The new study reveals that both past and present marijuana use is linked with worse verbal memory in middle age.
The researchers - led by Dr. Reto Auer of the University of Lausanne in Switzerland - publish their work in JAMA Internal Medicine.
Marijuana use in the US is quite common among adolescents and young adults. In fact, data from 2012 revealed that 37% of 12th-graders had used marijuana in the last year, 23% used it within the last 30 days and 6.5% used it daily.
Previous studies have shown an association between long-term marijuana use and cognitive impairment, especially when it comes to learning and remembering new information.
However, the researchers from the latest study say that this association with lower levels of marijuana exposure had not previously been demonstrated. As such, they set out to investigate.
They used data from the Coronary Artery Risk Development in Young Adults (CARDIA) study, which contains 25 years of marijuana exposure measurements, beginning in early adulthood.
In the final year, the study measured cognitive performance through standardized tests of verbal memory, processing speed and executive function.
Communicating risks is a 'public health challenge'
Using the measurements from the final year of the study, Dr. Auer and colleagues investigated the association between cumulative years of marijuana use and cognitive performance in middle age.
In total, 3,499 participants were assessed at the 25-year visit, and 3,385 had cognitive function data. Of these, 84.3% reported past marijuana use and 11.6% reported continuous marijuana use into middle age.
The researchers found that past marijuana exposure was linked with worse verbal memory. However, it did not appear to affect other areas of cognitive function.
In detail, for every 5 years of past exposure, verbal memory was 0.13 standardized units lower, which the researchers say corresponds to an average of 1 of 2 participants remembering one word fewer from a list of 15 words.
The researchers write that "with recent changes in legislation and the potential for increasing marijuana use in the US, continuing to warn potential users about the possible harm from exposure to marijuana seems reasonable."
"The public health challenge is to find effective ways to inform young people who use, or are considering using, marijuana about the cognitive and other risks of long-term daily use," write Wayne Hall, PhD, of the University of Queensland in Australia, and Michael Lynskey, PhD, of Kings College London in the UK, in a linked editorial.
"Young adults may be skeptical about advice on the putative adverse health effects of marijuana, which they may see as being overstated to justify the prohibition on its use. More research on how young people interpret evidence of harm from marijuana and other drugs would be useful in designing more effective health advice."
Despite the large study size, the researchers point to some limitations. For example, the self-reported information could be unreliable, as measurements were infrequent and age of marijuana exposure - particularly during adolescence and young adulthood - was not included.
Additionally, because the cognitive function measurements were only conducted at one point in time, this limits the researchers' ability to pinpoint exactly when a change in outcome could have occurred.
Still, the findings are significant, and the researchers note areas for further investigation: "Future studies with multiple assessments of cognition, brain imaging and other functional outcomes should further explore these associations and their potential clinical and public health implications."
Other recent studies investigating the potential effects of marijuana use have revealed that teens who use cannabis could be at risk of schizophrenia and high-potency cannabis damages a key brain structure.
Still, other studies have found positive outcomes related to cannabidiol, an extract of marijuana. One study found that it shows promise as an epilepsy treatment, while another found that the extract helps bones to heal.
Written by Marie Ellis