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

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Cannabis composition determines effects on the brain.

Some 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.

Levels 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 have tried as haven't, according to the 2008 report Statistics on Drug Misuse by the National Health Service — and concerns have been raised that 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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The results suggest that cannabidiol can mitigate THC's interference with memory formation. This is the first study in human to show such effects. One previous study, led by Aaron Ilan, a cognitive neuroscientist at the San Francisco Brain Research Institute in California, failed to find variations in cognitive effects with varying concentrations of cannabidiol3.

Ilan attributes the positive finding of Curran and her team to their more powerful methodology in analysing subjects' own smoking preferences. In the United States, government policy dictates that only marijuana provided by the National Institute on Drug Abuse can be used for research — and it "is notorious for being low in THC and of poor quality", says Ilan.

Lester Grinspoon, professor emeritus of psychiatry at Harvard Medical School in Boston, Massachussetts, who has studied the effects of marijuana on patients since 1967, says that Curran's study is important. "Cannabis with high cannabidiol levels will make a more appealing option for anti-pain, anti-anxiety and anti-spasm treatments, because they can be delivered without causing disconcerting euphoria," he says.