3. Trends in Marijuana Incidence - Initiation of Marijuana Use: Trends, Patterns, and Implications Report

Estimates of marijuana incidence, or the number of new marijuana users during a given year, provide an important measure of the Nation's marijuana use problem. They can suggest emerging patterns of use, give clues about the changes in the prevalence of use, identify at-risk subgroups for targeting prevention programs, and suggest substance abuse treatment needs for the Nation.

This chapter presents incidence estimates of marijuana use based on data from the 1999 and 2000 National Household Surveys on Drug Abuse (NHSDAs). These 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 year, month, and day of first use was determined for each substance used by each respondent. Because these data were collected on a retrospective basis, incidence estimates were always 1 year behind the data on current use. For age-specific incidence rates, the period of exposure was defined for each respondent and age group for the time that a respondent was in an age group during a calendar year.

The average age of new users in each year also was estimated. These rates are presented in this report as the number of new marijuana users per 1,000 potential new users because they indicate the rate of new use among persons who had not yet used the drug (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 drug in the year. 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 the time during the year that the person was in that age group. Persons who first used the drug in a prior year had zero exposure to first use in the current year, and persons who still had never used the drug by the end of the current year had 1 full year of exposure to the risk.

Because these incidence estimates were based on retrospective reports, they were subject to several biases, as discussed in Chapter 2. It is possible that some of these biases, particularly telescoping and underreporting because of fear of disclosure, may affect estimates for the most recent years more significantly. However, further analysis is needed to understand the magnitude of these biases. In addition, the estimates in this report were based on the new CAI data, and the estimation methodology for these estimates was different from that used in NHSDAs prior to 1999 (i.e., based on paper-and-pencil interviewing [PAPI] methodology). The revised methodology had an impact on age-specific rates (Gfroerer et al., in press). Thus, comparisons with prior NHSDA estimates should not be made.

Estimated trends and incidence rates of marijuana use reported in this chapter were based on the combined sample of 1999 and 2000 CAI data. These estimates are presented by the overall sample, combined age groups and gender (e.g., 12 to 14 male, 12 to 14 female, 15 to 17 male, 15 to 17 female, 18 to 20 male, 18 to 20 female, 21 or older male, and 21 or older female), and race/ethnicity (e.g., white, black, Hispanic, Asian/Pacific Islander/Native Hawaiian, American Indian/Alaska Native, and persons reporting more than one race).

Table 3.1 summarizes the estimated number of new marijuana users, mean age of first use, and annual incidence rates from 1999 and 2000 NHSDA data. An estimated 2.0 million Americans aged 12 or older used marijuana for the first time in 1999, which was fewer than the estimated number of new users in 1998 (approximately 2.5 million Americans), but still above the 1989 and 1990 levels (1.4 million each year). Figure 3.1 shows that the rate of marijuana initiation increased during the late 1960s and early 1970s, with a peak in 1976 and 1977 (21.0 per 1,000 potential new users). After that period, the rate of new marijuana use decreased to 8.5 in 1990, followed by an increase to 16.8 in 1996, then a decrease to 13.6 in 1999. The mean age at first use was 19 years in the early 1970s and decreased to 17 years in the 1990s.

Note: The numerator of each rate is the number of persons who first used marijuana in the year, while the denominator is the person-time exposure measured in thousands of years for persons aged 12 or older.

Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999 and 2000.

Over the years, rates of marijuana incidence were generally higher among males than among females (Tables 3.2 and 3.3, Figure 3.2). Among males, the rate increased dramatically from 4.9 in 1965 to 22.9 in 1971. The highest peak was noted in 1976-1977 (close to 24). After the late 1970s, incidence rates for males declined to around 10 in the late 1980s, followed by a period of increase during the 1990s to 19.3 in 1997. For females, the incidence rate increased steadily from 3.3 in 1965 to 18.9 in 1976. Similar to the pattern of males, the rate was lower during the 1980s, followed by an increase during the early 1990s. The most recent peak for females was in 1996 (15.5). For both genders, the rate in 1999 (15.5 and 12.1, respectively, for males and females) was lower than the rate in 1996-1998.

The estimated mean age at first marijuana use generally has been slightly younger in males than in females. For males, the mean age at first marijuana use ranged from 18-19 years during late 1960s to 16-17 years in recent years. For females, the mean age at first marijuana use decreased from 20 years during late 1960s to around 17 years in recent years. The average age of new marijuana users in 1999 was 16.4 years for males and 17.6 years for females.

