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.