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Why Marijuana Might Not Be Harmful to the Teen Brain

May 11, 2020

Cheech & Chong aside, commentary published by a Hartford HealthCare researcher declares that teen marijuana use may not affect one’s IQ as is generally assumed.

Dr. Godfrey Pearlson, director of the Olin Neuropsychiatry Research Center and research director at the Institute of Living, part of Hartford HealthCare, was the single author of a piece published recently in Scientific American examining several “rigorous new studies.”

“Physicians and lawmakers need a more accurate sense (of the) effects on adolescent minds so parents, teachers and social planners can respond preemptively to teenage marijuana use,” Dr. Pearlson said. “If cognitive effects are transient or better explained by sociological phenomena, we can all take a step back and direct our effects and resources elsewhere. Therefore, knowing the actual measure of the risks is crucial.”

Early work from a large community study in New Zealand suggested that heavy teen cannabis use caused IQ drops of as many as eight points. But, more recent studies came from California and Minnesota researchers who mined data from two large studies of adolescent twins. The twins’ IQ was measured between ages nine and 12, before using drugs, and again between the ages of 17 and 20. Marijuana users had lower test scores and showed “notable reductions in IQ over time.”

However, crucially, the researchers determined that the IQ changes were not related to marijuana use but dipped equally whether the twins had used marijuana or not.

“(They) noted that, prior to any substance involvement, future marijuana users … already had significantly lower IQ scores. Put another way, cannabis did not drag down their IQ; it was already low,” Dr. Pearson explained.

Digging further, he said the researchers uncovered other factors that could affect the twins’ IQ levels, such as impulsive behavioral traits and being raised in a family that did not value education.

“Delinquent kids received lower grades because of their tendency to skip school and use drugs,” Dr. Pearlson said. “So cannabis was not a culprit in cognitive decline. A welter of inherited and environmental factors seemed to explain both.”

These conclusions, he continued, are contrary to other earlier, large studies that point to marijuana as dulling intelligence, memory and other cognitive abilities. Dr. Pearlson pondered that the latter were conclusions arrived at after studying long-term marijuana users in middle age.

“Perhaps adolescent cannabis use has no detectable cognitive impact except at very high levels and/or over many years,” he asserted.

He is looking to the recently-launched Adolescent Brain and Cognitive Development study of 11,000 American 10-year-olds to address more of the unknowns. The study will examine serial IQ testing and brain imaging to capture the trajectories of normal brain and IQ development before substance use and to document any consequences.

“This research has the potential to settle the issue of the relationship of adolescent marijuana use to changes in cognition,” Dr. Pearlson said. “Scientists will begin to see meaningful results in the next few years, as these subjects reach their mid-teens. If long-term cognitive effects are shown to be real, this conclusion should result in appropriate plans to restrict use through educational efforts and tough legal sanctions.”

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