Heavy Pot Use Tied to IQ Drop
A small to medium decline in mean IQ between tests taken on the eve of adolescence and again at age 38 was seen in those diagnosed at least three times with cannabis dependence, according to Madeline Meier, PhD, of Duke University, and colleagues, who reported on data from 1,037 participants in a New Zealand birth cohort.
In the small group of participants who became cannabis dependent before age 18 -- a total of 23 cohort members -- the decline translated to an average of about 8 IQ points, whereas 14 participants who also showed heavy cannabis use but only beginning in adulthood showed only a very small drop in full-scale scores (P=0.02), Meier and colleagues indicated online in Proceedings of the National Academy of Sciences.
The researchers also reported that the size of the mean decline increased with the number of cannabis dependence diagnoses that participants had received in five evaluations conducted from ages 18 to 38 (P<0.0001 for trend).
"Collectively, [the study's] findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects," Meier and colleagues wrote.
Study participants were members of the Dunedin Multidisciplinary Health and Development Study. It attempted to track all children born in Dunedin, New Zealand, from April 1972 to March 1973 starting at age 3. At age 38, the investigators had data on IQ testing and cannabis dependence diagnoses on 874 cohort members.
Besides assessing cannabis use, the five structured interviews in adulthood also elicited information on use of other illicit drugs and alcohol. IQ tests were conducted initially at ages 7 to 13 and again at age 38.
Among participants included in the current analysis, 242 never reported cannabis use in the structured interviews nor were they ever diagnosed with dependence; 479 indicated some use but never received a diagnosis; and 80, 35, and 38 had received one, two, or three or more dependence diagnoses in the adult evaluations.
Baseline mean IQ scores were similar and close to the standardized population average of 100 in these cannabis-use subgroups. However, at age 38, the mean scores had diverged considerably. The change from baseline was as follows (P values not reported):
- 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
For participants with three or more diagnoses, the change was -0.38 units.
The researchers also stratified participants into two groups according to whether, at a given interview, they reported using cannabis at least once a week on average ("regular user").
Individuals classed as regular users at least three times in the study also showed the largest declines in IQ between tests (mean -5.23 IQ points, -0.35 standard deviation units, P value not reported), whereas little change was seen in those never reporting regular use.
Similar patterns were seen in IQ subdomain scores.
These small to medium declines were also seen in this participant group in tests of memory, processing speed, and executive function.
Meier and colleagues sought to rule out potential confounding factors, taking educational attainment and use of alcohol and other drugs into account.
When they restricted their analysis to 278 participants who did not get beyond high school, again those with three or more dependence diagnoses showed medium-level declines in IQ (mean -0.48 standard deviation units) whereas those in the never-used, never-diagnosed category had essentially no change (mean -0.03 units, P=0.0009 for trend).
The pattern also held up when the researchers excluded participants with persistent dependence on tobacco, alcohol, and "hard" drugs, and also those with diagnoses of schizophrenia.
However, the strongest relationships between persistent dependence and IQ decline applied to those who began heavy use in adolescence. Meier and colleagues suggested that, actually, the relationship between dependence and IQ decline may be entirely driven by this group.
The 23 participants with adolescent-onset dependence and at least three diagnoses overall showed a mean decline of about 0.53 standard deviation units, compared with a drop of about 0.13 among those with three diagnoses that all came during adulthood (P=0.02).
Nonsignificant trends toward greater declines in IQ were also seen in those teen-onset dependence but fewer total diagnoses of dependence.
"In fact, adult-onset cannabis users did not appear to experience IQ decline as a function of persistent cannabis use," Meier and colleagues wrote.
They cautioned that their data did not prove that heavy cannabis use actually caused the decline. "There may be some unknown 'third' variable that could account for the findings. The data also cannot reveal the mechanism underlying the association between persistent cannabis dependence and neuropsychological decline," they wrote.
They also acknowledged that their data relied on self-report of cannabis use, without confirmation from blood or urine testing.
But these caveats did not stop them from speculating on causal mechanisms and the implications of a causal relationship.
Meier and colleagues noted that animal studies as well as theories of neural development during adolescence support a neurotoxic effect of cannabis in the young adult brain.
Moreover, they argued, their findings should inform public health programs.
"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," they wrote.
Duke University, in a press release, was even less circumspect in a press release headlined, "Adolescent pot use leaves lasting mental deficits."
Its first sentence claimed that "the persistent, dependent use of marijuana before age 18 has been shown to cause lasting harm to a person's intelligence, attention and memory" in the study.
The Dunedin cohort study was funded by the New Zealand Health Research Council. The current analysis was supported by the U.K. Medical Research Council, the U.S. National Institutes of Health, and the Jacobs Foundation.
No potential conflicts of interest were reported.
No potential conflicts of interest were reported.
Primary source: Proceedings of the National Academy of Sciences
Source reference:
Meier M, et al "Persistent cannabis users show neuropsychological decline from childhood to midlife" Proc Natl Acad Sci USA 2012; DOI: 10.1073/pnas.1206820109.
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