Estimates of substance use incidence, or initiation (i.e., number of new users during a given year), provide another measure of the Nation's substance use problem. They can suggest emerging patterns of use, particularly among young people. In the past, increases and decreases in incidence have usually been followed by corresponding changes in the prevalence of use, particularly among youths.
The incidence estimates in this report are based on combined 1999 and 2000 CAI data and should not be compared to previously published data based on PAPI data. Not only is the mode of data collection different for the incidence estimates prior to the 1999 NHSDA, but the estimation methodology has been revised as well. The estimation methodology is described in Appendix B and summarized below.
The incidence estimates are based on the NHSDA questions on age at first use, year and month of first use for recent initiates, the respondent's date of birth, and the interview date. Using this information along with editing and imputation when necessary, an exact date of first use is determined for each substance used by each respondent. For age-specific incidence rates, the period of exposure was defined for each respondent and age group for the time that the respondent was in the age group during the calendar year. Incidents of first use were also classified by year of occurrence and age at the date of first use. By applying sample weights to incidents of first use, estimates of the number of new users of each substance for each year were made. These estimates include new users at any age (including ages under 12) and are also shown for two specific age groups (12 to 17 and 18 to 25). In addition, the average age of new users in each year and age-specific rates of first use were estimated. These rates are presented in this report as the number of new users per 1,000 potential new users, since they indicate the rate of new use among persons who have not yet used the substance (i.e., potential new users). More precisely, the rates are actually the number of new users per 1,000 person-years of exposure. The numerator of each rate is the number of persons in the age group who first used the substance in the year, while the denominator is the person-time exposure measured in thousands of years. Each person's exposure time ends on the date of first use. For age-specific estimates, exposure is limited to time during the year that the person was in the age group. Persons who first used the substance in a prior year have zero exposure to first use in the current year, and persons who still have never used the substance by the end of the current year had one full year of exposure to risk.
The incidence estimates are based on retrospective reports of age at first substance use by survey respondents interviewed during 1999 and 2000, and may therefore be subject to several biases, including bias due to differential mortality of users and nonusers of each substance, bias due to memory errors (recall decay and telescoping), and underreporting bias due to social acceptability and fear of disclosure. See Appendix B for a discussion of these biases. As is explained in Appendix B, it is possible that some of these biases, particularly telescoping and underreporting because of fear of disclosure, may be affecting estimates for the most recent years more significantly. To account for this bias in the interpretation of the trends, a more stringent standard for determining statistical significance involving estimates from the most recent years (1997 and later) is used in this chapter. Differences are reported to be statistically
significant only if they differ at the "=.01 level. The usual standard in the rest of the report is the "=.05 level. This is an arbitrary standard that provides some protection against incorrect conclusions in the face of potential biases that can fluctuate and even change direction from year to year. Some tentative analysis of this problem is discussed in Appendix B. A more thorough analysis of the problem using the data from 1999 to 2001 will be conducted next year.
Because the incidence estimates are based on retrospective reports of age at first use, the most recent year available for these estimates is 1999, based on the 2000 NHSDA. Estimates for the year 1999 are based only on data from the 2000 survey, while estimates for earlier years are based on the combined 1999 and 2000 data. For two of the measures, first alcohol use and first cigarette use, initiation before age 12 is common. A two-year lag in reporting of estimates is applied for these measures, because the NHSDA sample does not cover youths under age 12. The two-year lag insures that initiation at age 10 and 11 is captured in the estimation.
Marijuana
The estimated annual number of new marijuana users has declined from 2.6 million in 1996 to about 2.0 million in 1999. This was preceded by a period of significant increase from 1990 (1.4 million new users) to 1996.
In 1965, there were an estimated 0.5 million new users of marijuana. The annual number of marijuana initiates increased until reaching a peak in 1976-77 (two years before the past month prevalence rate among youth peaked in 1979) at around 3.2 million new users per year. After that, the number of initiates declined to 1.4 million in 1990 (two years before the youth past month prevalence rate reached a low point in 1992) and then increased again to 2.6 million in 1996, a recent high point.
