ARTICLE IN BRIEF
Investigators reported that teens who began smoking marijuana before age 18 and continued with the habit into adulthood lost an average of eight points on IQ tests between the ages of 13 and 38. The early, persistent marijuana users also showed deficits in learning, memory and executive functioning, and even those who eventually quit smoking pot as adults did not regain their full intellectual functioning.
Could smoking marijuana as a teen lead to a lower IQ as an adult? The answer is yes, according to a decades-long study that tracked more than 1,000 New Zealanders from adolescence into their late 30s.
The study found that those who began smoking marijuana before age 18 and continued with the habit into adulthood lost an average of eight points on IQ tests between the ages of 13 and 38. The early, persistent marijuana users also showed deficits in learning, memory, and executive functioning, and even those who eventually quit smoking pot as adults did not regain their full intellectual functioning.
A similar cognitive decline was not found among study participants who were already adults when they became regular marijuana users.
“Findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain and highlight the importance of prevention and policy efforts targeting adolescents,” the researchers reported in the Aug. 27 online edition of the Proceedings of the National Academy of Sciences. Madeline Meier, PhD — the study's lead author and postdoctoral research associate in the department of psychology and neuroscience at Duke University — said in written comments that the “extent of IQ decline among adolescent-onset persistent cannabis users is non-trivial.”
“For example, an average person has an IQ of 100, placing them in the 50th percentile for intelligence compared to same-age peers,” Dr. Meier noted. “If this average person loses eight IQ points, they drop from the 50th to the 29th percentile for intelligence.”
Researchers defined early-onset use as marijuana dependence before age 18 or weekly use before age 18. Persistence of use was defined as the total number of points in time (age 18, 21, 26, 32, and 38) that a participant met criteria for cannabis dependence.
The study found that early persistent users tended to have a decline in IQ score, even after statistical adjustments were made for factors such as alcohol or other drug use or reduced years of education. Their deficits tended to be apparent to those who filled out the questionnaire about their everyday functioning.
Still, the researchers noted that their study had some shortcomings. Even though use of alcohol and other drugs were factored into the statistical analysis, “There may be some unknown ‘third’ variable that could account for the findings,” the study authors wrote.
Dr. Meier said more research is needed to determine “how much cannabis needs to be consumed and across what ages before impairment occurs.” She said future research should also explore whether “cannabis-related IQ decline is reversible.”
Dr. Meier said her group plans to use the New Zealand cohort to look at the persistent use of alcohol on neuropsychological functioning.
WHAT DO THE FINDINGS MEAN?
While the findings of the New Zealand study do not prove that smoking pot causes long-term cognitive decline, researchers not connected with the study told Neurology Today that the study makes a convincing case that marijuana use is harmful to the young brain, which continues to develop into the mid twenties.
Karen Bolla, PhD, associate professor of neurology at Johns Hopkins Bayview Medical Center, said the current study was noteworthy because it involved a large sample size that was maintained over a long number of years. The participants were given baseline IQ tests at age 13, which then were compared to tests given at age 38. Interviews were conducted at age 18, 21, 26, 32 and 38 to determine marijuana use. Five percent of the study participants fell into the category of early-onset users.
“The more they used, the more there was this negative effect,” Dr. Bolla said, noting that the linear association strongly suggests that marijuana smoking may be causally related to the cognitive decline.
Dr. Bolla is involved in a study of more than 900 Kentucky adolescents that is looking for risk factors for marijuana use, including whether kids with sleep problems are more apt to smoke pot. She said drug-prevention efforts aimed at teens should focus on the point that they “could be damaging something they really need — their brain.”
Anthony Feinstein, MD, PhD, professor of psychiatry at the University of Toronto, told Neurology Today that the new findings out of New Zealand “are pretty powerful data.” In his own work with multiple sclerosis patients, some of whom use marijuana to help with pain or other problems, he said he has found that the drug interferes with people's cognitive well-being. Also, he said the psychiatry literature is strong when it comes to the connection between marijuana and the risk for psychosis.
Dr. Feinstein said the refining process that the brain goes through during adolescence and into young adulthood is like “a tree pruning itself.” When marijuana enters the equation, he said, “development may be less than perfect.”
Archived articles on marijuana and neurologic disorders in Neurology Today: http://bit.ly/Om216N.
THE STUDY: IN GREATER DETAIL
* This latest research is based on data collected as part of the Dunedin Multidisciplinary Health and Development Study, a prospective study of the health and behavior of 1,037 individuals of varying socioeconomic status born in 1972–1973 in Dunedin, New Zealand.
* Researchers compared the results of IQ tests (Wechsler Intelligence Scale for Children-Revised, WISC-R) given in 1985–1986 to those from IQ tests (WAIS-IV) given in 2010–2011.
* They also analyzed the results of other neuropsychological tests administered at age 38, including the Wechsler Memory Scale- III, the Trail-Making Test, the Cambridge Neuropsychological Test Automated Battery and the Rey Auditory Verbal Learning Test.
* In addition, at the 38-year mark, the study collected information from people who knew the participants that focused on whether they had noticed attention or memory problems.