People’s verbal memory – which refers to memory of words or the ability to remember various words – may be affected by long-term marijuana use, according to a new study.
Researchers found that for every additional five years of marijuana use, about one out of two people could remember one word fewer from a total of 15 words that were presented to them.
However, other cognitive functions did not decrease significantly due to long-term marijuana use, the findings suggested.
For the new study – published Monday (Feb. 1) in the journal JAMA Internal Medicine – the researchers looked at the effects of marijuana use among people who were already enrolled in a study, called the Coronary Artery Risk Development in Young Adults (CARDIA).
More than 5,000 adults who were ages eighteen to thirty took part in the Coronary Artery Risk Development in Young Adults study. During follow-up visits, the participants would report whether they had used marijuana in the previous month. They took cognitive tests at the 25-year follow-up, which were meant to observe the verbal memory, executive function, and processing speed of the study participants.
The results showed that long-term marijuana use decreased the performance of people on all three tests. However, when the researches accounted for other factors – like depression and use of other substances – they found that the only statistically significant connection was the one between verbal memory and long-term marijuana use. The other two associations may have been due to chance.
According to the researchers, the more marijuana a person used, the higher the effects on their verbal memory were.
One explanation as to why marijuana use affects verbal memory, is that THC (tetrahydrocannabinol) – the the principal psychoactive constituent of cannabis and the chemical responsible for most of the psychological effects triggered by marijuana – may impact the way in which the hippocampus processes information.
A previous study conducted in New Zealand over a longer period, found that long-time marijuana users appeared to have a greater decline in IQ scores, compared with individuals who had stopped using marijuana, or those who had never used it.
On the other end of the spectrum, another study conducted earlier the year found quite the opposite: marijuana use was not linked with a decrease in IQ (intelligence quotient) in teenagers.
So far, no cause and effect was found between decline in cognitive abilities and long-term marijuana use, but researchers still advise people to be aware of the potential harms associated with this drug.
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