Published in full in Scientific American.
Cannabis composition determines effects on the brain.
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 October 1 in the British Journal of Psychiatry.
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.
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