Youths aged 12 to17 have constituted about two-thirds of the new users of marijuana in recent years, with young adults aged 18 to 25 constituting most of the remaining third. Recent rates of new use among youth in 1996-1998 (averaging 86.4 initiates per 1000 potential new users) were higher than they have ever been. Rates of new use for both youth and young adults decreased between 1998 and 1999 (from 85.2 to 73.0, and from 44.1 to 31.7, respectively) (Figure 5.1).
The average age of initiation of marijuana use in 1999 was 17.0 years. Since 1992, the average age has ranged from 16.5 to 17.4. The average age of marijuana initiates has generally declined since 1965; during 1965-1969 it ranged from 19.0 to 20.4 years of age, and during 1970-1991 it ranged from 17.4 to 19.2 years of age.
Cocaine
The highest number of initiates of cocaine (including crack) occurred in the late 1970s and early 1980s, when there were approximately 1.0 to 1.5 million new users each year. After falling to recent lows in the early 1990s (e.g., 531,000 in 1991), the total number of new initiates of cocaine rose to 882,000 in 1998. The total increased between 1991 and 1998 both for youths aged 12 to 17 and young adults ages 18 to 25; however, the number increased more for youths than for young adults. In 1991, among youths there were only 92,000 new initiates of cocaine. By 1998, the number of new cocaine initiates among youth had risen to 339,000. This represents a higher rate of increase and a higher absolute increase than for young adults, which rose from an estimated 284,000 new initiates to 444,000 during the same period. Since 1965, the highest annual rate of first use among youth occurred in 1998 (14.5 per 1,000 potential new users), while the rate for young adults in 1998 (17.9 per 1000 potential new users) was only about three-fifths of its highest level (29.0 per 1000 potential new users) attained in 1983.
The estimates of the number of cocaine initiates and age-specific rates for 1999 appear to be generally lower than the corresponding estimates for 1998; however, the differences are not statistically significant.
The average age of cocaine initiates in 1999 was 19.5 years. This is younger than the average age of cocaine initiates for any year since 1973. From 1980 to 1993, the average age of cocaine initiates generally remained above 22 years.
Heroin
There were an estimated 104,000 new users of heroin in 1999. This number of new initiates is similar to the number in 1998 (140,000). Comparisons for youth and young adults show no statistically significant difference between the 1998 and 1999 numbers of new initiates. The number of new initiates among those aged 18 to 25 (53,000) was larger than the number among those aged 12 to 17 (34,000), as has been the historic pattern.
Hallucinogens
In 1998, the estimated number of new users of hallucinogens (including LSD and PCP) was 1.2 million, which is the highest estimate since 1965. The number of new users in 1999 (1.4 million) appears to be even higher than in 1998, but this increase is not statistically significant. The estimated number of new users among youths aged 12 to 17 (669,000) and young adults aged 18 to 25 (604,000) in 1999 are similar to the all-time high numbers of initiates in 1998.
Inhalants
In 1999, the estimated number of new users of inhalants was 1.0 million, not significantly different than the estimates for 1998 (918,000) or 1997 (975,000). However, these estimates are the highest annual number of inhalant initiates since 1965.
Psychotherapeutics
This category includes nonmedical use of any prescription-type pain reliever, tranquilizer, stimulant, or sedative. It does not include over-the-counter substances. Among the psychotherapeutics, pain relievers had the highest number of new users in 1999, a total of approximately 1,469,000 persons. This number has been increasing since the mid-1980s, when there were fewer than 400,000 initiates annually (Figure 5.2). Youths aged 12 to17 constitute the majority of this increase, from 78,000 in 1985 to 722,000 in 1999. The number of young adult initiates aged 18 to 25 increased from 166,000 to 492,000 during the same period.