Detailed data on age- and gender-specific incidence rates are summarized in Table 3.4. The data indicate that trends of incidence rates peaked at different periods for youths and adults. Among youths aged 12 to 17, annual incidence rates reached peaks during the late 1970s and late 1990s, and the pattern was similar for both genders. Among adults, particularly males, a peak rate of initiation was reached during the late 1960s, with rates remaining high throughout the 1970s, before dropping significantly in the 1980s. In addition, among adults aged 21 or older, the data did not show a peak in new use during the late 1990s, while persons aged 18 to 20 did.

Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999 and 2000.

The trends of marijuana incidence also varied across racial/ethnic groups (Tables 3.5 to 3.7). In 1999, an estimated 1.4 million new marijuana users were white; there were 0.25 million black initiates, 0.25 million Hispanic initiates, 0.04 million Asian initiates (including other Pacific Islanders and Native Hawaiians), 0.03 million American Indian/Alaska Native initiates, and 0.03 million initiates who reported more than one race. Except for American Indians/Alaska Natives, the estimated numbers of new users were lower in 1999 than in 1998. In recent years, American Indians/Alaska Natives appeared to have a younger mean age of first marijuana use (14.1 years in 1999) than members of other racial/ethnic groups. In 1999, the mean age of marijuana initiation was 17.2 years for whites, about 16.4 years for blacks and Hispanics, 18.8 for Asians (including other Pacific Islanders and Native Hawaiians), and 15.8 years for persons reporting more than one race.

Among whites, the trend pattern was generally consistent with the overall trend seen in Table 3.1. Probably because of small samples, more variation was noted for non-Hispanic minority groups (American Indians/Alaska Natives, Asians/Pacific Islanders/Native Hawaiians, and persons reporting more than one race) and for years before 1990. Incidence rates between 1990 and 1999 for the racial/ethnic groupings are displayed in Figure 3.3.

Among blacks, the annual incidence rate (per 1,000 potential new users) increased from 8.0 in 1966 to 16.7 in 1968, reached a peak at about the same time as whites (19.4 in 1976), then remained high throughout the late 1970s. Following the low rates in the 1980s, rates among blacks rose again in the early 1990s, reached a peak in 1997 and 1998 (19.2 and 19.1, respectively), then dropped to 14.0 in 1999. Similar to the general pattern for whites and blacks, Hispanics' annual incidence rate rose during late 1970s and 1990s, with a peak in 1998 (17.8).

Asians (including other Pacific Islanders and Native Hawaiians) typically had lower annual incidence rates than the other racial/ethnic groups. However, the sample size did not allow for the generation of reliable estimates for trend data prior to 1985. Among recent initiates, rates of first marijuana use by racial/ethnic groups were generally lower in 1999 than in 1998, with the exception of American Indians/Alaska Natives. Estimates from Table 3.7 suggest a higher rate of new marijuana use in recent years among American Indians/Alaska Natives and among persons reporting more than one race. The annual incidence rate among American Indians/Alaska Natives was 21.2 (per 1,000) in 1995 and had risen over these years to a rate of 46.5 in 1999. Similar to the rates for American Indians/Alaska Natives, incidence rates among persons reporting more than one race were higher than among other racial/ethnic groups during the 1990s. Their incidence rate ranged from 26.2 to 35.4 between 1995 and 1999 compared with a rate below 20.0 among whites, blacks, Hispanics, and Asians/Pacific Islanders/Native Hawaiians.

Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999 and 2000.

