Illicit drug use and dependence in a New Zealand birth cohort

Objective: To describe the patterns of illicit drug use in a birth cohort studied to the age
of 25 years.
Method: The data were gathered during the Christchurch Health and Development Study.
In this study a cohort of 1265 children born in the Christchurch, New Zealand urban region
in mid-1977 have been studied to the age of 25 years. Information was gathered on
patterns of illicit drug use and dependence during the period 15–25 years.
Results: By age 25 years, 76.7% of the cohort had used cannabis, while 43.5% had used
other illicit drugs on at least one occasion. In addition, 12.5% of the cohort met DSM-IV
criteria for dependence on cannabis, and 3.6% of the cohortmet criteria for dependence on
other illicit drugs at some time by age 25. There was also evidence of substantial poly-drug
use among the cohort, with hallucinogens and amphetamines being the most commonly
used illicit drugs (excluding cannabis). Illicit drug use and dependence was higher in
males, in M¯aori, and in those leaving school without qualifications. Key risk factors for illicit
drug use and dependence included adolescent risk-taking behaviours including cigarette
smoking and alcohol consumption, affiliation with substance-using peers, novelty-seeking,
and conduct problems in adolescence. Other key risk factors included parental history of
illicit drug use and childhood sexual abuse.
Conclusions: Levels of cumulative illicit drug use in this cohort were relatively high, with
the majority of respondents having tried illicit drugs by age 25. For the majority of illicit
drug users, drug use did not lead to problems of dependence. Nonetheless, nearly 15%
of the cohort showed symptoms of illicit drug dependence by the age of 25 years, with
cannabis dependence accounting for the majority of illicit drug dependence.

In recent years there has been growing concern about
the use of illicit drugs in young New Zealanders. This
concern has been reflected in a number of government
and local initiatives aimed at reducing the use of illicit
drugs and minimizing the harm caused by illicit drug use
[1,2]. These plans acknowledge the extent to which the
use and misuse of illicit drugs represents a critical social
issue with implications for the health and safety of a large
number of New Zealanders.
The importance of the issue of illicit drug use is illustrated
by periodic cross-sectional surveys of illicit drug
use in New Zealand. These surveys have provided evidence
that illicit drug use is common among New Zealanders,
is particularly common in young people, and may
be increasing. For example, Wilkins et al. [3] reported
that approximately 52% of respondents aged 15–45 had
reported using cannabis on at least one occasion in their
lives. For illicit drugs other than cannabis, 4% of respondents
reported having used opiates, 15% reported
using hallucinogens and 11.9% reported using stimulants
(including cocaine, amphetamine/methamphetamine and
derivatives of these) at least once in their lives [3]. Furthermore,
Wilkins et al. [3] reported that patterns of
use appeared to be age dependent with the highest incidence
of use in the previous year being reported by
those in the 18–24 years age range. In this group, approximately
35% of the sample had reported using cannabis
on at least one occasion in the previous year, and 21%
reported having used other illicit drugs on at least one
occasion in the previous year [3]. Comparisons between
1998 and 2001 surveys suggested that there was an increase
in the rates of other illicit drug use, particularly
for hallucinogens and stimulants [3]. The percentage of
the sample reporting having used hallucinogens on at
least one occasion increased from 12.8% to 15%, while
the percentage of those reporting having used stimulants
increased from 9% to 11.9% [3].
It should also be noted that a number of studies examining
the prevalence and incidence of illicit drug use
in New Zealand and overseas have been cross-sectional
studies. Although these have provided valuable data on
illicit drug use, it would be of particular importance to
examine prospective, longitudinal data on illicit drug use.
Longitudinal data provide more accurate estimates of cumulative
use over the life span than retrospective reports
of past use [4], as well as more accurate estimates of such
factors as age of onset and the timings of the occurrence
of other risk factors [5].
In light of these concerns and methodological issues,
there is a need for further detailed information on patterns
of illicit drug use among adolescents and young adults.
To address this, the present paper reports on the results
of analyses of cumulative measures of illicit drug use
and dependence in a birth cohort of young New Zealanders
studied to the age of 25. The specific aims of this
investigation were:
1. To derive life table estimates of the probability that,
by a given age, a young person: (i) would have used
an illicit drug; and (ii) will report dependence upon
an illicit drug.
