London: While cannabis has long been associated with poor short-term memory, researchers have found that it is a particular strain that determines its effects on the brain.
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, reports Nature.
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.