Despite its illegal status in most states, marijuana is commonly used, especially among young adults. In 2012, a study showed that 37% of 17-18 year old had used marijuana in the last year, and 23% had used it within 30 days. Though marijuana does not have many short term effects on health, the long-term effects are not well known. Though long-term, frequent used has shown cognitive impairment when it comes to learning new information and retaining it, there is not a lot of evidence showing how significant these impairments can be.
A recent study called the CARDIA study, which stands for the Coronary Artery Risk Development in Young Adults study, observed the effects of marijuana exposure over 25 years. Ultimately, the study wanted to find any associations between marijuana use over many years, starting in young adulthood, and middle age cognitive performance. The study was an epidemiologic study that looked at 5115 adults in total. The baseline ages ranged from 18-30 years of age, and the participants were followed from June 1986 to August 2011. Participants came from Birmingham, Chicago, Minneapolis, and Oakland California. A variation of sex, age, race, and education was maintained through the group. Participants were followed up at the start of the study, and then 2, 5, 7, 10, 15, 20, and 25 years later. At each visit, they were asked about their marijuana use. Using questions like “About how many times in your lifetime have you used marijuana?”, their cumulative marijuana use was estimated through direct self-reporting. At the 25 year visit, participants took 3 cognitive tests. The Rey Auditory Verbal Learning Test (RAVLT) assesses verbal memory. The Digit Symbol Substitution Test determines visual motor speed, sustained attention, working memory, and executive function. These factors culminated in what the study called “processing speed”. Finally, the Stroop Interference Test looks at complex visual stimuli to asses what the study defines as “executive function”.
By the 25 year visit, 3499 patients were reassessed. 3385 yielded cognitive function data, while only 3326 had data completed from all three tests. Including or excluding the current marijuana user data, lifetime marijuana exposure showed reduced scores in all three cognitive tests. Lifetime exposure had a significant association with worse scores on the RAVLT, indicating deficient verbal memory. For every 5 years of exposure, an average of 1-2 participants remembered 1 word less from a 15 word list. However, cumulative exposure did not show significant associations when looking at processing speed or executive function. This study is one of the first that shows a negative association with cognitive function and marijuana use at low levels of cumulative marijuana use. Other studies have shown how long-term heavy use has had negative associations. However, this extended tracking of marijuana use through middle age, especially with such frequent and detailed measurements, has allowed researchers to show the negative effects marijuana can have on cognitive abilities, even if cumulative use is not relatively high.
Lena Chatterjee is the 2015-2016 Research Highlights Editor.