Convergent evidence suggests a link between cannabis use and
psychosis.1However, cannabis comprises a combination of
cannabinoids and these different constituents may have distinct
effects, not all of which are detrimental to mental health. The
main component of smoked cannabis is D9-tetrahydrocannabinol
(D9-THC), which is thought to be responsible for the majority of
the psychotomimetic effects of the drug: it has been shown to
elevate levels of anxiety and psychotic symptoms in healthy
individuals.2In contrast, cannabidiol (CBD), another major con-
stituent of some strains of cannabis, has been found to be anxio-
lytic and to have antipsychotic properties,3,4and may be
neuroprotective in humans.5The ratio of these two compounds
in smoked cannabis varies – there are higher levels of D9-THC
in ‘skunk’ or genetically modified strains of the plant.6Cannabis
users are often unaware of the ratio of CBD to D9-THC because
CBD has no psychotomimetic effect in humans.3Elevated levels
of psychosis proneness and delusions have been found in people
who use cannabis regularly.7Despite suggestions about the differ-
ent psychological properties of these two constituents of smoked
cannabis, no prior research has examined the link between
psychosis proneness and delusions and the CBD/D9-THC ratio
in those who use cannabis. This study aimed to use hair
analytic techniques to examine levels of D9-THC and CBD, and
relate these objective indices of cannabis use to measures of
psychosis proneness and delusional thinking.
Our sample consisted of 140 individuals who were taking part in
categorised as current and former ketamine users, other drug users
and non-users. Inspection of the hair analysis results from the
sample revealed that 54 individuals screened positive for cannabis.
Confirmatory screens showed both CBD and D9-THC in the hair
of 26 of these individuals but D9-THC alone in the hair of 20
others. Only individuals who showed evidence of D9-THC–
carboxylic acid in hair as well as D9-THC were included, because
this indicated actual consumption rather than passive use. The
remaining 8 people who screened positive showed evidence of
CBD alone in their hair samples, but they were excluded from this
study as the group size was too small for statistical analysis. Rather
than correlating the exact levels of CBD and D9-THC with symp-
toms – as concentrations have been shown to be vulnerable to
other factors (e.g. hair washing or dyeing)9– we divided the
sample into three groups: those with D9-THC only in hair
(‘THC only’: n=20; mean age 26.1 years, s.d.=6.21; 13 males);
those with D9-THC and CBD in hair (‘THC+CBD’: n=27; mean
age 27.8 years, s.d.=6.26; 21 males); and those with no cannabi-
noids in hair (n=85; mean age 26.7 years, s.d.=6.59; 58 males).
Hair analysis was performed by gas chromatography/mass
spectrometry (GC/MS). Following addition of THC-d3 (Tricho-
Tech, UK; www.tricho-tech.co.uk) as the internal standard, 50
mg of the powdered hair was dissolved in 1ml of 0.1mol/l sodium
hydroxide (1008C for 30 min). The solution was adjusted to a pH
of 5.5 with 1.0mol/l hydrochloric acid, and the D9-THC and CBD
were extracted into n-hexane and quantified by GC/MS after
silyation. The lower limit of detection was 0.025ng/mg for both
D9-THC and CBD.
The short form of the Oxford Liverpool Inventory of Life
Experiences (OLIFE) questionnaire was used to assess psychosis
proneness.10This measure yields four factors: unusual experiences
(an analogue of positive symptoms in schizophrenia, including
hallucinations and delusions); cognitive disorganisation (roughly
corresponding to thought disorder); introvertive anhedonia
(negative symptoms such as social withdrawal); and impulsive
non-conformity (relates to behavioural impulsivity).
Peter’s Delusion Inventory (PDI) was used to index delusional
One-way analysis of variance (ANOVA) showed that there was no
significant difference in age or in drug use (other than cannabis)
reported in the three groups. Chi-squared tests revealed no differ-
ence in gender. The mean CBD level in the THC+CBD group was
0.15ng/mg (s.d.=0.27). A Mann–Whitney U-test (as variance was
heterogeneous) found no significant difference in the mean level
of D9-THC in the THC only group (0.17ng/mg, s.d.=0.07) and
the THC+CBD group (0.19ng/mg, s.d.=0.33).