Table 3.1 Estimated Numbers (in Thousands) of Persons Who First Used Marijuana During the Years 1965 to 1999, Their Mean Age at First Use, and the Annual Incidence Rates of First Use (Per 1,000 Person-Years of Exposure), for All Ages
Year	Number of Initiates (1,000s)	Mean Age at First Use	Incidence Rates¹
1965	553	20.4	4.0
1966	975	19.2	7.0
1967	1,385	19.5	9.7
1968	1,738	19.4	12.0
1969	2,123	19.0	14.5
1970	2,592	18.7	17.5
1971	2,789	18.7	18.7
1972	2,819	18.8	18.8
1973	2,854	18.6	19.0
1974	2,853	17.9	18.9
1975	2,874	18.3	19.0
1976	3,184	18.5	21.0
1977	3,163	18.3	20.9
1978	2,967	18.1	19.5
1979	2,859	18.1	18.7
1980	2,522	19.2	16.4
1981	1,867	17.9	12.0
1982	2,021	18.8	12.8
1983	1,865	18.2	11.7
1984	2,012	18.3	12.4
1985	1,865	18.1	11.4
1986	1,753	17.6	10.6
1987	1,588	17.6	9.5
1988	1,550	17.4	9.2
1989	1,447	17.7	8.7
1990	1,407	18.3	8.5
1991	1,485	18.0	9.1
1992	1,599	16.7	9.8
1993	1,954	17.2	12.2
1994	2,187	16.7	13.8
1995	2,357	16.5	15.1
1996	2,590	17.1	16.8
1997	2,494	17.0	16.5
1998	2,488	17.4	16.7
1999²	2,028	17.0	13.6
¹ The numerator of each rate is the number of persons who first used marijuana in the year, while the denominator is the person-time exposure measured in thousands of years for persons aged 12 or older. ² Estimated using 2000 data only. Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999 and 2000.

Table 3.2 Estimated Age-Gender Numbers (in Thousands) of Persons Who First Used Marijuana During the Years 1965 to 1999
Year	Number of Initiates (1,000s)
Year	Males 12-14	Females 12-14	Males 15-17	Females 15-17	Males 18-20	Females 18-20	Males 21+	Females 21+
1965	41	*	114	36	72	84	70	98
1966	64	*	159	86	271	129	142	102
1967	113	54	200	98	312	149	314	132
1968	98	38	248	109	552	195	293	184
1969	115	78	372	216	488	261	266	291
1970	197	187	435	328	496	279	333	303
1971	266	210	405	283	486	320	358	395
1972	264	148	496	453	414	308	385	326
1973	261	225	565	385	365	320	353	306
1974	245	271	584	468	329	301	253	307
1975	309	275	469	493	339	270	235	404
1976	213	208	665	603	414	317	303	420
1977	292	272	633	559	396	309	291	354
1978	263	221	691	542	317	341	230	296
1979	287	237	627	522	362	300	176	274
1980	184	165	486	531	215	297	249	312
1981	156	144	357	383	212	203	120	221
1982	189	132	385	391	254	215	154	258
1983	182	152	394	329	197	172	241	128
1984	237	176	382	385	209	207	160	215
1985	184	155	370	371	232	194	204	118
1986	155	134	361	382	212	183	159	118
1987	85	109	340	386	250	189	75	124
1988	132	80	348	327	210	164	112	136
1989	122	96	326	280	175	175	116	99
1990	130	94	309	240	197	135	103	148
1991	154	96	302	265	180	171	160	101
1992	185	159	347	258	222	173	104	82
1993	244	222	364	355	229	210	124	136
1994	276	261	450	394	242	234	123	121
1995	336	274	510	401	226	256	137	141
1996	350	294	523	523	235	268	138	202
1997	329	313	547	478	266	227	145	139
1998	334	313	519	467	236	250	154	175
1999¹	291	255	446	399	151	175	124	159
* Low precision; no estimate reported. ¹ Estimated using 2000 data only. Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999 and 2000.

