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The British Journal of Psychiatry (2008) 192, 470–471. doi:
10.1192/bjp.bp.107.045740
Jouko Miettunen, Sari Tormanen, Graham K. Murray, Peter B. Jones, Pirjo
Maki, Hanna Ebeling, Irma Moilanen, Anja Taanila, Markus Heinimaa, Matti
Joukamaa and Juha Veijola
Summary
Recent interest has focused on the association between cannabis use and risk
of psychosis. In the largest unselected, population-based study on this
topic to date, we examined cannabis use and prodromal symptoms of psychosis
at age 15–16 years among 6330 adolescents. Those who had tried cannabis
(n=352; 5.6% of the total sample) were more likely to present three or more
prodromal symptoms even after controlling for confounders including previous
behavioural symptoms (OR=2.23; 95% CI 1.70–2.94). A dose–response effect was
seen. We conclude that cannabis use is associated with prodromal symptoms of
psychosis in adolescence.
Declaration of interest
None.
Heavy cannabis use is strongly suspected to be associated with both
psychotic symptoms and schizophrenia. 1
Whether cannabis causes psychoses beyond intoxication, however, remains
controversial, as some studies describing associations between cannabis use
and psychosocial harm are subject to bias or confounding. 2
Our aim was to examine associations between cannabis use and prodromal
symptoms of psychosis in a prospective general population-based birth cohort
study.
Method
The sample was composed of a prospective mother–child birth cohort collected
from the two northernmost provinces in Finland. 3 There were 9340 children
(4806 boys) alive at the beginning of the latest field study conducted
during 2001–2002, when the birth cohort was 15–16 years old. The adolescents
were invited to a clinical examination, at which they were given
self-completion questionnaires on prodromal symptoms of psychosis and on
drug use. Only those who gave informed consent and answered questions on
cannabis use were included in the analyses (n=6298; 3043 boys). The study
was approved by the ethical committee of Oulu University Hospital. Boys (64%
v. 72% girls), those from urban areas (64% v. 72%) and those from non-intact
families (60% v. 70%) were less likely to participate (P50.05).
For screening for prodromal symptoms of psychosis, we used the PROD-screen,
a recently developed questionnaire. 4 The scale has 21 items; however, we
used only the 12 symptom items specifically probing for psychotic-like
experiences. 5 These rate feelings that something strange or inexplicable is
taking place in oneself or in the environment, feelings that one is being
followed or influenced in some special way, and experience of thoughts
running wild or difficulty in controlling the speed of thoughts, among other
symptoms. We recorded whether symptoms had been experienced (‘no/yes’) in
the past 6 months. A recommended screening cut-off point of three specific
PRODscreen symptoms was used to define cases potentially at risk of
developing a psychotic disorder. 4
Information on early emotional and behavioural symptoms was collected using
the Rutter B2 questionnaire for teachers, which measures children’s
behaviour during the past year. 6 The scale was completed when the
participants were 8 years old. The continuous score from this scale was used
as a covariate. Other covariates included gender, family type (both parents
all the time v. others), parental social class based on occupation (the
highest of mother’s or father’s social class; professionals, entrepreneurs
and other white-collar workers v. others), regular tobacco use history
(‘no/yes’), use of other drugs (‘no/yes’) and parental substance misuse
disorder (‘no/yes’; information obtained from the nationwide hospital
discharge register 1972–2000). We present results using both categorised and
continuous symptom variables for a versatile presentation of the data.
Results
Of the participating adolescents, 352 (5.6% of the sample) reported that
they had ever used cannabis once or more; 59 (0.9%) had used cannabis more
than five times. Girls reported cannabis use more commonly than boys (6.1%
v. 4.9%, P=0.04). Those who had tried cannabis had a higher mean number of
prodromal symptoms (3.11 v. 1.88; t-test=8.68, P50.001). When we adjusted
for covariates, the difference remained statistically significant. When we
analysed categorised prodromal symptoms, those who had tried cannabis were
more likely than other adolescents to report three or more prodromal
symptoms (crude OR=2.79; 95% CI 2.24–3.46). For adjusted tests, see Table 1.
