As expected with the legalization of marijuana in several areas of the country, teen and adolescent use of the product is on the rise, and one of the reasons is that most teens and adolescents in the nation view marijuana as not harmful.

But a new 25-year study, beginning with over 5,000 participants and cited on cnn.com, says the use of marijuana is associated with poorer verbal memory skills and slower memory processing skills later in the life of regular users.  In fact, lifetime use of marijuana was associated with worse performance in all three areas of cognitive function, including verbal memory, processing speed, and executive function.

In this new study, the researchers found participants failed to remember of one additional word from a list of 15 words for every five years of marijuana use.  The team also says that while past exposure to marijuana was associated with poorer verbal memory, it was also unclear if occasional marijuana use earlier in life impacted other cognitive functions or led to long-term memory loss.

After initially selecting adults aged 18-30, the study finished with 3,400 still in the program after the 25-year mark.  Those participants were evaluated periodically over the course of the study with standardized testing for memory functions.

Dr. Reto Auer said the study involved as many women as men, black as whites, and lower education as higher education, so the research provided a better sense of the association across the overall population.  Dr. Auer added there was a lack of well-performed studies on the topic and more research is needed before drawing firm conclusions.

The findings are in line with an earlier study by researchers in New Zealand that found frequent marijuana users over decades had less cognitive function performance than their non-user counterparts.

The relaxing of marijuana laws in the United States is making it easier for adolescents to become more frequent users of the substance, and is making it more readily available to first-time users as well, according to commentary published in the JAMA Internal Medicine, where the findings of the new study can also be found.

The commentary also adds while the available evidence does not establish beyond a reasonable doubt that heavy use of marijuana impairs verbal memory and lowers IQ’s, considerable weight should be given to the findings from this and other related studies.