The number of new users of stimulants was about 646,000 in 1999. This number is similar to the estimates for 1997 and 1998 (about 700,000 new users in each of those years). Since 1994, there have been more new users among youths aged 12 to 17 (322,000 in 1999) than among young adults aged 18 to 25 (213,000 in 1999).
There were approximately 642,000 new users of tranquilizers in 1999. While the number of new users of all ages in 1999 appears to be slightly lower than the number in 1998 (814,000), both the numbers of users for youth and young adults were similar to corresponding numbers in 1998. For youth, these estimated numbers of new users are the highest since 1965.
The estimated number of new initiates of sedatives was the smallest among the psychotherapeutics, at 143,00 new users in 1999. The number of new users of sedatives was significantly higher in the early 1970s (about 300,000 to 500,000). The number of initiates has been relatively lower since the early to mid 1980s with a reported low of about 42,000 in 1991.
Alcohol
In 1998, approximately 5.1 million persons initiated the use of alcohol. With reported data back to 1965, this puts the number of new users as high or higher than any estimate since the early 1970s. The largest contributors to this rise are youths aged 12 to 17, who now constitute about 67 percent of total new initiates. The late 1980s and early 1990s were a recent low for the number of new initiates. Estimates of new users of alcohol among youth at that time were about 1.7 to 1.8 million per year, and initiates among young adults aged 18 to 25 were 0.9 to 1.1 million. In 1998, the number of new users among youth grew dramatically to 3.4 million, while the initiates among young adults increased slightly to 1.2 million. The 3.4 million new users aged 12 to 17 represents about 15 percent of all youth in the nation.
Cigarettes
The incidence rate for cigarette use among youth aged 12 to 17 decreased between 1998 and 1999, from 141.4 to 120.0 persons per 1,000 potential new users. The numbers and rates among young adults aged 18 to 25 remained stable between 1998 and 1999. The overall annual number of persons who first tried a cigarette had increased between 1991 and 1996 from about 2.4 million to 3.4 million, then decreased to 2.9 million in 1998 (Figure 5.3).
The average age at first use of cigarettes was 15.4 years in 1998. While there have been some fluctuations, the average age has generally changed very little since 1965, ranging only from 14.9 to 16.2.
New use of cigarettes on a daily basis has decreased since its recent peak in 1997 at 1.9 million new users. In 1998, the number of initiates dropped to about 1.7 million and it dropped again in 1999 to about 1.4 million. Contributing to this decrease was the smaller number of new daily smokers among youths aged 12 to 17, falling from about 1,163,000 in 1997 to 783,000 in 1999. Translated to a per-day basis, the number decreased from 3,186 per day in 1997 to 2,145 per day in 1999.
The average age at first daily smoking was 17.7 years in 1999. While there have been some small variations in this average age, it has changed little since 1965, ranging from 17.6 to 19.3.
Smokeless Tobacco
The estimated annual number of new users of smokeless tobacco was stable during 1997 to 1999 at about a million per year (996,000 in 1997, 972,000 in 1998, and 982,000 in 1999) (Figure 5.3). More than half of smokeless tobacco initiates in 1999 were aged 12 to 17.
Cigars
The estimated number of new users of cigars fell dramatically between 1998 and 1999, from 4.6 million to 3.6 million. In 1998, the number of new cigar users had been at its highest level since 1965. The number had been only 1.4 million in 1991. The incidence rates for those aged 12 to 17 and 18 to 25 also declined significantly between 1998 and 1999, from 94.2 to 74.0 and from 83.5 to 60.7, respectively. During 1999, 415,000 fewer youth and 407,000 fewer young adults initiated cigar use than initiated use in 1998 (Figure 5.3).
This page was last updated on June 03, 2008.
SAMHSA, an agency in the Department of Health and Human Services, is the Federal
Government's lead agency for improving the quality and availability of
substance abuse prevention, addiction treatment, and mental health
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