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The Scary Way Long-Term Marijuana Use May Impact Memory

"Come again?"

Years of puffing on a joint might cause memory troubles when you're older, researchers believe. A new study says that the effects of marijuana use can last for decades, particularly when it comes to your memory. 

A team led by researchers at the University of Lausanne in Switzerland found that for every five years of marijuana use, middle-aged people could have poorer and poorer verbal memory.

The study, published in the JAMA Internal Medicine journal, followed 3,500 young adults, aged 18-30 at the start of the study period, for 25 years, into middle age. 

At a follow-up, 84 percent of the participants admitted some past marijuana use, but only around 12 percent said they continued to use the drug into middle age.

The men and women were tested on their ability to remember words, their mental processing speed and executive function.

Those who were actively using marijuana scored worse on verbal memory and mental processing. But in even those who had given up the drug, lifetime use still had an effect. For every five years of reported use, half of participants were able to recall one fewer word on a 15-word memory test. Tests showed that other areas of cognitive function weren't significantly affected. 

"We did not expect to find such a consistent association with verbal memory for chronic exposure to marijuana,” lead author, Reto Auer of the University of Lausanne, told Reuters. 

The authors say it's an important warning for all recreational marijuana users, especially as marijuana becomes legalized in more and more places. Recreational marijuana is now legal in Alaska, Oregon, Colorado and Washington. Among post 50s, the use of the drug has increased in the past decade.

The study does have limitations, as the marijuana use was self-reported and no brain scans were used, the authors say. Participants also did not report the amount of the drug used, only how often they used it. 

Researchers say more work needs to be done on the effects of the drug, as findings are sometimes mixed. Some positive studies have found that low doses of marijuana can slow down the progression of Alzheimer's disease, and even PTSD. Other findings shed a negative light on the drug's use, including one study which found that heavy marijuana use can shrink the amygdala and the hippocampus, the brain's memory center.

"Cannabis is a drug," co-author Wayne Hall of the University of Queensland, Australia told Reuters, "and like all drugs, it can harm users when used in particular ways."

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