Smoking pot every day for the next five years will make remembering certain words difficult for you, and consequently ruin your verbal memory, a new study in the United States has revealed.

Past studies have been warning marijuana smokers — particularly teenagers — that long-term use of the stimulant can lead to a decline in intelligence.

The new study, however, said long-term use of marijuana was not significantly associated with decreases in other measures of cognitive functions, such as processing speed or executive functions. The latter includes the ability to plan and focus.

"We did not expect to find such a consistent association with verbal memory for chronic exposure to marijuana," says Dr. Reto Auer, lead author of the study and an expert from the University of Lausanne in Switzerland.

Auer said this was because the link to verbal memory was present even when factors such as cigarette use, alcohol consumption and other behavioral factors related to marijuana use were taken into account.

Auer and his colleagues examined the long-term effects of marijuana use by evaluating participants enrolled in the study called Coronary Artery Risk Development in Young Adults (CARDIA), which involved more than 5,000 adults who joined the study when they were 18 to 30 years old, during the 1980s.

In a series of follow-ups, the participants reported if they used marijuana in the previous month. The follow-up lasted for 25 years, with different time intervals. The study participants answered several cognitive tests that examined verbal memory, processing speed and executive function.

As years of marijuana use added up, verbal memory scores decreased, researchers said. For every five years of marijuana use, one out of five people failed to remember one word from a list of 15 words. It was two words less every decade.

Auer said recreational marijuana users smoke the drug to get high, but this exact effect might have long-term consequences on brain processing, as well as direct toxic effects on neurons.

However, Auer added that they have yet to know whether lower verbal memory is a cause or consequence of marijuana use, especially because they only employed self-reporting techniques and did not use any brain imaging scanning methods.

The study was published online in the journal JAMA Internal Medicine.

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