1996 National Household Survey on Drug Abuse: Preliminary Results

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8. TRENDS IN INITIATION OF DRUG USE

Estimates of drug use incidence, or initiation, provide another measure of the Nation's drug problem. They can suggest emerging patterns of use among young people. In the past, increases and decreases in incidence have usually been followed by corresponding changes in the prevalence of use. SAMHSA recently released a detailed report on incidence trends based on 1991-93 data, covering the period 1919 through 1992. Updated estimates and new estimates for 1993 and 1994 were included in Advance Report 18, which summarized the results of the 1995 NHSDA. Using the 1994, 1995, and 1996 NHSDA data, it is now possible to update those earlier estimates and develop estimates for 1995.

Details of the methodology are available in Trends in the Incidence of Drug Use in the United States, 1919-1992, released in 1996. Briefly, the estimates are based on the NHSDA questions on age at first use. Using each respondent’s reported age at first use in conjunction with his/her age and interview date, the respondent’s year of first use of each drug was determined by subtracting their age from the interview year and then adding the age at first use. By combining all respondents and applying sample weights, estimates of the number of new users of each drug for each year were made. These estimates include new users at any age, including under age 12. 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 person-years of exposure. The numerator of each rate is the number of persons in the age group who first used the drug in the year (times 1,000), while the denominator is the number of persons who were exposed to the risk of first use during the year, adjusted for their estimated exposure time in years. Persons who first used the drug in a prior year have zero risk of first use in the current year, and persons who still have never used the drug by the end of the current year had 1 year of exposure to risk. Persons who first used during the year are assumed to have a half year of exposure to risk.

The incidence estimates are based on retrospective reports of age at first drug use by survey respondents interviewed during 1994-96, and may therefore be subject to several biases, including bias due to differential mortality of users and nonusers of each drug, bias due to memory errors (recall decay and telescoping), and underreporting bias due to social acceptability and fear of disclosure. See Appendix 2, Section III for a discussion of these biases. As is explained in Appendix 2, 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. However, analyses have not clearly shown the magnitude of these biases.

Marijuana

Undisplayed Graphic
An estimated 2.4 million Americans used marijuana for the first time in 1995, about the same number as in 1994. The number had been increasing since 1991, after a long-term decrease that had been occurring since 1975. It is interesting to note that the decrease in prevalence of marijuana use that occurred in the 1980s did not begin to occur until several years after the peak in incidence. The rising incidence during the 1990s seems to have been fueled largely by the increasing rate of new use among youths age 12-17 years (from 39 per 1,000 person years in 1991 to about 75 per 1,000 person years in 1994 and 1995). This is in contrast with the epidemic of the late 1960s and early 1970s, which involved substantial increases among young adults as well as youths. The rates of marijuana initiation for youths in 1994 and 1995 are similar to the estimated rates in the late 1970s, the peak years for marijuana incidence and prevalence among youth (Figure 13).

Heroin

There were an estimated 141,000 new heroin users in 1995. Estimates of heroin incidence are subject to wide variability and usually do not show any clear trend, although there is a statistically significant upward trend in the number of new heroin users from 1992 to 1995, a finding that is consistent with anecdotal reports of increasing numbers of new heroin users. By combining 1991-1996 NHSDA data (Appendix 5 incidence tables are based on 1994-95 data), a more stable estimate of the long term trend emerges, showing that the recent increases in new heroin use are comparable to the increases seen in the epidemic of the late 1960s. The rate of heroin initiation for the age group 12-17 increased from around 0.5 during the 1980's to 2.5 in 1995 (Figure 14).

Undisplayed Graphic

A large proportion of the recent heroin initiates are young and are smoking, sniffing, or snorting heroin. Among recent initiates found in the 1995 and 1996 NHSDAs, 90 percent were under age 26 and 77 percent had never injected heroin. A similar analysis of new heroin users in the 1991 and 1992 NHSDAs showed that only 61 percent were younger than age 26 and only 46 percent had never injected (questions about smoking, snorting, and sniffing were not included in the NHSDA until 1993).

Cocaine and Crack Cocaine

The annual number of new cocaine users rose between 1992 and 1995, but was at a lower level than during the early 1980's. In 1995 there were an estimated 652,000 new users, while during 1980-1984 there had been about 1.3 million cocaine initiates per year. The rate of initiation by different age groups, however, has been changing in recent years. The rate among youths age 12-17 increased from 4.6 in 1991 to 10.6 in 1995. Historically, most initiation of cocaine use has taken place among young adults age 18-25. The rate for that age group fell from a high of 28.6 in 1980 to 10.2 in 1992. Since 1992 there has been no significant increase in this rate, but the rate in 1995 was 13.8. With the age group 18-25 showing a decrease in the rate of first use after 1980, the rate of first use for that group is now similar to that for the 12-17 age group. For crack cocaine, the estimated annual number of new users has remained stable in recent years.

Hallucinogens

There were an estimated 1.2 million new hallucinogen users in 1995, approximately twice the average annual number during the 1980s. The rate among youths age 12-17 increased between 1991 and 1995, from 10.4 to 27.5 per 1,000 person years. Over the same period, the rate for ages 18-25 years increased from 13.1 to 24.3.

Inhalants

There were an estimated 676,000 new inhalant users in 1995, up from 401,000 in 1991. The rate of first use among youths age 12-17 rose significantly from 1991 to 1995, from 10.7 to 21.8 per 1,000 person years.

Cigarettes

An estimated 3 million people tried their first cigarette in 1994 (1995 estimate not available). The rate of initiation among youths age 12-17 increased from 1991 to 1994. An estimated 1.7 million people began smoking on a daily basis in 1995, and there was no statistically significant change in the rate of youth initiation of daily smoking from 1991 to 1995. The annual number of new daily smokers has remained stable since 1982.

Alcohol

In 1994 there were approximately 4.1 million new users of alcohol, while in 1991 there were only 3.3 million users. The rate of new usage among the 18-25 age group was flat in recent years (240 per 1000 person years in 1994), but the rate among the 12-17 age group increased from 119 per 1000 person years in 1991 to 161 in 1994.

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