2. To describe the range of illicit drugs used by young
people, and derive life table estimates of the probability
that by age 25 a young person would have used
or become dependent upon a particular kind of drug.
3. To determine the demographic distribution of illicit
drug use and dependence by gender, ethnicity and
education level.
4. To explore the extent to which family and social
factors may contribute to an increased the risk of
illicit drug use and dependence.
Method
Data were collected as part of the Christchurch Health and Development
Study (CHDS), which is a longitudinal study of a birth cohort of
1265 children born in the Christchurch, New Zealand urban region during
mid-1977. This cohort has been studied at birth, 4months, 1 year,
at annual intervals up to age 16, and at ages 18, 21 and 25 years.
Measures
Cannabis and other illicit drug use
At each assessment (15, 16, 18, 21 and 25 years), sample members
were questioned about their use of cannabis and other illicit drugs,
including the age at which they first reported using these drugs, and
the different types of other illicit drugs they had used in each year
from age 14–15 to age 24–25 years. The cohort members were questioned
about their use of a range of illicit drugs, including cannabis,
solvents (glue, petrol, paint); amphetamine-type stimulants (including
methamphetamine and amphetamines); barbiturates; prescription medications
that were illicitly obtained; opiates, including both heroin and
morphine; cocaine (in any form); hallucinogens including ecstasy, LSD
and PCP; and any other substances, primarily plant extracts, including
psilocybin mushrooms and datura. The questions regarding the use
of individual classes of drugs were non-specific to allow comparison
across assessment periods. Reports of cannabis and other illicit drug
use were used to derive life table estimates of the cumulative risk of
cannabis and other illicit drug use (and overall illicit drug use) over the
period 14–25 years.
Cannabis and other illicit drug dependence
In addition, respondents were questioned about symptoms of
cannabis and other illicit drug dependence using questions based on the
generic DSM-IV [6] criteria for substance dependence derived from the
Composite International Diagnostic Interview (CIDI) [7]. This questioning
was not started until age 16 years; thus the earliest estimates
of cannabis and other illicit drug dependence are reported from age
18 years.
Demographic factors
At age 21 years, an assessment of the ethnic identification of M¯aori
members of the cohort was conducted using the 1996 New Zealand
census questions on ethnicity as well as a questionnaire designed by the
Ngai Tahu M¯aori Health Research unit. Fifteen per cent of the cohort
reported M¯aori descent, while 11% reported M¯aori cultural identification.
In this paper those reporting a M¯aori cultural identification were
classified as M¯aori.
At age 18, cohort members were assessed as to the extent of their
educational qualifications. Those who reported having left secondary
school without achieving qualifications were classified as having left
without qualifications (19% of the sample). Gender was recorded at
birth.
Risk factors
To examine predictors of illicit drug use, measures of social, family
and childhood circumstances were considered. These included family
social background, family functioning, individual characteristics, adolescent
behaviours and peer affiliations. Initial analyses revealed that
the following measures were significant predictors of illicit drug use or
dependence:
1. Peer substance use – Assessed on the basis of participant reports
of the extent to which their friends used tobacco, alcohol, or illicit
drugs or had problems resulting from alcohol or illicit drugs,
α =0.69–0.77.
2. Parental history of illicit drug use – Parental illicit drug use was
assessed at age 11 (24.9% of the sample were thus classified) via
parent self-report and scored as a dichotomous measure.
3. Novelty-seeking (age 16) – Assessed at age 16 using the noveltyseeking
items from the Tridimensional Personality Questionnaire
[8], α =0.76.
4. Frequency of cigarette smoking (age 14) – Assessed at age 14 on
a five-point scale ranging from non-smoker to daily smoker via
young person self-report.
5. Frequency of alcohol use (age 14) – Assessed at age 14 via selfreported
number of occasions of alcohol use over the previous
3months via young person self-report.
6. Childhood sexual abuse – Assessed via young person self-report
at ages 18 and 21 for the period up to and including 15 years,
spanning an array of abusive experiences, resulting in a four-level
classification of severity [9].
7. Conduct problems (age 14) – Assessed via parent and child reports
of child behaviour issues at age 14 using items from the Rutter et al.
