ANN ARBOR, Mich. — Marijuana use over time is associated with remembering fewer words from a list, but it did not appear to affect other areas of cognitive function in a study of men and women followed up over 25 years, according to an article published online by JAMA Internal Medicine.

Marijuana use is common among adolescents and young adults, with 19.8 million current users according to federal estimates, and growth in use in recent years. But it remains unclear whether there are long-term effects from low-intensity or occasional marijuana use earlier in life and whether the magnitude and persistence of impairment depends on the duration of marijuana use or the age of exposure.

The researchers used data from the long-term Coronary Artery Risk Development in Young Adults (CARDIA) study, which includes 25 years of repeated measures of marijuana exposure starting in early adulthood. In year 25, CARDIA measured cognitive performance using standardized tests of verbal memory, processing speed and executive function.

Reto Auer, M.D., M.A.S., formerly of the University of California-San Francisco and now the University of Lausanne, Switzerland, and coauthors used those measurements to study the association between cumulative years of exposure to marijuana use and cognitive performance in middle age among study participants who had marijuana exposures typical to the communities in which they live.

The research team included Deborah Levine, M.D., MPH, an assistant professor at the University of Michigan Medical School who studies effects of aging, lifestyle and disease on different aspects of thinking ability, also called cognitive function. Levine is a member of the U-M Institute for Healthcare Policy and Innovation and the VA Center for Clinical Management Research, and an affiliate of the Michigan Alzheimer’s Disease Center. She practices at both the U-M Health System and VA Ann Arbor Healthcare System.

“This study is timely because marijuana use has increased, and many states have legalized marijuana use or decriminalized its possession,” says Levine. “We need to better understand the risks of marijuana use, particularly its effects on brain health, to inform decision making by teenagers and adults as well as policymakers.”

Of the 3,499 participants assessed at the year 25 visit, 3,385 (96.7 percent) had data on cognitive function. Among the 3,385 participants, 2,852 (84.3 percent) reported past marijuana use but only 392 (11.6 percent) continued to use marijuana into middle age.

Past exposure to marijuana was associated with worse verbal memory but does not appear to affect other domains of cognitive function. For every five years of past exposure, lower verbal memory corresponded to an average of 1 of 2 participants remembering one word fewer from a list of 15 words, according to the results.

Limitations to the study include self-reported information that is not always reliable.

"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. In the meantime, with recent changes in legislation and the potential for increasing marijuana use in the United States, continuing to warn potential users about the possible harm from exposure to marijuana seems reasonable," the study concludes.

Reference: JAMA Intern Med. Published online February 1, 2016. doi:10.1001/jamainternmed.2015.7841