Table 3.3 Estimated Numbers (in Thousands) of Persons Who First Used Marijuana During the Years 1965 to 1999, Their Mean Age at First Use, and the Annual Incidence Rates of First Use (Per 1,000 Person-Years of Exposure), by Gender
Year	Number of Initiates (1,000s)		Mean Age at First Use		Incidence Rates¹
Year	Males	Females	Males	Females	Males	Females
1965	315	239	18.1	23.4	4.9	3.3
1966	642	333	18.8	19.9	9.8	4.5
1967	952	433	19.1	20.4	14.4	5.7
1968	1,212	527	19.0	20.1	18.1	6.8
1969	1,264	859	18.6	19.5	18.7	10.9
1970	1,479	1,112	18.6	19.0	21.7	13.9
1971	1,570	1,218	18.4	19.0	22.9	15.1
1972	1,560	1,258	19.2	18.3	22.7	15.5
1973	1,587	1,267	18.6	18.6	23.1	15.5
1974	1,493	1,360	17.7	18.1	21.7	16.6
1975	1,405	1,469	17.7	18.9	20.4	17.8
1976	1,625	1,559	18.2	18.8	23.6	18.9
1977	1,647	1,517	18.0	18.5	23.9	18.4
1978	1,556	1,411	17.6	18.7	22.5	17.0
1979	1,507	1,352	17.5	18.7	21.7	16.2
1980	1,187	1,335	19.0	19.4	17.0	15.9
1981	896	971	17.2	18.6	12.6	11.4
1982	1,014	1,007	17.9	19.7	14.1	11.7
1983	1,049	815	18.9	17.4	14.4	9.4
1984	1,020	992	18.3	18.2	13.8	11.3
1985	1,021	844	18.2	17.9	13.6	9.5
1986	925	828	17.8	17.4	12.1	9.2
1987	773	815	17.3	17.9	10.0	9.0
1988	834	716	17.1	17.9	10.8	7.9
1989	787	660	17.5	17.8	10.3	7.3
1990	774	633	17.5	19.4	10.2	7.1
1991	837	648	18.1	17.8	11.2	7.3
1992	909	690	16.6	16.8	12.3	7.8
1993	1,009	945	16.8	17.6	13.8	10.8
1994	1,152	1,035	16.7	16.8	16.0	11.9
1995	1,254	1,103	16.4	16.7	17.7	12.9
1996	1,284	1,306	16.4	17.7	18.5	15.5
1997	1,318	1,176	17.0	16.9	19.3	14.1
1998	1,268	1,220	17.6	17.2	18.9	14.9
1999²	1,034	993	16.4	17.6	15.5	12.1
¹ The numerator of each rate is the number of persons who first used marijuana in the year, while the denominator is the person-time exposure measured in thousands of years. ² Estimated using 2000 data only. Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999 and 2000.

Table 3.4 Estimated Annual Age-Gender Specific Incidence Rates of First Use (Per 1,000 Person-Years of Exposure) of Persons Who First Used Marijuana During the Years 1965 to 1999
Year	Age-Gender Specific Incidence Rates¹
Year	Males 12-14	Females 12-14	Males 15-17	Females 15-17	Males 18-20	Females 18-20	Males 21+	Females 21+
1965	7.2	*	21.5	5.9	18.2	16.0	3.0	3.4
1966	11.1	*	30.5	14.7	60.9	22.3	5.8	3.3
1967	19.7	8.8	37.7	17.2	68.0	24.0	12.4	4.1
1968	17.1	6.1	46.9	18.8	125.6	34.0	11.0	5.4
1969	19.1	11.9	71.2	37.4	121.7	49.1	9.6	8.2
1970	30.9	28.6	86.4	57.5	124.6	54.8	11.6	8.2
1971	41.3	32.3	83.0	48.9	125.5	64.1	12.2	10.3
1972	43.6	22.5	99.5	77.0	108.3	64.5	12.8	8.2
1973	43.9	33.5	110.6	68.0	98.2	68.3	11.4	7.5
1974	40.0	42.0	118.6	84.6	89.7	64.1	8.0	7.3
1975	48.3	44.4	99.4	88.5	93.2	58.2	7.2	9.4
1976	34.1	34.0	142.5	109.6	114.5	71.3	9.1	9.5
1977	49.9	44.5	131.6	108.3	117.5	72.6	8.5	7.8
1978	48.4	37.6	141.2	110.5	100.0	81.3	6.6	6.4
1979	57.0	41.6	133.2	106.3	114.3	73.7	4.9	5.8
1980	37.2	29.9	110.3	109.1	66.0	78.0	6.8	6.5
1981	30.9	26.2	82.9	79.7	60.3	54.4	3.2	4.5
1982	36.3	23.7	93.4	83.0	72.3	57.6	4.0	5.2
1983	34.5	26.9	96.2	69.6	58.4	44.9	6.2	2.5
1984	46.5	31.4	90.7	82.2	63.5	53.3	4.0	4.1
1985	38.1	28.8	85.8	79.2	73.7	50.5	5.0	2.2
1986	33.4	26.5	83.6	80.6	67.2	47.5	3.8	2.2
1987	18.5	22.0	80.8	81.6	76.0	50.0	1.8	2.2
1988	29.0	16.7	85.9	71.4	62.2	43.0	2.6	2.4
1989	26.3	19.6	82.9	64.3	51.7	45.3	2.6	1.7
1990	27.2	19.1	79.7	55.3	58.6	34.5	2.3	2.5
1991	31.5	18.6	77.3	62.2	56.5	44.3	3.5	1.7
1992	36.1	29.3	88.3	59.6	70.9	46.6	2.2	1.4
1993	45.6	39.6	90.5	82.5	75.2	57.4	2.6	2.2
1994	49.6	46.8	112.7	89.1	80.5	67.3	2.5	1.9
1995	61.2	49.9	126.8	90.3	76.9	74.6	2.8	2.2
1996	65.1	55.0	127.0	117.8	80.5	80.4	2.8	3.1
1997	60.2	59.2	130.5	110.3	94.0	67.7	2.8	2.1
1998	59.9	58.8	127.4	111.5	83.9	75.2	3.0	2.6
1999²	51.8	48.1	112.2	97.9	53.0	52.1	2.3	2.4
* Low precision; no estimate reported. ¹ The numerator of each rate is the number of persons who first used marijuana in the year, while the denominator is the person-time exposure measured in thousands of years. ² Estimated using 2000 data only. Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999 and 2000.

