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Marijuana use impairs verbal memory

Posted on 05-04-16 in Cognition, Memory, Substance Abuse

Marijuana use impairs verbal memory

Few would argue that smoking marijuana makes people smarter (except maybe for persons with schizophrenia, but even those results are mixed). The degree to which marijuana impairs cognitive function remains unclear. Adolescents are of particular concern because their brains are considered to be undergoing critical neurodevelopment until about age 25.

Impairments are obvious during acute marijuana intoxication, but long-term outcomes vary. Part of the reason for this ambiguity is the variations in how researchers design their studies lead to conflicting findings. For example, a 2012 study found frequent, heavy marijuana use to be associated with a decline in IQ. The same data were re-analyzed in a follow-up study adjusting for other factors, such as economic status, alcohol use and mental illness. Other factors were found to possibly explain the decline in IQ initially reported, rather than marijuana use.

Long-awaited results are in

Results from a large project on lifestyles of thousands of patients aged 18 to 30 years were recently reported. Researchers asked detailed questions about lifestyle habits and performed standardized cognitive testing to measure verbal memory, processing speed and executive function. Cognitive scores were examined relative to cumulative lifetime exposure to marijuana. Analyses adjusted for various other factors that could affect results, such as demographics, depression, physical activity, and alcohol and drug use.

About 84 percent had used marijuana during the 25 years of the study, but only 12 percent continued to use it into middle age. Current use was associated with worse verbal memory and processing speed. Higher cumulative lifetime marijuana exposure was associated with worse cognitive performance as well, including executive processing. When the analysts excluded current users and adjusted for other factors, only verbal memory was affected by cumulative use.
In other words, those who used marijuana daily for years had poorer verbal memory in middle age than those who did not. The authors illustrate the loss of verbal memory as “one or two participants remembering one word fewer from a list of 15 words for every 5 years of use.”

Results match

The findings from this important study corroborate those of a New Zealand study that measured IQ at age 13 and again at age 38. Those who used marijuana earlier and persistently had an average decline in IQ scores of 8 points compared with nonusers and those who stopped using it. (It should be noted, however, that IQ scores are not exact and typically can change between 5 to 20 points over time.)

Putting research into action

Heavy, long-term use of marijuana in adolescence and young adulthood may actually cause some cognitive decline by middle age. Middle age is generally the time when people should function at their peak performance. During this time, careers and incomes usually peak, and many have influential roles as parents, mentors and leaders. Middle age is the time to function at the highest possible level, not to be held back by cognitive deficits.

Any habit practiced daily for many years will be reflected in a person’s life, for better or for worse. Failed attempts to cut down or quit any habit usually indicate a problem exists that is negatively impacting daily functioning. Such problems typically remain or get worse unless something ends the cycle. Ending the cycle allows those who are stuck to break free and live up to their greatest potential.

The Sovereign Health Group treats individuals with substance abuse, mental health problems and dual diagnosis. We emphasize cognitive rehabilitation in our brain wellness program and use state-of-the-art science and technology that allow accurate diagnosis and customized therapy. Comprehensive treatment and ongoing aftercare provide the support clients need to recover from addiction and all of its consequences. To find out more about specialized programs at Sovereign Health, please call our 24/7 helpline.

About the author

Dana Connolly, Ph.D., is a senior staff writer for the Sovereign Health Group, where she translates current research into practical information. She earned her Ph.D. in research and theory development from New York University and has decades of experience in clinical care, medical research and health education. The Sovereign Health Group is a health information resource and Dr. Connolly helps to ensure excellence in our model. For more information and other inquiries about this article, contact the author at