Yet Another Study Finds That Cannabis Use Is Not Independently Linked With IQ Decline

Marijuana researchCannabis use by teens is not independently linked with adverse changes in intelligence quotient or executive functioning, according to longitudinal data published online ahead of print in the journal Addiction.

A team of investigators from the United States and the United Kingdom evaluated whether marijuana use is directly associated with changes over time in neuropsychological performance in a nationally representative cohort of adolescent twins. Authors reported that “family background factors,” but not the use of cannabis negatively impacted adolescents’ cognitive performance.

They wrote: “[W]e found that youth who used cannabis … had lower IQ at age 18, but there was little evidence that cannabis use was associated with IQ decline from age 12 to 18. Moreover, although cannabis use was associated with lower IQ and poorer executive functions at age 18, these associations were generally not apparent within pairs of twins from the same family, suggesting that family background factors explain why adolescents who use cannabis perform worse on IQ and executive function tests.”

Investigators concluded, “Short-term cannabis use in adolescence does not appear to cause IQ decline or impair executive functions, even when cannabis use reaches the level of dependence.”

Their findings are consistent with those of several other studies – including those here, here, here, and here – finding that cannabis use alone during adolescence does not appear to have a significant, direct adverse effect on intelligence quotient.

widely publicized and still often cited New Zealand study published in 2012 in The Proceedings of the National Academy of Sciences reported that the persistent use of cannabis from adolescence to adulthood was associated with slightly lower IQ by age 38. However, a follow up review of the data published later in the same journal suggested that the observed changes were likely due to socioeconomic differences, not the subjects’ use of cannabis. A later study by the initial paper’s lead investigator further reported that the effects of persistent adolescent cannabis use on academic performance are “non-significant after controlling for persistent alcohol and tobacco use.”

Study: Alcohol Use, But Not Cannabis, Associated With Changes In Brain Structure

marijuana_seedlingAlcohol consumption is associated with negative changes in gray matter volume and in white matter integrity, while cannabis use is not, according to data published online ahead of print in the journal Addiction.

Investigators from the University of Colorado, Boulder and the Oregon Health & Science University evaluated neuroimaging data among adults (ages 18 to 55) and adolescents (ages 14 to 18). Authors identified an association between alcohol use and negative changes in brain structure, but identified no such association with cannabis.

“Alcohol use severity is associated with widespread lower gray matter volume and white matter integrity in adults, and with lower gray matter volume in adolescents,” they concluded. By contrast, “No associations were observed between structural measures and past 30-day cannabis use in adults or adolescents.”

Researchers acknowledged that their findings were similar to those of prior studies “suggesting that regionally specific differences between cannabis users and non-users are often inconsistent across studies and that some of the observed associations may actually be related to comorbid alcohol use.”

A 2015 brain imaging study published in The Journal of Neuroscience similarly reported that cannabis use was not positively associated with adverse changes in the brain, but that alcohol “has been unequivocally associated with deleterious effects on brain morphology and cognition in both adults and adolescents.”

Longitudinal data published in June in the British Medical Journal reported, “Alcohol consumption, even at moderate levels, is associated with adverse brain outcomes including hippocampal atrophy.”

An abstract of the study, “Structural Neuroimaging Correlates of Alcohol and Cannabis Use in Adolescents and Adults,” appears online here.

Study: Medical Cannabis Use Associated With Improved Cognitive Performance, Reduced Use Of Opioids

Marijuana researchMedical cannabis administration is associated with improved cognitive performance and lower levels of prescription drug use, according to longitudinal data published online in the journal Frontiers in Pharmacology.

Investigators from Harvard Medical School, Tufts University, and McLean Hospital evaluated the use of medicinal cannabis on patients’ cognitive performance over a three-month period. Participants in the study were either naïve to cannabis or had abstained from the substance over the previous decade. Baseline evaluations of patients’ cognitive performance were taken prior to their cannabis use and then again following treatment.

Researchers reported “no significant decrements in performance” following medical marijuana use. Rather, they determined, “[P]atients experienced some improvement on measures of executive functioning, including the Stroop Color Word Test and Trail Making Test, mostly reflected as increased speed in completing tasks without a loss of accuracy.”

Participants in the study were less likely to experience feelings of depression during treatment, and many significantly reduced their use of prescription drugs. “[D]ata revealed a notable decrease in weekly use across all medication classes, including reductions in use of opiates (-42.88 percent), antidepressants (-17.64 percent), mood stabilizers (-33.33 percent), and benzodiazepines (-38.89 percent),” authors reported – a finding that is consistent with prior studies.

Patients in the study will continue to be assessed over the course of one-year of treatment to assess whether these preliminary trends persist long-term.

Full text of the study, “Splendor in the grass? A pilot study assessing the impact of marijuana on executive function,” appears online here.

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