Table 3.5 Estimated Numbers (in Thousands) of Persons Who First Used Marijuana During the Years 1965 to 1999, by Racial/Ethnic Subgroups
Year	Number of Initiates (1,000s)
Year	White	Black	Hispanic	Asian / Pacific Islander / Native Hawaiian	American Indian / Alaska Native	More Than One Race
1965	427	*	*	*	*	*
1966	804	113	*	*	*	*
1967	1,180	128	49	*	*	*
1968	1,417	246	62	*	*	*
1969	1,834	175	63	*	*	*
1970	2,264	180	73	38	*	21
1971	2,313	228	177	*	14	*
1972	2,413	244	111	12	22	17
1973	2,442	260	91	*	*	*
1974	2,343	256	213	12	*	19
1975	2,377	296	171	12	*	*
1976	2,615	317	172	*	31	*
1977	2,608	277	163	74	*	28
1978	2,370	297	206	77	*	12
1979	2,388	275	127	*	8	23
1980	2,067	235	168	*	*	*
1981	1,518	195	120	*	6	*
1982	1,640	164	165	*	16	7
1983	1,459	149	187	39	11	*
1984	1,633	232	98	*	8	7
1985	1,437	179	165	65	7	12
1986	1,375	186	130	25	24	13
1987	1,242	140	134	52	10	12
1988	1,222	137	132	20	24	15
1989	1,074	137	183	21	*	16
1990	1,057	144	144	49	8	5
1991	1,092	164	146	50	14	20
1992	1,154	199	178	27	13	27
1993	1,388	256	225	34	23	28
1994	1,582	273	250	41	16	24
1995	1,711	282	259	54	16	35
1996	1,848	303	307	75	19	39
1997	1,733	345	308	44	20	43
1998	1,702	336	320	80	19	31
1999¹	1,436	248	246	39	30	28
* Low precision; no estimate reported. ¹ Estimated using 2000 data only. Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999 and 2000.

