HealthDay News — Persistent marijuana use is associated with neuropsychological decline, with those who start regularly using marijuana as teenagers experiencing greater declines in IQ scores than those who start later, study findings indicate.

Individuals who were diagnosed at least three times with cannabis dependence experienced a small to medium decline in mean IQ score at age 38 years compared with scores recorded on the eve of adolescence, data from 1,037 participants in a New Zealand birth cohort reveal.

Among the 23 participants who became cannabis dependent prior to 18 years of age, the decline in mean IQ score was greater than the drop observed among 14 study participants who reported heavy cannabis use in early adulthood (0.53 standard deviation units vs. 0.13; P=0.02), Madeline H. Meier, PhD, from Duke University in Durham, N.C., and colleagues reported in the Proceedings of the National Academy of Sciences.

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Furthermore, average declines in IQ scores increased along with the number of cannabis dependence diagnoses participants received during the course of five evaluations administered from age 18 to 38 years (P<0.0001 for trend).

“Findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain and highlight the importance of prevention and policy efforts targeting adolescents,” Meier and colleagues wrote.

The researchers tracked participants in the Dunedin Multidisciplinary Health and Development Study, which included all children born in Dunedin, New Zealand, from April 1972 to March 1973. IQ testing was conducted at ages 7 to 13 years and again at age 38 years, and five structured interviews were conducted during the study period to assess the use of cannabis, alcohol and other illicit drugs.

Data on IQ and cannabis use was available for a total of 874 cohort members at age 38. Overall, 242 participants never reported cannabis use nor were diagnosed with dependence; 479 reported some cannabis use, but were never diagnosed with dependence; and 80, 35, and 38 had received one, two, or three or more dependence diagnoses.

Neuropsychological testing across five domains was conducted before starting cannabis (at 13 years old) and again after a consistent pattern of cannabis use had developed (at 38 years old).

At age 38, changes in baseline IQ scores among the groups were as follows:

  • Never used, never diagnosed: +0.80
  • Used, never diagnosed: -1.07
  • One diagnosis: -1.62
  • Two diagnoses: -2.47
  • Three or more diagnoses: -5.75

The researchers found that, even after controlling for education and use of other drugs, including alcohol, persistent cannabis users had broad neuropsychological declines across all domains of functioning. Informants who knew persistent cannabis users reported greater cognitive problems in attention and memory. Those who started using cannabis as adolescents showed greater neuropsychological impairment, with greater declines for persistent users. In addition, neuropsychological function was not fully restored even among those who quit or reduced their cannabis use.

“Prevention and policy efforts should focus on delivering to the public the message that cannabis use during adolescence can have harmful effects on neuropsychological functioning, delaying the onset of cannabis use at least until adulthood, and encouraging cessation of cannabis use particularly for those who began using cannabis in adolescence,” the researchers wrote.

Study limitations include the self-reported nature of cannabis use without confirmation from blood or urine tests. More studies are needed to determine the mechanism underlying persistent cannabis use and neuropsychological decline.


  1. Meier M et al. Proc Natl Acad Sci USA 2012; doi: 10.1073/pnas.1206820109.