[10] and Conners [11,12] behaviour scales, and from the Diagnostic
Interview Schedule for Children [13], α =0.90.
Statistical analyses
All analyses were based on all cohortmembers assessed at each point
of observation. Sample sizes were as follows: 15 years (965); 16 years
(953); 18 years (1025); 21 years (1011); and 25 years (1003). These
samples represented between 75% and 81% of the original cohort of
1265 participants and over 85% of study participants resident in New
Zealand at each age.
Rates of cannabis and other illicit drug use by cohort members were
used to calculate life table estimates of cannabis, other illicit drug, and
any illicit drug (either cannabis or other illicit drug) use and dependence,
in order to estimate the cumulative risk of using or being dependent upon
cannabis and other illicit drugs by ages 15, 18, 21 and 25. These life
table estimates were then tested for demographic differences in illicit
drug use and dependence (gender, M¯aori and education level) using a
univariate log-rank test. Finally, proportional hazards regression models
were fitted to the data to identify childhood and family factors that were
predictive of the onset of illicit drug use or dependence.
Results
The development of illicit drug use
Table 1 presents life table estimates of risks of cannabis and other
illicit drug use and dependence over the period from 15 to 25. The table
shows a high level of use of both cannabis and other illicit drugs. By
the age of 25, almost 77% of the cohort had used cannabis and 43.5%
had used other illicit drugs. Overall rates of dependence were relatively
high with nearly one in seven (13.6%) meeting diagnostic criteria for
substance use dependence; 12.5% met criteria for cannabis and 3.6%
met criteria for other drug dependence. The table also shows that there
was a rapid growth in illicit drug use and dependence over the period
from 15 to 18.
Types of illicit drugs used
Table 2 shows the types of other illicit drugs used by the cohort
and the percentage of cases in which each type of drug was used in
cases of dependence by age 25. Other illicit drug use in the cohort was
dominated by hallucinogens, including LSD and ecstasy (36.2%) and
amphetamine-type stimulants (26.9%). However, a substantial minority
had used harder drugs including cocaine (9.1%) and opiates (3.7%).
There was also a high rate of use of other substances, primarily plant
extracts including psilocybin mushrooms and datura.
It will be noted that the percentages of other illicit drugs used exceeds
the percentage (43.5%) of those using any other illicit drugs. This
reflects the fact that those using other illicit drugs tended to use more
than one other illicit drug. Those using other illicit drugs reported a
mean of 2.4 other illicit drugs used by age 25.
Differences in illicit drug use by gender, ethnicity
and education
Table 3 shows life table estimates of the rates for use and dependence
on cannabis and other illicit drugs by age 25, classified by:
(i) gender; (ii) ethnicity; and (iii) education level. The table shows
that:
1. Males were significantly more likely than females to report: (i)
cannabis dependence (p<0.0001); (ii) using other illicit drugs
(p<0.05); (iii) using any illicit drugs (p<0.05); and (iv) dependence
on any illicit drugs (p<0.0001).
2. Cohort members identifying themselves as M¯aori were significantly
more likely than non-M¯aori to report: (i) using cannabis
(p<0.0001); (ii) cannabis dependence (p<0.05); (iii) using any
illicit drugs (p<0.0001); and (iv) dependence on any illicit drugs
(p<0.05).
3. Those cohort members who left school without qualifications were
significantly more likely than those achieving qualifications to report:
(i) using cannabis (p<0.0001); (ii) cannabis dependence
(p<0.0001); (iii) dependence on other illicit drugs (p<0.01); (iv)
using any illicit drugs (p<0.0001); and (v) dependence on any
illicit drugs (p<0.0001).
Risk factors for illicit drug use and dependence
The results above raise issues about the extent to which patterns of
illicit drug use and dependence could have been predicted from factors
present by age 15 years. This issue was explored by fitting proportional
hazards regression models in which the hazards or instantaneous risks
of onset of cannabis and other illicit drug use and dependence by age
25 years were modelled as log-linear functions of a range of social,
childhood and related risk factors. The results of these regression models
are presented in Table 4, which shows parameter estimates, standard
errors, and significance levels for the statistically significant demographic
factors and risk factors for any illicit drug use and dependence.