Subjective estimates of cannabis use in these two groups did
not differ in days per month of use (THC only, mean=19.4 days,
s.d.=10.0; THC+CBD, mean=21.1 days, s.d.=10.1); age at which
the participant became a regular user (THC only, mean=16.5
years, s.d.=3.07; THC+CBD, mean=5.48 years, s.d.=4.69) or days
since last use (THC only, mean=3.89 days, s.d.=8.56; THC + CBD,
mean=2.67 days, s.d.=3.96). There was a significant difference in
number of days taken to smoke 3.5g of cannabis (the standard
quantity in which cannabis is sold in the UK, used as a more
Effects of cannabidiol on schizophrenia-like
symptoms in people who use cannabis
Celia J. A. Morgan and H. Valerie Curran
Cannabis contains various cannabinoids, two of which have
almost opposing actions: D9-tetrahydrocannabinol (D9-THC)
is psychotomimetic, whereas cannabidiol (CBD) has
antipsychotic effects. Hair samples were analysed to
examine levels of D9-THC and CBD in 140 individuals. Three
clear groups emerged: ‘THC only’, ‘THC+CBD’ and those with
no cannabinoid in hair. The THC only group showed higher
levels of positive schizophrenia-like symptoms compared
with the no cannabinoid and THC+CBD groups, and higher
levels of delusions compared with the no cannabinoid group.
This provides evidence of the divergent properties of
cannabinoids and has important implications for research
into the link between cannabis use and psychosis.
Declaration of interest
The British Journal of Psychiatry (2008)
192, 306–307. doi: 10.1192/bjp.bp.107.046649
reliable indicator than amount smoked per session), with the THC Download full-text
only group taking longer (mean=10.2 days, s.d.=8.61) to smoke 3.5g
than the THC+CBD group (mean=5.0 days, s.d.=6.03); F(1,46)=5.59,
P=0.023. Subjective estimates of cannabis use were not correlated
with levels of D9-THC and CBD obtained from hair analysis.
One-way ANOVA yielded significant differences between the three
groups in scores on the OLIFE factor of unusual experiences:
F(2,129)=12.86, P50.001 (Fig. 1). Post hoc Scheffe’s test showed
that this was due to greater scores in the THC only group
compared with the no cannabinoid group (P50.001) and the
THC+CBD group (P=0.021). Significant differences also emerged
for the factor of introvertive anhedonia (F(2,129)=7.45, P=0.001),
with significantly lower scores in the THC+CBD group compared
with the no cannabinoid group (P=0.001) and the THC only
One-way ANOVA revealed significant group differences in scores on
the PDI: F(2,129)=5.90, P=0.004. Compared with the no cannabinoid
group (mean score 5.48, s.d.=3.58) there were significantly higher
scores in the THC only group (mean score 8.15, s.d.=3.16;
P=0.012) and a trend for greater scores in the THC+CBD group
(mean score 7.22, s.d.=3.23; P=0.088).
Our results show higher levels of unusual experiences – an
analogue of hallucinations and delusions – in individuals who
had evidence of only D9-THC in their hair compared with those
with both D9-THC and CBD, and those with no cannabinoid.
There were also greater levels of delusions in this THC only group
compared with individuals who showed no evidence of cannabi-
noids in their hair, with a similar trend in the THC+CBD group.
The THC+CBD group reported less anhedonia than the other two
groups. This study is the first to demonstrate that hair analytic
techniques can be used to define subsets of cannabis users. The
implications of these findings are that people who smoke different
strains of cannabis manifest different psychological symptoms.
These preliminary findings may support previous work show-
ing the antipsychotic properties of CBD in the laboratory.3,4
Moreover, this suggests that smoking strains of cannabis contain-
ing CBD in addition to D9-THC may be protective against the
psychotic-like symptoms induced by D9-THC alone. This is
further evident from the findings that participants with both
D9-THC and CBD in their hair had significantly less anhedonia
than the other groups in this study. However, another potential
explanation of the results of our study is that pre-existing dif-
ferences in psychosis proneness between people who use cannabis
may draw them to smoke different strains of the drug. In spite of
this, the former explanation seems more plausible in light of the
absence of differences in any other recreational drug use between
these groups, and the emerging evidence of neuroprotective effects
of CBD. A further limitation of this research is that the mechanisms
by which cannabinoids are incorporated into hair are not well
understood, and thus we cannot directly infer the ratio of CBD to
D9-THC. Despite this, our study highlights the importance of dis-
tinguishing between different cannabinoids and has implications
in the debate over the link between cannabis use and psychosis.
Celia J. A. Morgan, PhD, H. Valerie Curran, PhD, DClinPsy, Clinical
Psychopharmacology Unit, University College London, UK
Correspondence: Celia Morgan, Clinical Psychopharmacology Unit, Sub-
Department of Clinical Health Psychology, University College London, Gower
Street, London WC1E 6BT, UK. Email: firstname.lastname@example.org
First received 22 Oct 2007, final revision 14 Jan 2008, accepted 16 Jan 2008
H.V.C. and C.J.A.M. were supported by a grant from the Economic and Social Research
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Cannabinoids and schizophrenia symptoms
CBD and THC
Experiences factors categorised by cannabis group. CBD,
cannabidiol; THC, D9-tetrahydrocannabinol.
Scores on the Oxford Liverpool Inventory of Life