifetime use. The consistency between NSDUH and MTF trend data is found not only in terms of the specific drugs showing decreases, but also in terms of the magnitude of the decreases. Despite the higher levels of prevalence estimated from MTF, the two surveys show very similar rates of change in past month prevalence, especially for the three substances used most commonly by youths: alcohol, cigarettes, and marijuana. Between 2002 and 2007, the rate of current alcohol use among youths declined 10 percent according to both NSDUH and MTF. Current cigarette use prevalence rates in 2007 were 25 percent lower in both NSDUH and MTF compared with 2002 rates. For past month marijuana use, the NSDUH decline from 2002 to 2007 was 18 percent, and the MTF decline was 24 percent.

Data on young adults also show similar trends in the two surveys, although not as consistent as for the youth data.  Potential reasons for differences are the relatively smaller MTF sample size for young adults and possible bias in the MTF sample due to noncoverage of school dropouts and a low overall response rate, considering nonresponse by schools, by students in the 12th grade survey, and in the follow-up mail survey. Both surveys show declines from 2002 to 2007 in past year and past month cigarette and marijuana use among young adults, although the decline in past month marijuana use in NSDUH was not statistically significant. However, the NSDUH rates of decline in current cigarette and marijuana use for young adults were less than the declines in NSDUH for youths and for young adults in MTF. Past month marijuana prevalence among young adults declined 5 percent according to NSDUH and 12 percent according to MTF. For past month cigarette use, declines were 11 percent in NSDUH and 18 percent in MTF. Both surveys show no significant change from 2002 and 2007 in the rate of current alcohol use among young adults. A significant decline between 2006 and 2007 in past month cocaine use is seen in the NSDUH data, and the MTF data show a similar drop in use (although not statistically significant).

Both NSDUH and MTF generally show substantial decreases for both youths and young adults in the past year use of Ecstasy and LSD between 2002 and 2004, then a leveling in 2005. The 2006 data from both surveys had suggested the start of a possible resurgence in the past year use of Ecstasy among youths and young adults, but the 2007 data do not indicate any major increase or decrease since 2005, in general. The only statistically significant change in past year Ecstasy use between 2005 and 2007 was for use among youths in NSDUH (from 1.0 to 1.3 percent). NSDUH also showed a statistically significant increase between 2005 and 2006 in past year initiation of Ecstasy use for young adults (from 322,000 initiates in 2005 to 494,000 initiates in 2006), but the increase did not continue in 2007 (414,000 initiates).

Another source of data on trends in the use of drugs among youths is the Youth Risk Behavior Survey (YRBS), sponsored by the Centers for Disease Control and Prevention. YRBS surveys students in 9th through 12th grades in classrooms every other year during the spring (Eaton et al., 2008). The most recent survey was completed in 2007. YRBS has generally shown higher prevalence rates but similar long-term trends when compared with NSDUH and MTF. However, comparisons between YRBS and NSDUH or MTF are less straightforward because of the different periodicity (i.e., biennially instead of annually) and ages covered, the limited number of drug use questions, and smaller sample size in the YRBS. For the substances for which information on current use is collected in the YRBS, including alcohol, cigarettes, marijuana, and cocaine, the YRBS trend results between 2001 and 2007 are consistent with NSDUH and MTF (Eaton et al., 2008; Grunbaum et al., 2002). YRBS data for the combined grades 9 through 12 showed no significant change in alcohol use (47.1 percent in 2001 and 44.7 percent in 2007), a decrease in cigarette use (28.5 percent in 2001, 20.0 percent in 2007), a decrease in marijuana use (23.9 percent in 2001, 19.7 percent in 2007), and a decline in cocaine use (4.2 percent in 2001, 3.3 percent in 2007).

Although changes in NSDUH preclude direct comparisons of recent estimates with estimates from before 2002, it is important to put the recent trends in context by reviewing longer term trends in use. NSDUH data (prior to the design changes in 1999 and 2002) on youths aged 12 to 17 and MTF data on high school seniors have shown substantial increases in youth illicit drug use during the 1970s, reaching a peak in the late 1970s. Both surveys then showed significant declines throughout the 1980s until about 1992, when rates reached a low point. These trends were driven by the trend in marijuana use. With the start of annual data collection in NSDUH in 1991, along with the biennial YRBS and the annual 8th and 10th grade samples in MTF, trends among youths are well documented since the low point that occurred in the early 1990s. Although they employ different survey designs and cover different age groups, the three surveys are consistent in showing increasing rates of marijuana use during the early to mid-1990s, reaching a peak in the late 1990s, followed by consistent declines in use since the turn of the 21st century (Figure 9.1).

Below is a line graph. Click here for the text describing this graph.

Figure 9.1 Past Month Marijuana Use among Youths in NSDUH, MTF, and YRBS: 1971-2007