How do you warn people about the potentially serious risks of a recreational drug that many users think is harmless? This is the challenge now facing health officials hoping to inform and warn the public about an increasingly legal recreational drug.

A new study indicates lifetime use of marijuana is linked to poorer verbal memory at middle age, and its authors want to spread the word.

“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,” wrote Dr. Wayne Hall, University of Queensland, Australia, and Dr. Michael Lynskey, Kings College London, in an editorial accompanying the new study. They say the challenge, then, is finding effective ways to inform pot smokers (and wannabe smokers) about the risks if they decide to smoke long-term.

Starting Age and Duration

During 2012, roughly 37 percent of 12th grade American students reported using marijuana within the last year, 23 percent within the last 30 days, and 6.5 percent daily, the study authors explain. Meanwhile, past studies indicate heavy, long-term use is linked to mental impairment, particularly in learning and remembering new information, the researchers noted. 

But what are the consequences of occasional use over the years? And how much do other factors matter, such as what age you are when you start?

Dr. Reto Auer, formerly of UC San Francisco and now of the University of Lausanne, Switzerland, and his co-researchers turned to National Institutes of Health-funded heart disease research to answer these questions. Repeatedly over 25 years, the Coronary Artery Risk Development in Young Adults (CARDIA) study has measured marijuana use beginning in early adulthood among its participants. Because the study is not a laboratory experiment, CARDIA provides an opportunity to assess the long-term effects of the drug as used by people within a community.

During year 25, the study measured participants' cognitive performance using standardized tests of verbal memory, processing speed, and executive function. Looking at the data for the total 3,499 participants, Auer and his colleagues discovered that nearly 97 percent had the necessary comparison data on cognitive function from earlier years in their files. Among these participants, about 84 percent had reported past marijuana use, while only about 11 percent continued to use marijuana into middle age.

“We used current and lifetime use to compute marijuana-years, with one year of exposure equivalent to 365 days of marijuana use,” wrote the researchers, who also factored participants’ use of other drugs, including tobacco, alcohol, cocaine, and antidepressants, into their calculations. Analyzing the data, the researchers discovered a relationship between cumulative lifetime use of cannabis and worsening verbal memory in middle age. 

Specifically, for each additional five marijuana-years of exposure, verbal memory was 0.13 standardized units lower than for those who never used it; this corresponds to a mean of one of two participants remembering one word fewer from a list of 15 words for every 5 years of use. 

“We found no significant associations of cumulative exposure with executive function or processing speed,” wrote the researchers.

In their accompanying editorial, Hall and Lynskey refer to a New Zealand study which assessed IQ at age 13 (pre-marijuana use) and again at age 38. Early initiators and persistent users showed the largest decline in IQ scores — 8 points compared to those who had never used or those who had used and then stopped.

Though the new study provides some evidence to discourage early and regular marijuana use, Hall and Lynskey amplify this warning by noting that people who start young and continue to use are more likely to become dependent. And here, past research has shown that dependence on marijuana is linked to an increased risk of leaving school early, experiencing symptoms of psychosis, using other illicit drugs, developing depression and anxiety disorders, and receiving a diagnosis of schizophrenia and bipolar disorder.

Sources: Auer R, Vittinghoff E, Yaffe K, et al. Association Between Lifetime Marijuana Use and Cognitive Function in Middle Age The Coronary Artery Risk Development in Young Adults (CARDIA) Study. JAMA Intern Med. 2016.

Hall W, Lynskey M. Long-Term Marijuana Use, Cognitive Impairment in Middle Age. JAMA Intern Med. 2016.