As marijuana becomes more accessible to young and old alike in the U.S., researchers warn that long-term use of the drug may cause lasting harm to at least one type of brain function.

A new study based on following thousands of young adults into middle age finds that long-term marijuana use is linked to poorer performance on verbal memory tests, but other areas of brain function do not appear to be affected.



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“We did not expect to find such a consistent association with verbal memory for chronic exposure to marijuana,” especially since the link held even when other factors such as cigarette smoking, alcohol use and other behavioural factors associated with marijuana use were accounted for, said lead author Dr. Reto Auer of the University of Lausanne, Switzerland.

Mr. Auer and colleagues analyzed data from a 25-year U.S. study of young adults, which included repeated measures of marijuana exposure over time and a standardized test of verbal memory, processing speed and executive function in year 25. Almost 3,500 participants completed the standardized tests.

At the beginning of the study period in the 1980s, participants were 18 to 30 years old and more than 80 per cent reported past marijuana use. Just 12 per cent continued to use marijuana into middle age, according to the results in JAMA Internal Medicine.

Researchers found that as past years of marijuana use increased, verbal memory scores decreased. In practical terms, the results meant that for every additional five years of exposure, 50 per cent of marijuana users would remember one less word from a list of 15 tested words.

“Recreational marijuana users use it to get high, to benefit from the transient change it produces,” Mr. Auer told Reuters Health by e-mail. “But this transient effect might have long term consequences on the way the brain processes information and could also have direct toxic effects on neurons.”

But, he said, it is unclear from this observational study if lower verbal memory is a cause or a consequence of marijuana use.

The study only included self-reported marijuana use and did not employ brain imaging to measure structural changes, the authors note.

“Unfortunately, as with all recreational drug exposures, it would not be feasible to perform a long term randomized controlled trial to verify causation,” Mr. Auer said. “We are left with trying to do the best we can in epidemiological studies and adjust for potential confounders to estimate the potential causal effect of marijuana on health outcomes.”

Several studies have found that cannabis users who have used daily over years, and especially those who have used daily for decades, perform worse on various cognitive tasks than their peers who have either not used cannabis or have used it less frequently, for shorter periods, then stopped, said Wayne Hall, of the University of Queensland, Australia, who co-authored a related commentary.

“People who used occasionally in their 20s and discontinue as most cannabis users do, are at low risk of developing any cognitive impairment,” Mr. Hall told Reuters Health by e-mail.

“But cannabis is a drug, and like all drugs, it can harm users when used in particular ways,” Mr. Hall said. “This message needs to be communicated to all cannabis users and especially those in U.S. states where medical or recreational cannabis use by adults is now legal.”

The new results did not include information on how the marijuana was consumed, only on the number of days of exposure in the month before each exam, Mr. Auer said.

All marijuana users and the overall population interested in knowing the health consequences of marijuana should have access to balanced, high quality information from independent researchers, but no study has repeatedly measured brain structure and marijuana use over time, Mr. Auer said.

“There is a severe lack of well performed studies on the topic, especially considering the proportion of the population who have been exposed, or are exposed to marijuana,” he said.

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10 thoughts on “Long-term pot use linked to reduced middle-age verbal memory, study says

  1. come Useless article. How frequently was pot used and how much per day? Alcohol is toxic too if ingested at higher levels over many years. The key is any psychoactive drug must be used in moderation. That’s not exactly news.

  2. It would be more surprising if there were no effects from using marijuana. But this is a particularly weak study. The self-reported use is unquantified. There are no physical measurements, only a “verbal memory” test, which is just recall of a list of random words, hardly a vital life skill. Let us know if researchers find something important, like cancer.

  3. thatll be reassuring to my buddy who last his family, job, and almost his life because he drank alcohol or to all the millions who died of cancer caused by smoking or from processed meat and pop.

  4. Hardcore pot use will have adverse effects to be sure.

    But personally, I could care less if someone’s vocabulary regresses over time because they abused the substance. People can abuse absolutely anything to their detriment. I like to smoke occasionally to release stress. My dad prefers a couple glasses of whiskey. My mom prefers indulging in sweets and a cigarette. I don’t see how my preference is worse (actually it’s probably better). I am aware the side-effects are not all butterflies and rainbows – but that shouldn’t be reason enough to make it illegal.

    I have a client that has a government license to grow – their operations are incredible. World-class botanists that grow *high* quality product free of mould and other nasty additives that you get from black-market dealers. The different strains have accurate labels of the % of THC and CBD so users can moderate their use more effectively. I would be interested to know how many studies on pot use only top-notch product grown by professionals in sanitary environments like my client does, rather than tracking individuals in these longitudinal studies where the user may be smoking illegal product that is filled with all these extra chemicals.

  5. Toronto I was a daily pot smoker for 27 years until I developed COPD. I have been clean for 10 years now. I also have a terrible memory.

    My lungs have improved a little since I quit, memory not so much.

  6. So,,,,,, if you lose 1 word every 5 years from a list of 15, it will take you 75 years to forget them all.

    Given the harm caused by other legal substances, it seems fairly minor. Certainly not enough to justify the current prohibition regime with its’ attendant social consequences.

  7. This was an accepted fact in the late ’60’s, glad the research community is catching up, however slowly.

    PS – today’s weed is something like 10 times more potent than Acapulco gold was back then so we should add that to the math when stretching qualitative data over such a long period.

    It will be interesting to see how marketers treat this subject when it becomes legal. Connisseurs enjoying high priced organically grown and hand sorted flower tops before an evening or Opera while the hoi polio sit around smoking friendly cafe’s toking and exchanging texts and messages on social media? Can’t wait!

  8. What a load of poppycock; why just yesterday I was saying to my wife….umm.. I said.. “Honey..” no.. no.. uhhhmm…. I think I said.. ummm….. hmmmmm

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