Long-term marijuana smokers may suffer a decrease in their verbal memory by the time they are middle-aged, according to a new study. Appearing in JAMA Internal Medicine, the paper reveals how other elements of cognition, such as processing speed and executive function, do not appear to be affected by cannabis use.

The research was conducted by surveying participants of the Coronary Artery Risk Development in Young Adults (CARDIA) study, all of whom were aged 18 to 30 during the period between March 1985 and June 1986. Subjects were then interviewed about their marijuana use after 2, 5, 7, 10, 15, 20 and 25 years, before being asked to undertake a series of cognitive tests in August 2011.

Among these was the Rey Auditory Verbal Learning Test, which assesses verbal memory by asking participants to memorize lists of 15 words. The Digit Symbol Substitution Test, meanwhile, was employed in order assess attributes such as visual motor speed, working memory and sustained attention, all of which contribute to processing speed. Finally, the Stroop Interference Test was used to evaluate executive function, by asking participants to focus on and respond to a single stimulus while viewing a complex assortment of visual stimuli.

Of 3,385 subjects, the majority reported marijuana use at some point during their lives, although only 11.6 percent continued this into middle age. Initial analysis of the results appeared to show reduced performance on all three tests by everyone who had smoked marijuana, although this data was later adjusted to account for other factors, such as alcohol and cocaine use, age, and physical fitness.

Re-evaluating the data, the study authors noted an association between sustained marijuana use and a decrease in verbal memory. For every 1,825 days (5 years) of cannabis smoking, participants were found to have a 1 in 2 chance of being able to remember one word fewer from the list of 15 words. However, no correlation was discovered between marijuana use and processing speed or executive function.

Although the study authors are unable at this stage to explain how marijuana smoking brings about these alterations, they point to other studies that have associated exposure to tetrahydrocannabinol – the plant’s psychoactive ingredient – with functional changes in the hippocampus, the brain's learning and memory hub. However, with observational studies such as this, it is virtually impossible to discern cause from effect.

The researchers also acknowledge that their study has numerous limitations, such as the fact that self-reported drug use is not always reliable. Regardless, they insist that this information should be taken into account by policymakers and public health officials when deciding how to educate the public on the potential dangers of smoking marijuana.