Past marijuana exposure linked to impaired verbal memory

1. Based on observational data, cumulative lifetime marijuana exposure was associated with worse performance on verbal memory testing, but not processing speed or executive function testing.

2. Verbal memory worsened in a dose-dependent manner, with a step-wise decrease in memory for each additional 5 marijuana-years of exposure (1825 days of use).

Evidence Rating Level: 2 (Good)

Study Rundown: Marijuana use is common amongst the American adolescent and young adult population, but evidence on long-term health outcomes is currently lacking. The described study used data from a cohort of young adults aged 18 to 30 with a 25-year follow-up. Survey data from the cohort was used to assess for marijuana and other drug exposure, as well as other psychiatric comorbidities, and cardiovascular risk factors. Cognition was formally assessed at 25-year follow up using examinations of verbal memory, processing speed, and executive function.

The results of the study showed that prior exposure to marijuana was associated with decreased performance on verbal memory testing, but not processing speed or executive function testing. Additionally, verbal memory worsened in a dose-dependent manner, with a step-wise decrease in memory for each additional 5 marijuana-years of exposure (1825 days of use). The strengths of the study included a large cohort size, long follow-up, and data on possibly confounding comorbid conditions and demographics. Limitations of the study included reliance on self-reported exposures, using different tests to interpret cognitive function at the beginning compared to the end of the observational period, and only evaluating cognitive function at one follow up time period rather than at regular intervals.

Click to read the study, published today in JAMA Internal Medicine

Relevant Reading: Persistent cannabis users show neuropsychological decline from childhood to midlife

In-Depth [prospective cohort]: This study used data from the Coronary Artery Risk Development in Young Adults (CARDIA) cohort, which comprised of 5115 adults aged 18 to 30 from March 25, 1985 to June 7, 1986. The CARDIA cohort was followed up at 2, 5, 7, 10, 15, 20, and 25 years with assessment of cardiovascular risk factors, drug and alcohol use, physical activity, and psychiatric disorders. A total of 3385 patients were followed for 25 years, of which 84.3% reported prior marijuana exposure. Initial cognitive function was assessed at 2-years using a “mirror star tracing test” and at 25-years using three different formal assessments of verbal memory, processing speed, and executive function.

After excluding participants with current marijuana use or self reported history of schizophrenia, linear regression analysis was performed. Lifetime exposure to marijuana was associated with decreased performance on verbal memory testing (p = 0.02). Association was dependent on time of marijuana exposure: every 5 marijuana-years decreased performance by 0.13 (95%CI -0.24 to -0.02) standardized units. After controlling for possible confounders, no significant association was identified between past marijuana use and changes in executive function or processing speed.

Image: PD

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