The table shows that:
1. Illicit drug use was predicted by: (i) male gender (p<0.0001); (ii)
M¯aori identification (p<0.05); (iii) association with substanceusing
peers (p<0.0001); (iv) a parental history of illicit substance
use (p<0.001); (v) novelty-seeking (p<0.0001); (vi) frequency
of cigarette smoking at age 14 (p<0.0001); and (vii) frequency of
alcohol consumption at age 14 (p<0.0001).
2. Illicit drug dependence was predicted by: (i) gender (p<0.0001);
(ii) association with substance-using peers (p<0.0001); (iii) a
parental history of illicit substance use (p<0.05); (iv) childhood
sexual abuse (p<0.0001); (v) novelty-seeking (p<0.0001); and
(vi) conduct problems at age 14 (p<0.05).
The results suggest that the association between education, ethnicity,
and illicit drug use and dependence reported in Table 3 were partially
mediated by the other risk factors shown in Table 4. In particular, after
adjustment for these factors, education level was no longer a predictor
of illicit drug use while ethnicity remained predictive of illicit drug use.
In contrast, both education level and ethnicity were no longer predictive
of illicit drug dependence after risk factors had been taken into account.
However, gender remained a predictor of illicit drug use and dependence
in the proportional hazards regression model, even after control for risk
factors, suggesting that males were more susceptible to use and misuse
of illicit drugs.
Similar proportional hazards regression analyses were carried out
for cannabis use and dependence, and other illicit drug use and
dependence. A similar pattern of findings emerged for cannabis
and other illicit drug use and dependence as were reported.
The results of the fitted regression models for illicit drug use and
dependence are illustrated in Figs 1 and 2, which depict rates of cannabis
and other illicit drug use (Fig. 1) and dependence (Fig. 2) classified by
the number of risk factors present in cohort members. Risk factors were
dichotomized as: (i) present or absent, in the case of (male) gender,
Ma¯ori identification, parental history of illicit substance use, and sexual
abuse; or (ii) the cohort member scoring in the highest decile on the
risk factor measure, in the case of peer substance use, novelty-seeking,
frequency of cigarette smoking and alcohol use, and conduct disorder
at age 14. The dichotomized risk factor scores were summed to give a
total number of risk factors present for each cohort member. The results
for illicit drug use show clear trends in which increased exposure to
risks is associated with increased use of both cannabis and other illicit
drugs. Those with three or more risk factors (38% of the cohort) had
over a 95% chance of using cannabis and a better than 60% chance of
using other illicit drugs. The results for illicit drug dependence show
a similar pattern of increased exposure to risks being associated with
increased dependence. Those with four or more risk factors (11% of the
cohort) had a 50% risk of cannabis dependence and a greater than 15%
risk of dependence on other illicit drugs.
Discussion
This paper has presented a longitudinal description of
patterns of illicit drug use by members of the CHDS
cohort up to the age of 25 years, and an examination of
the risk factors for illicit drug use and dependence among
cohort members. A number of major themes and issues
emerged from the analyses.
The principal findings of this study were of high lifetime
rates of illicit drug use, with nearly 80% of the
cohort using an illicit drug by the age of 25. This high
lifetime rate of illicit drug use is consistent with that
found in other New Zealand studies. Thus, the Dunedin
Multidisciplinary Health and Development Study [14,15]
found that by age 26, 70.1% of cohort members had
used cannabis at some point in their lives (compared
with 76.7% in the current study). Similarly, the 2001
National Drug Use surveys [3] found that nearly 60% of
New Zealanders aged 18–24 reported using cannabis on
at least one occasion. Because the National Drug Use
survey is based on retrospective reports derived from
cross-sectional data, it may underestimate the lifetime
use of illicit drugs by young adults. In comparison with
observed international rates of illicit drug use, drug use
by young New Zealanders appears to be relatively high.
For example, US and European studies suggest rates of
illicit drug use in young people that range from 44% to
55% [16–19]. The high rate of illicit drug use in New
Zealand is largely explained by the high rate of cannabis
use.
