People who smoke potent strains of cannabis that are low in
cannabidiol (such as skunk) are putting themselves at far greater
risk of acute memory loss than people who smoke other types of the
to new research
published in the October issue of the
British Journal of Psychiatry
Now the study’s authors are calling for
cannabis users to be made aware of the risks associated with using
some strains of cannabis – and encouraged to use strains with
higher levels of cannabidiol instead.
Two major constituents of cannabis are
Δ9-tetrahydrocannabinol (THC) and cannabidiol. THC is the main
psychoactive ingredient which makes users feel ‘stoned’, and
produces psychotic-like symptoms such as delusions and paranoia. In
contrast, cannabidiol does not induce these symptoms and may
counteract the effects of THC. Levels of cannabidiol in cannabis
can range from virtually none, to up to 40%.
The researchers, from UCL (University College
London), tested 134 cannabis users aged between 16 and 23 years.
The participants were invited to take a series of memory tests on
two separate occasions – once while they were smoking their own
preferred type of cannabis and were intoxicated, and once when they
had not smoked for the last 24 hours and were sober.
A sample of the cannabis each participant
smoked was taken on the intoxicated day and analysed for levels of
THC and cannabidiol. Saliva samples were also taken from each
participant, and analysed for levels of THC and cannabidiol.
The researchers found that people smoking
cannabis containing a low percentage of cannabidiol performed much
worse on the memory tests when they were intoxicated than when they
were sober. In contrast, those smoking cannabis high in cannabidiol
performed just as well on the tests when they were intoxicated as
when they were sober. There were no differences in the THC content
of the cannabis smoked by any of the participants.
Professor Valerie Curran, from UCL, said: "We
asked the participants to recall a short passage of prose,
immediately after hearing it and again after a short delay. We
found that people who were smoking cannabis low in cannabidiol
experienced memory impairment and remembered less of the prose than
they could when sober. However, people who were smoking high
cannabidiol strains were still good at these tests. Indeed, their
performance when intoxicated was virtually indistinguishable from
when they were drug-free.
"Over the last 20 years, the constituents of
street cannabis have changed, with low-cannabidiol strains like
skunk now dominating the market. But our study suggests that these
strains can increase the risk of cognitive harm. On the back of
this study, we believe users should be made aware of the risk of
memory impairment from smoking low-cannabidiol strains. They should
be encouraged to use strains containing higher levels of
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Morgan CJA, Schafer G, Freeman TP and Curran HV (2010) Impact of cannabidiol on the acute memory and psychomimetic effects of smoked cannabis: naturalistic study, British Journal of Psychiatry, 197:285-290