Use of Pot in Teen Years Linked to IQ Decline by Age 30

As recent studies have shown that marijuana use is extremely popular with adolescents, more so than alcohol and tobacco, further research, just released in the Proceedings of the National Academy of Sciences, finds that habitually smoking marijuana before age 18 showed an eight-point drop in IQ between the ages of 13 and 38, a considerable decline. The average IQ is 100 points. A drop of eight points represents a fall from the 50th percentile to the 29th percentile in terms of intelligence.

Researchers tested the IQs of all of the study subjects at age 13 before any habitual marijuana use. Researchers then split the study into five “waves” during which time they assessed cannabis use — ages 18, 21, 26, 32, and 38. They again tested IQ at age 38. The authors also controlled for alcohol use, other drug use and education level.

The eight-point drop in IQ was found in subjects who started smoking in adolescence and persisted in “habitual smoking” — that is, using cannabis at least four days per week — in three or more of the five study waves.

People who started smoking in adolescence but used marijuana less persistently still had a hit to their IQ’s, but it was less pronounced than the group that used it early and persistently.

In contrast, those who never used marijuana at all gained nearly one IQ point on average.

“Collectively, these findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects,” Meier writes in the study.

Of particular worry is the permanence of these effects among people who began smoking marijuana in adolescence. Even after these subjects stopped using marijuana for a year, its adverse effects persisted and some neurological deficits remained. People who did not engage in marijuana smoking until after adolescence showed no adverse effects on intelligence.

“Frontal lobe myelination is not fully completed until age 25 years or so, and the pre-myelinated brain is more susceptible to damage from neurotoxins,” says Dr. Richard Wahl, director of adolescent Medicine at the University of Arizona. “Cannabis, most likely, is a neurotoxin in high and continuous doses.”

The study appears to lend credence to “stoner” stereotypes in popular media. However, no previous studies can  provide data for this phenomenon, since establishing whether a drop in IQ has actually occurred requires that a baseline IQ be obtained before a person ever started using marijuana.  This study did just that.

“[The findings] provide evidence for the actual — rather than ideological and legal — basis for concerns regarding cannabis use,” said Dessa Bergen-Cico, an assistant professor of public health, food studies and nutrition at Syracuse University.  “These findings reinforce recommendations on the importance of primary prevention, evidence based drug education and policy efforts targeting not only adolescents, but elementary age children before they start.”
(Source:ABCnews.go) Full article here.

Comment from Director of Outpatient Counseling Services on this article:

As the director of ACS’ Adolescent Substance Abuse Treatment Program, marijuana is the number one substance our teens are habitually smoking on a daily, chronic basis. We see a continual decline in our teens lives – their social skills, family relationships, academic successes and overall feeling off health and well-being.

The adolescent brain is growing and changing – this study has reported evidence that teens 13-18 who are using marijuana on a regular basis suffer long-lasting IRREVERISBLE negative brain effects. Our teens, community and at times parents, are bombarded with information about the harmless effects of  the “herbal” drug that ‘everybody’ is doing. Our goal at ACS is to educate, prevent and treat teens who are using all drugs, especially marijuana, to learn how to live healthy adult lives.

To learn more about marijuana and its effects on the brain, we provide monthly substance abuse prevention workshops, named Esther’s Pledge Substance Abuse Prevention Workshops, for parents and teens through our Adolescent Substance Abuse Treatment Program.

We also provide SAIL (Substance Abuse Info Line) a “free” substance abuse information, resource and referral line for teens and parents, operated by professional treatment counselors. The line is available Monday-Friday 5 pm to 7pm. Just dial 650.384.3094.

To attend a workshop or if you have questions about substance abuse please contact Connie Mayer at 650.424.0852 ext 104 or

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