A new study based on a sample from thousands of participants ranging from young adults into middle age has found that there is a correlation between long term marijuana use and poorer performance on verbal memory tests — though other areas of brain function appear to not be affected.
According to Reuters, researchers analyzed data from a 25-year U.S. study in order to examine the long-term use effects of the drug. Participants in the study were given standardized tests of verbal memory, processing speed and executive functions during the 25th year of the study.
Repeated exposure to marijuana over time was linked to poor performance on verbal memory tests. As marijuana use increased, the researchers noticed, verbal memory test scores deceased.
“We did not expect to find such a consistent association with verbal memory for chronic exposure to marijuana,” said the study’s lead author Dr. Reto Auer of the University of Lausanne, Switzerland.
For every five additional years of marijuana exposure, 50 percent of regular users would typically remember one word less from a list of 15 tested words.
“Recreational marijuana users use it to get high, to benefit from the transient change it produces,” said Auer in an interview with Reuters Health. “But this transient effect might have long term consequences on the way the brain processes information and could also have direct toxic effects on neurons.”
Long-term use of other factors was accounted for in the study as well: cigarette smoking, alcohol use, and other behavioral factors associated with marijuana use were accounted for. However, Auer acknowledges potential limitations to the test.
Only self-reported marijuana use was accounted for in the study. Brain imaging techniques, used to measure potential structural changes in a participants brain, were not used.
“It would not be feasible to perform a long term randomized controlled trial to verify causation,” said Auer.