The proportion of those with high scores increased linearly (OR for linear
trend 1.42, 95% CI 1.23–1.64) by cannabis use category.
Discussion
Lifetime cannabis use was associated with the incidence of prodromal
symptoms of psychosis. This association remained after controlling for
confounding factors such as behavioural problems in childhood and other drug
use. There was also a dose–response relationship between frequency of
cannabis use and increased prodromal symptoms of psychosis. There are some
studies supporting this finding of dose–response. 2
The issue of the prevalence of prodromal symptoms of psychosis among people
who use cannabis has not previously been extensively investigated. Schiffman
et al found increased schizotypy among undergraduate college students who
used cannabis. 7 They also reported that schizotypal symptoms generally
preceded the onset of cannabis use. 7 In our study, the association between
cannabis use and prodromal symptoms remained even after controlling for
early behavioural symptoms at 8 years old. The PROD-screen use of the term
‘prodromal symptoms’, the symptom concept and relevant symptom descriptions
employed in this paper are in agreement with current international research
of clinical markers of psychosis risk. 5 We note that positive responses on
the PROD-screen are not diagnostic of the prodrome of first-episode
psychosis since this is a concept that can be established only
retrospectively. Nevertheless, the symptoms we measured are consistent with
and similar to the prodromal symptoms of established psychosis. There is
evidence that psychotic symptoms are much more common in the general
population than the incidence of cases of diagnosed psychotic disorder. This
can be explained by the continuum hypothesis, according to which psychotic
symptoms are expressed on a continuum from mild, clinically irrelevant forms
to manifestly psychotic symptoms. 8 This phenomenon was present in our
general population-based study: a large number of the adolescents reported
three or more prodromal symptoms. Studying the aetiology of these
‘psychotic-like experiences’ in the community can illuminate factors
contributing to the aetiology of psychotic illness. 5
Apart from the Swedish conscript study (which included only men), we believe
this is the first study in which the association between cannabis use and
prodromal symptoms of psychosis has been studied in a large, general
population-based sample of adolescents. The participants were adolescents
aged 15–16 years, which is a good age at which to investigate the
association between cannabis use and psychotic symptoms, as there is
evidence that cannabis use at that age is a stronger risk factor for
schizophreniform psychosis than later exposure to this substance. 9 We were
able to adjust for several potential confounders including the use of other
drugs and early emotional and behavioural problems, and the associations
remained significant. The childhood problems were measured at age 8 years by
teachers and do not adjust for all subsequent psychopathology that may still
pre-date cannabis use. However, teachers’ reports have been shown to have
good predictive value in earlier studies. 10 Our sample is more likely to be
representative of the general population than many previous studies.
Cannabis use was associated with prodromal symptoms of psychosis in this
large Finnish sample of adolescents. We showed these effects are not
secondary to confounding effects of other drug use, childhood
emotional/behavioural problems or family background. The association is
therefore unlikely to be caused by these or any closely related factors,
supporting the hypothesis that cannabis use may be causal in terms of
subsequent psychotic symptoms. In future studies, we aim to follow the
sample and study causal associations between cannabis consumption and
psychiatric disorders such as schizophrenia.
Table 1 Prodromal symptoms of psychosis categorised by cannabis use in the
sample
PROD-screen sum score b PROD-screen, 3 symptoms c
Cannabis use a n Mean (s.d.) n (%)
Never 5948 1.88 (1.94) 1749 (29.4)
Once 180 2.97 (2.53) 94 (52.2)
2–4 times 111 3.08 (2.61) 60 (54.1)
5 times or more 50 3.68 (2.90) 29 (58.0)
a. Participants were asked the question ‘Have you ever tried or used
marihuana or hashish? b. Test of analysis of covariance (never v. ever):
F=38.73 (P50.001). c. Logistic regression analysis (never v. ever): OR=2.23
(95% CI 1.70–2.94). Tests are adjusted for the following covariates: early
emotional/behavioural symptoms (Rutter B2 total score), gender, family type,
social class, regular tobacco use history, use of other drugs and parental
substance use disorder.
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