(CNN)People who smoked weed regularly as teenagers remembered fewer words as they entered middle age, according to a new study published online by JAMA Internal Medicine.
Marijuana use is increasingly common among adolescents and young adults in the United States. In fact, a recent survey of high school students found more teens use marijuana than tobacco. One reason: a perception that it's not harmful
For this study, researchers randomly selected more than 5,000 young adults from 18 and 30 and followed up with them at varying points over 25 years. At the end of the 25 years, there were more than 3,400 participants still in the study. Their cognitive function was measured using standardized tests of verbal memory, processing speed and executive function.
They found current marijuana use was associated with poorer verbal memory and processing speed, and lifetime exposure to marijuana was associated with worse performance in all three areas of cognitive function. Although past exposure to marijuana was associated with worse verbal memory, it does not appear to affect other domains of cognitive function.
It is unclear whether there are long-term effects on memory from occasional marijuana use earlier in life and whether there is an impact on other areas of cognitive function.
For every five years of past marijuana exposure, verbal memory was lower -- about half of participants remembered one word fewer from a list of 15 words, according to the study.
"In this study, there are as much women as men, as much black as white, as much lower education as higher education," said Dr. Reto Auer, one of the study's authors.
"It provides a better sense of what the association is in the overall population."
Auer said there is a lack of well-performed studies on this topic. As is often the case, more research is needed, he said.
Previous studies show similar and consistent results about the potential effects of using marijuana over time.
Researchers in New Zealand found people who used marijuana frequently over several decades had poorer cognitive function than those who did not participate in regular use. "Regular" reflects those with five cumulative years of marijuana use over the lifetime. The study analyzed IQ at 13, before marijuana was used, and again at 38. The results showed that the early initiators and constant users demonstrated the largest decline in IQ scores.
With the liberalization of medical marijuana laws in the United States, it is becoming easier for adolescents to participate in persistent, daily use of marijuana, according to a commentary also published in JAMA Internal Medicine.
This change is making marijuana more readily accessible and could possibly impact the increase of marijuana use in the United States, it said.
"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," wrote authors Wayne Hall of the Center for Youth Substance Abuse Research at the University of Queensland in Australia and Michael Lynskey of the National Addiction Centre at King's College London.
"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," the authors write. "More research on how young people interpret evidence of harm from marijuana and other drugs would be useful in designing more effective health advice."