High-potency cannabis and the risk of psychosis

During the last quarter of the 20th century recreational use of
cannabis increased greatly across the world.1 Cannabis consumption
came to be seen as a normal leisure activity, and was regarded
as safe even by the medical establishment.2 However, in recent
years there has been considerable controversy over the use of
cannabis, with, for example, the UK government repeatedly
reviewing its safety.3 This concern has arisen from large prospective
epidemiological studies which have reported that use of
cannabis increases the risk of schizophrenia-like psychosis.4,5
However, these studies have not collected detailed data on the
patterns of use or potency of the cannabis used, which may be
important factors moderating the associated risk.6
The principal constituents of cannabis are D9-tetrahydrocannabinol
(D9-THC) and cannabidiol. The former is the main
psychoactive ingredient and in experimental studies it produces
transient psychotic symptoms and impaired memory in a dose dependent
manner.6,7 In contrast, cannabidiol does not induce
hallucinations or delusions, and it seems to antagonise the cognitive
impairment and psychotogenic effects caused by D9-THC.6
Until the early 2000s the most freely available type of cannabis
in the UK was cannabis resin (‘hash’), which had approximately
70% of the ‘street’ market, followed by traditional imported herbal
cannabis and then sinsemilla (‘skunk’). Cannabis resin contains
2–4% D9-THC and a similar proportion of cannabidiol, whereas
herbal cannabis contains a similar percentage of D9-THC but no
cannabidiol.8,9 However, sinsemilla (skunk) has increasingly taken
over the UK market and its THC concentration, and to a lesser
extent that of imported herbal cannabis, has been consistently
rising. For example, seizures of cannabis on the streets of England
in 2008 by the police showed that sinsemilla had a market share
of more than 70%, and had reached a D9-THC concentration of
12–18% with virtually no cannabidiol.8,9

Smith has suggested that such high-potency cannabis might be
especially harmful to mental health.10 We therefore compared
patterns and types of cannabis use in people experiencing their
first episode of psychosis and in a healthy control sample.
Specifically, we sought to test the hypothesis that daily use of
high-potency cannabis is associated with a particularly high risk
of psychosis.

We approached all patients aged 18–65 years who presented with a
first episode of psychosis to the Lambeth, Southwark and Croydon
adult in-patient units of the South London & Maudsley Mental
Health National Health Service (NHS) Foundation Trust between
December 2005 and October 2008. We validated clinical diagnosis
by administering the Schedules for Clinical Assessment in
Neuropsychiatry (SCAN).11 Patients who met ICD–10 criteria
for a diagnosis of psychosis (codes F20–F29 and F30–F33)12 were
invited to participate in the study; cases with a diagnosis of
organic psychosis were excluded. During the same period we
recruited a healthy control group (n = 174) from the local
population living in the area served by the Trust, by means of
internet and newspaper advertisements, and distribution of
leaflets at train stations, shops and job centres. Cannabis was
not mentioned in these advertisements. Particular attention was
directed to attempting to obtain a control sample similar to the
patient sample in age, gender, ethnicity, educational qualifications
and employment status. Those who agreed to participate were
administered the Psychosis Screening Questionnaire,13 and
excluded if they met criteria for a psychotic disorder or reported
a previous diagnosis of psychotic illness.
Ethical permission was obtained from the Trust and the
Institute of Psychiatry research ethics committee. All study
participants signed a consent form allowing publication of data
originating from the study.

People who use cannabis have an increased risk of
psychosis, an effect attributed to the active ingredient D9-
tetrahydrocannabinol (D9-THC). There has recently been
concern over an increase in the concentration of D9-THC in
the cannabis available in many countries.

To investigate whether people with a first episode of
psychosis were particularly likely to use high-potency

We collected information on cannabis use from 280 cases
presenting with a first episode of psychosis to the South
London & Maudsley National Health Service (NHS) Foundation
Trust, and from 174 healthy controls recruited from the local

There was no significant difference between cases and
controls in whether they had ever taken cannabis, or age at
first use. However, those in the cases group were more
likely to be current daily users (OR = 6.4) and to have smoked
cannabis for more than 5 years (OR = 2.1). Among those who
used cannabis, 78% of the cases group used high-potency
cannabis (sinsemilla, ‘skunk’) compared with 37% of the
control group (OR 6.8).

The finding that people with a first episode of psychosis had
smoked higher-potency cannabis, for longer and with greater
frequency, than a healthy control group is consistent with
the hypothesis that D9-THC is the active ingredient
increasing risk of psychosis. This has important public health
implications, given the increased availability and use of highpotency

Source: The British Journal of Psychiatry (2009)
195, 488–491. doi: 10.1192/bjp.bp.109.064220

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