As might be expected from high lifetime rates of illicit
drug use, life time rates of illicit drug dependence
in this cohort were comparatively high, with 12.5%
meeting DSM-IV criteria for cannabis dependence and
3.6% meeting criteria for other illicit drug dependence
by age 25. These figures are similar to those reported
by the Dunedin Multidisciplinary Health and Development
Study, who found that 9.4% of the sample had met
DSM-III and DSM-IV criteria for cannabis dependence
by age 26 [15]. There has been growing international
interest in rates of illicit drug dependence, and in particular
cannabis dependence. Comparisons suggest that
Australia and New Zealand tend to have higher rates of
illicit drug dependence, ranging from 12% to 9% [15,20–
22], than the US and Europe, with rates ranging from
3.4% to 2.2% [19,23,24]. Although the precise reasons
for these differences are unknown, it is possible that certain
cultural or social factors unique to Australasia lead
to higher rates of illicit drug dependence in this area. In
the present cohort, high levels of substance use may be
one reason for the observed high levels of dependence,
as suggested by Degenhardt et al. [25].
The results also show that the CHDS cohort members
had experience with awide array of drugs. The most commonly
used drugs, aside from cannabis, were hallucinogens
(36.2%) and amphetamine-type stimulants (26.9%),
with a minority of the cohort using ‘hard’ drugs such as
cocaine (9.1%) and opiates (3.7%). In addition, there was
clear evidence of poly-drug use among those using other
illicit drugs. In this group the average number of different
types of illicit drugs (other than cannabis) used by age 25
was 2.4.
Overall these results reinforce concerns about the apparently
growing utilization of illicit drugs by adolescents
and young people. The findings of this study suggest:
1. The use of illicit drugs by young people has reached
a point where drug use at some point in the life span
is part of normal experience.
2. Although the majority of illicit drug users are occasional
recreational users who do not develop dependence,
nearly one in seven young people develop
dependence on an illicit drug by age 25. In most
cases this dependence involved cannabis.
In agreement with a number of other studies, illicit drug
use and dependence was more common among males,
M¯aori and young people lacking formal educational qualifications
[15,22,26–28].
Subsequent analyses of this cohort suggested that while
gender remained a predictor of illicit drug use and dependence,
the associations between ethnicity and education
level and illicit drug use and dependence reflected the
influence of a series of mediating factors. These factors
included family and childhood factors, peer factors, and
personality factors. Specifically, those factors that were
found to predict illicit substance use and dependence
included having parents who used illicit drugs, experiencing
sexual abuse as a child, affiliating with substanceusing
peers during early adolescence, cigarette smoking
and alcohol consumption by age 14, novelty-seeking behaviours
and conduct problems at age 14. Examination
of the role of these factors in the development of illicit
drug use and dependence suggests a cumulative risk
model in which risks of use and dependence increased
with increasing exposure to risk factors in childhood and
adolescence.
Although the present study provides an overview of
illicit drug use and dependence and the correlates of such
use and dependence, the findings are subject to a number
of caveats.
1. Measurement – First, all measurements were based
on self-report data, and the accuracy of the reports
depends upon the willingness of respondents to
disclose illicit drug use and dependence. It is possible
that this limitation may have led to some underreporting
of illicit drug use and dependence, so the
results of this study should best be interpreted as
giving lower limit estimates of the actual incidence
of illicit drug use and dependence among the CHDS
cohort.
2. Sampling – This paper is based on the results for
a cohort born at a specific point in time and in a
specific geographic region, and measured at specific
ages. The extent to which these findings can be generalized
to other cohorts in other regions of New
Zealand or elsewhere is unclear. In particular, there
has been concern on the part of many about the increasing
use of illicit drugs by adolescents, and it
may be that findings from this study have underestimated
the problem of the use of illicit drugs in the
contemporary adolescent population.
These concerns notwithstanding, the present study suggests
a disturbingly high level of use of, and dependence
on, illicit drugs by young people in New Zealand,
with particularly high rates of cannabis use. The findings
should serve to alert psychiatrists and others dealing
with this population that issues regarding illicit drug use
are perhaps more pervasive than previously believed, and
that more attention should perhaps be given to the role
of cannabis in the development of drug problems. Recent
advances in brief interventions for cannabis and other
substance dependence [29–31] would suggest that, although
the problems of drug use and dependence may be
underestimated, they may indeed be tractable.