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Heavy Pot Use Tied to IQ Drop

Reviewed by Robert Jasmer, MD; Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, BSN, RN, Nurse Planner

 

Action Points

  • Individuals repeatedly diagnosed with cannabis dependence during young adulthood had noticeable declines in IQscores by age 38, especially when the heavy use started in their teens.
  • Note that the study findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects.[/background]

 

Individuals repeatedly diagnosed with cannabis dependence during young adulthood had noticeable declines inIQ scores by age 38, especially when the heavy use started in their teens, researchers said.

A small to medium decline in mean 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 points, whereas 14 participants who also showed heavy cannabis use but only beginning in adulthood showed only a very small 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 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. 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 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

Meier and colleagues reported other changes over time in "standard deviation units," in which changes of 0.20, 0.50, and 0.80 should be considered small, medium, or large, respectively.

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 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 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 (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 decline applied to those who began heavy use in adolescence. Meier and colleagues suggested that, actually, the relationship between dependence and 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 of about 0.13 among those with three diagnoses that all came during adulthood (P=0.02).

Nonsignificant trends toward greater declines in 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 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.

 

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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Unbelievable reefer-madness stuff!

 

i only think it is reefer madness in how some news outlets such as Faux News will use it as a bludgeon. It is not the best of tests. I Would look at it as a small step in a certain direction but as noted it could have been improved. I do have some questions about the number of people who were heavy pot smokers and young. I suspect it would be more relevant if the study went on to find as many people who fit the bill as possible to find out if the results still held true. There must be a magic statistical number that would weed out (I am just so funny!) random circumstance from the results. Also it did reveal how the end groups iq s differed from the norm and also the sex of the participants should be noted. if any of the young pot smokers were above average versus a control group. Certainly the study needs to be tightened up and replicated by an independent group.

Edited by mrd
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http://www.huffingtonpost.com/2011/07/22/medical-marijuana-study-w_n_907521.html

 

take a look at this article it explains who's really using pot according to the huffington post survey---

 

I find the article at the top of this thread to be last minute right wing/republican propaganda before election to put and keep a negative spin on mmj....JMO

 

did no-one see the reward our illustrious AG got for his tough hard core right wing policies....he opened the RNC!

 

and to tell you the truth the 'kidz' enjoy smoking chronic-chronic-chronic herb for recreational....IMO its the perfect escape for them in these tough and uncertain times....same thing we did when we were young...and every generation

 

point is the youth embraces the strongest herb as the safest and best way to inestitize theirselves from everyday life...and I am of the same opinion....icon pared to other things

 

this study seems to convey that long term use lowers IQ.....in New Zealand!

what are they smoking???

 

I have seen studies saying most users are of higher IQ and educated that use cannabis...I will keep searching for that study I have seen it numerous times in years past...

 

 

but I do know of certain strains of weed that WILL make you 'stupid' ....but not sure if it's permanent..

 

 

and I absolutely know alcohol KILLS brain cells and believe with all my heart marijuana stimulates brain activity--JM2c

 

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does anyone have a link to the actual study and not some press release from duke ?

 

never defined what 'cannabis dependence' is. and how one is 'diagnosed' as having a dependence on it.

werent there some studies showing marijuana does not cause any dependence at all?

 

and yet further evidence that people will admit to using marijuana instead of admitting they use harder drugs, or even sniffing paint/glue, etc? especially because marijuana will get you 2 years max, but crack cocaine will get you 10years minimum in some areas...

 

i'd love to see if they had anyone that did admit to sniffing glue/paint and put those stats against the marijuana users, maybe it would show just how much of a problem sniffing glue is haha

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Most relevant conclusions from this study:

 

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.

 

The fact that the study is flawed "did not stop them from speculating..." Typical unscientific gobbledegook from the fear mongers.

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