Table 3.6 Estimated Mean Ages at First Use of Persons Who First Used Marijuana During the Years 1965 to 1999, by Racial/Ethnic Subgroups
Year	Mean Age at First Use
Year	White	Black	Hispanic	Asian / Pacific Islander / Native Hawaiian	American Indian / Alaska Native	More Than One Race
1965	21.3	*	*	*	*	*
1966	19.3	19.2	*	*	*	*
1967	19.7	18.5	22.0	*	*	*
1968	19.3	19.6	20.2	*	*	*
1969	19.0	19.0	18.6	*	*	*
1970	18.9	17.1	17.1	18.0	*	*
1971	18.8	18.4	17.9	*	*	*
1972	18.8	18.0	21.2	*	19.5	*
1973	18.7	18.4	18.0	*	*	*
1974	17.9	18.4	17.1	*	*	16.6
1975	18.5	17.3	18.1	*	*	*
1976	18.4	19.8	17.7	*	16.8	16.4
1977	18.4	18.0	18.1	17.7	16.6	*
1978	17.9	18.5	20.1	17.9	*	*
1979	18.2	17.7	16.7	*	14.5	15.2
1980	19.4	18.8	17.9	17.1	*	*
1981	18.0	18.6	15.9	*	*	*
1982	18.8	20.8	17.0	*	18.1	*
1983	18.2	18.2	18.8	18.5	16.2	14.0
1984	18.0	20.3	14.7	29.5	10.8	13.5
1985	18.2	17.6	17.4	17.1	14.8	19.1
1986	17.8	17.4	16.9	16.1	14.5	14.9
1987	17.7	16.6	17.3	19.2	19.6	18.3
1988	17.4	16.5	18.0	17.9	21.8	15.1
1989	17.9	17.2	17.0	17.3	16.5	15.5
1990	18.6	17.8	17.7	17.5	19.3	14.9
1991	18.3	17.5	16.0	17.7	22.2	16.5
1992	16.8	17.0	15.7	17.5	15.4	16.0
1993	17.1	17.9	16.7	18.1	22.7	14.4
1994	16.8	16.9	16.2	16.5	15.8	15.2
1995	16.5	16.8	16.0	18.3	15.1	14.9
1996	17.1	17.6	16.7	17.3	14.5	16.0
1997	17.0	17.7	16.0	16.6	15.8	15.9
1998	17.8	17.2	16.0	17.6	14.6	17.0
1999¹	17.2	16.5	16.4	18.8	14.1	15.8
* Low precision; no estimate reported. ¹ Estimated using 2000 data only. Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999 and 2000.

Table 3.7 Estimated Annual Incidence Rates of First Use (Per 1,000 Person-Years of Exposure) of Persons Who First Used Marijuana During the Years 1965 to 1999, by Racial/Ethnic Subgroups
Year	Racial/Ethnic Specific Incidence Rates¹
Year	White	Black	Hispanic	Asian / Pacific Islander / Native Hawaiian	American Indian / Alaska Native	More Than One Race
1965	4.0	*	*	*	*	*
1966	7.5	8.0	*	*	*	*
1967	10.8	8.9	4.1	*	*	*
1968	12.8	16.7	4.9	*	*	*
1969	16.4	11.7	4.9	*	*	*
1970	20.2	11.8	5.4	7.3	*	21.3
1971	20.6	14.7	12.7	*	20.0	*
1972	21.5	15.5	7.7	2.2	31.6	17.7
1973	21.8	16.3	6.2	*	*	*
1974	21.0	15.9	14.1	2.0	*	18.8
1975	21.3	18.2	11.1	2.0	*	*
1976	23.5	19.4	10.9	*	41.5	34.0
1977	23.5	16.8	10.1	11.7	*	27.0
1978	21.4	17.8	12.5	12.2	*	11.6
1979	21.6	16.3	7.5	*	10.3	21.3
1980	18.6	13.7	9.7	*	*	*
1981	13.5	11.1	6.8	*	*	*
1982	14.5	9.1	9.2	*	19.7	5.8
1983	12.8	8.1	10.1	5.7	14.1	*
1984	14.2	12.4	5.2	*	9.3	5.5
1985	12.4	9.4	8.6	9.1	8.0	9.2
1986	11.7	9.6	6.6	3.4	28.5	9.2
1987	10.5	7.1	6.7	7.0	11.4	8.2
1988	10.3	6.9	6.5	2.7	28.8	10.3
1989	9.2	6.9	9.1	2.9	*	10.9
1990	9.1	7.3	7.3	6.6	10.2	3.4
1991	9.5	8.4	7.4	6.9	16.9	13.9
1992	10.1	10.3	9.1	3.7	17.1	19.5
1993	12.3	13.4	11.6	4.7	29.3	20.9
1994	14.2	14.5	13.1	5.7	21.8	18.3
1995	15.6	15.2	13.7	7.6	21.2	27.1
1996	17.1	16.6	16.5	10.5	25.8	31.2
1997	16.3	19.2	16.8	6.2	28.8	35.4
1998	16.3	19.1	17.8	11.6	28.6	26.2
1999²	13.8	14.0	13.5	5.6	46.5	26.4
* Low precision; no estimate reported. ¹ The numerator of each rate is the number of persons who first used marijuana in the year, while the denominator is the person-time exposure measured in thousands of years. ² Estimated using 2000 data only. Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999 and 2000.

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 services in the United States.

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