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Adolescent cannabis use linked to neuropsychological decline later on
Those who persistently used cannabis from adolescence showed an average decline in IQ of eight points, according to new study results. Cessation of cannabis use did not fully restore neuropsychological functioning.
That degree of loss in IQ points represents a disadvantage in education, income, health and longevity, according to study researcher Madeline H. Meier, PhD.
Madeline H. Meier
“Somebody who loses eight IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come,” Meier said in a press release.
Meier, of Duke University, and colleagues examined data on 1,037 participants of the Dunedin Multidisciplinary Health and Development Study, a long-term investigation of the health and behavior of a complete birth cohort from New Zealand.
Participants completed IQ tests at age 13 years, before cannabis use began, and again at age 38 years, when a pattern of persistent cannabis use had emerged. Informant reports completed by friends and family of the study participants were also used to assess neuropsychological function. Cannabis dependence in the past year was ascertained through interviews using DSM-IV criteria.
Even after controlling for education, persistent cannabis use was significantly associated with neuropsychological impairment across a broad range of mental function — especially in the domains of executive functioning and processing speed. Third-party informants reported observing more attention and memory problems among those with more persistent dependence.
Those who started using cannabis persistently before aged 18 years showed a greater IQ decline in adulthood compared with adult-onset users, who did not appear to experience IQ decline as a result of using cannabis. This effect was particularly pronounced among those who were diagnosed with dependence at three or more follow-ups during the study period.
Cessation of cannabis use did not appear to reverse the long-term effects of adolescent-onset use. Among those who started using cannabis persistently from adolescence, IQ decline was apparent, although the drug was used infrequently a year before testing.
According to the researchers, the impairment of neuropsychological function could not be explained by alcohol, tobacco or hard drug use. Schizophrenia was also ruled out.
“Adolescents should not use marijuana,” Meier told Healio.com.
Disclosure: The researchers report no relevant financial disclosures.
Perspective
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Worldwide, marijuana is (after alcohol and tobacco) the most commonly used psychoactive drug. In the US, use among adolescents is again increasing, after a decade-long decline that began in the mid-1990s. Although more than 40% of the US population aged 15 to 64 years has used marijuana at some point in their lives (and as many as 14% in the past year), identifying and quantifying adverse effects specific to marijuana use has proven very difficult: How does one disentangle effects associated with use — but perhaps due to innate and environmental factors that may lead both to marijuana use and a variety of adverse outcomes — from effects specifically caused by use?
In a recent longitudinal study, Meier and colleagues have reported that, as compared to neuropsychological function assessed before onset of marijuana use, long-term, persistent use of marijuana was associated with a decline in neuropsychological function across a range of domains. The study addressed most of the confounds that plague this area of research; and (as one would expect) there appears to be a relationship between degree of decline and amount of use over a 20-year period.
This study adds to the growing literature indicating that early onset (in particular) of marijuana use is associated with a number of risks — of development of later mental disorder, of addiction to other substances, and of long-term cognitive decline. Given the prevalence of its use, the evidence is mounting that even modest elevation in risk of such outcomes is associated with significant public health burden.