Published online 1 October 2010 | Nature | doi:10.1038/news.2010.508

News

Key ingredient staves off marijuana memory loss

Cannabis composition determines effects on the brain.

Arran Frood

Smoking cannabisSome forms of cannabis may contain an ingredient that helps to
diminish the drug's negative effects on memory.iStockphoto.com/ annedde

Smoking cannabis has long been associated with poor short-term memory, but a
study now suggests that the strain of cannabis makes all the difference. In a test of
short-term memory skills, only users of 'skunk'-type strains exhibited impaired recall
when intoxicated, whereas people who smoked hashish or herbal cannabis blends
performed equally well whether they were stoned or sober.

The findings suggest that an ingredient more plentiful in some types of marijuana
than in others may help to reduce the memory loss that some users suffer.

The key difference between the types of cannabis is the ratio of two chemicals found
in all strains. Tetrahydrocannabinol (THC) is the primary active ingredient, and is
responsible for the effects associated with the classic 'high', including euphoria and
giddiness but also anxiety and paranoia. The second chemical, cannabidiol, has more
calming effects, and brain-imaging studies have shown that it can block the
psychosis-inducing effects of THC2. Skunk-type strains of cannabis contain a higher
ratio of THC to cannabidiol than do hashish or herbal types.

Valerie Curran, a psychopharmacologist from University College London who led the
latest study, says that if habitual users must partake they should be encouraged to
use strains with higher levels of cannabidiol, rather than using skunk. She also
argues that studying cannabidiol could provide insight into the mechanics of memory
formation, and that it may have therapeutic benefits for disorders involving memory
deficits. The findings are published in the British Journal of Psychiatry today1.
marijuana leafLevels of THC in 'skunk' marijuana are higher than in other
varieties.Getty Images

Cannabis use has increased in recent years — almost as many 16–24-year-olds in the
United Kingdom haveave tried as haven't, according to the 2008 report Statistics on
Drug Misuse by the National Health Service — and concerns have been raised thhat
increased levels of THC in 'skunk' varieties owing to agressive plant breeding over the
past decade are responsible for a rise in the number of young users displaying mild-
to-severe cognitive impairment. However, links to a possible higher incidence and
earlier onset of psychotic conditions such as schizophrenia remain controversial, as do
associations with long-term psychological problems. Researchers suspect any effects
of the drug on mental health could be a result of an increased ratio of THC to
cannabidiol in cannabis, because levels of cannabidiol have not kept pace with rising
THC concentrations.

Total recall

To test this hypothesis, Curran and her colleagues travelled to the homes of 134
volunteers, where the subjects got high on their own supply before completing a
battery of psychological tests designed to measure anxiety, memory recall and other
factors such as verbal fluency when both sober and stoned. The researchers then
took a portion of the stash back to their laboratory to test how much THC and
cannabidiol it contained.

The subjects were divided into groups of high (samples containing more than 0.75%
cannabidiol) and low (less than 0.14%) cannabidiol exposure, and the data were
filtered so that their THC levels were constant. Analysis showed that participants who
had smoked cannabis low in cannabidiol were significantly worse at recalling text
than they were when not intoxicated. Those who smoked cannabis high in
cannabidiol showed no such impairment.

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