Early, Persistent Cannabis Use Linked to Marked Drop in IQ

Pam Harrison

August 27, 2012

August 27, 2012 — Adolescents who start using cannabis persistently before the age of 18 years show a marked decline in measures of intelligence in adulthood even if they stop using the drug along the way.

Madeline Meier, PhD, from Duke University in Durham, North Carolina, and multicenter investigators found that cannabis users who start using the drug in adolescence and who use it for years thereafter had an average 8-point decline in IQ in adulthood compared with persons who never used the drug; those who never used the drug showed a modest increase in IQ during the same interval.

In contrast, adults who start using cannabis in adulthood had no decline in IQ over time.

"Quitting or reducing cannabis use did not appear to fully restore intellectual functioning among adolescent-onset, former persistent cannabis users either," Dr. Meier told Medscape Medical News. "And findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects."

The study was published online August 27 in the Proceedings of the National Academy of Sciences.

Reduction in IQ "Not Trivial"

Previous studies have suggested that adolescents may be particularly vulnerable to the effects of cannabis on IQ. Until now, however, research had not been able to rule out the possibility that poorer IQ test performance among adolescent-onset cannabis users predated cannabis use.

In the present study, the authors investigated the association between persistent cannabis use prospectively assessed during a 20-year period and neuropsychological functioning in a birth cohort of 1037 individuals born from 1972 to 1973 in Dunedin, New Zealand.

Participants underwent neuropsychological testing prior to onset of cannabis use at the age of 13 years; 96% of the sample took part in IQ testing again at the age of 38 years.

"The extent of IQ decline among adolescent-onset persistent cannabis users of approximately 8 points is not trivial," Dr. Meier noted. For example, an average person has an IQ of 100, which places them in the 50th percentile for intelligence compared peers of the same age.

"If an average person loses 8 IQ points, they drop from the 50th to the 29th percentile for intelligence," she added. "And individuals who lose 8 points in their teens and 20s may be disadvantaged relative to their same-age peers in most of the important aspects of life and for years to come."

Investigators measured cannabis use in 2 ways: cannabis dependence and regular cannabis use. Persistence of cannabis dependence was defined as the total number of study "waves" out of 5 done at the ages of 18, 21, 26, 32 and 38 years at which participants met criteria for cannabis dependence.

Persistence of regular cannabis use was defined as the total number of study waves at which participants reported using cannabis on the majority of days in a week.

Results were similar for persistent cannabis dependence and persistent regular cannabis use.

Education Not a Factor

Investigators also asked whether it was the cannabis itself that caused the decline in IQ or whether there was some other, alternative explanation.

Dr. Meier noted that cannabis users tend to be less educated than nonusers, and the observed decline in IQ among cannabis users might simply reflect lower levels of education.

This did not prove to be the case.

"Even after equating study members on years of education, persistent cannabis use was associated with IQ decline, so even among study members with a high school education or less, the more persistent cannabis users showed greater IQ decline.

"This suggests that reduced years of education among persistent cannabis users cannot alone explain the association between persistent cannabis use and IQ decline," said Dr. Meier.

Commenting on the study for Medscape Medical News, Laurence Steinberg, PhD, from Temple University in Philadelphia, Pennsylvania, said the fact that investigators were able to control for any cognitive impairment that might have existed prior to cannabis use was very important because it ruled out the possibility that cognitive differences may have existed between users and nonusers that predated their cannabis use.

He also said it was interesting to note that adult-onset cannabis use, even if persistent, was not associated with cognitive impairment.

"This finding is important in my view because it is consistent with emerging research suggesting that the adolescent brain is more susceptible to environmental influences than the adult brain and that exposure to drugs and not just cannabis is worse when it occurs early in adolescence," Dr. Steinberg said.

The study was funded by the US National Institute on Aging, the UK Medical Research Council, the Health Research Council of New Zealand, the US National Institute of Mental Health, the US National Institute on Drug Abuse, and the Jacobs Foundation. Dr. Meier and Dr. Steinberg have disclosed no relevant financial relationships.

Proc Natl Acad Sci. Published online August 27, 2012.