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This article claims that marijuana (cannabis) is linked to a drop in IQ. Specifically:

Participants who used cannabis heavily in their teens and continued through adulthood showed a significant drop in IQ between the ages of 13 and 38—an average of 8 points for those who met criteria for cannabis dependence....By comparison, those who never used marijuana showed no declines in IQ.

It also claims that

Socio-economic status (SES) could not account for the findings they observed, because adolescent cannabis use was not more prevalent in populations with lower SES

Is it true that marijuana use in teens is associated to an average of 8 points of IQ loss? If so, has this been proven to be causative?

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    There’s a disconnect between title and text: “long-term” ≠ “heavy use”. At least they’re not necessarily the same. – Konrad Rudolph Feb 14 '18 at 14:24
  • Also curious if you're asking about adolescent use or adult use as it's very plausibly different depending on how complete brain development is. – William Grobman Feb 15 '18 at 17:23
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    @KonradRudolph Actually there isn't a disconnect. I quote: Participants who used cannabis heavily in their teens and continued through adulthood. I.e. the article is about people who: 1) started using marijuana in their teens, 2) used it heavily in their teens, 3) continued to use it in their adulthood (possibly less heavily?), so it's about people that are both heavy users and long-term users and moreover started taking marijuana when their brain wasn't fully matured. – Bakuriu Feb 15 '18 at 22:59
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Cannabis is not without dangers. But these are really small in the bigger perspective. And with all drugs, dosis facit venenum. High as a kite all day and every day leads to behaviour that is unlikely to maintain the training of the brain "as a muscle". Especially THC alone in high concentrations can be quite nasty.

First, many studies were specially motivated to reveal 'dangers'. And less motivated to follow proper research methodology and design either experimental studies or properly control for possible confounders in observationals. One of the biggest is like 'having an allergy', in that the overwhelmingly vast majority tolerates cannabis products quite well, while a select few are vulnerable and do not as well.

We critically reviewed the literature in order to propose a model that would be able to summarize the complex relationship between premorbid IQ and cannabis use, in determining different outcomes. Longitudinal studies that assessed IQ measures before and after cannabis consumption had yielded discordant findings. We, however, observed a relationship between higher premorbid IQ and recreational or discontinued use, while a lower premorbid IQ resulted as a predictor for regular or heavy cannabis use. Using these observations as a starting point, we sustain the hypothesis of a shared genetic vulnerability to psychosis and cannabis use, reflected on premorbid IQ, and able to influence the contact with the substance. In turn, cannabis may be a trigger for psychosis toward this continuum of neurodevelopmental vulnerability.

L. Ferraro & L. Sideli & D. La Barbera: "Chapter 24 - Cannabis Users and Premorbid Intellectual Quotient", Handbook of Cannabis and Related Pathologies Biology, Pharmacology, Diagnosis, and Treatment 2017, Pages 223-233. (DOI)

As there are different routs of administrations, very rarely a study controls for these routes and in effect produce the desired results. Results that are unfortunately largely worthless, as they might have measured a correlation for negative outcomes with for example just 'smoking'. Or including users who smoked adulterants, like pot laced with lead.
That limits the reach of conclusions from these studies for the substances at hand, as they can be 'clean', vaped, drunk or eaten as well. Getting a repeated effect from inhaling too much carbon monoxide isn't easy if just eat cookies (Example for a brilliant study that is fond of confounding in 'Nature'.)

These findings suggest that adolescent cannabis use is not associated with IQ or educational performance once adjustment is made for potential confounds, in particular adolescent cigarette use. Modest cannabis use in teenagers may have less cognitive impact than epidemiological surveys of older cohorts have previously suggested.

C Mokrysz & R Landy & SH Gage et al.: "Are IQ and educational outcomes in teenagers related to their cannabis use? A prospective cohort study", Journal of Psychopharmacology, First Published January 6, 2016, (DOI)

Unfortunately, most systematic reviews fail to address this and present the evidence as "under-researched" or with "conflicting results". (Samantha J.Broyda et al.: "Acute and Chronic Effects of Cannabinoids on Human Cognition—A Systematic Review", Biological Psychiatry, Volume 79, Issue 7, 1 April 2016, Pages 557-567, DOI).

The under-researched part is quite true, though, as the moral panics policies of the last century have handicapped science quite severely.

Although mild executive control deficits in adolescent frequent users and a relation between early cannabis initiation and cognitive performance are partially consistent with prior research, cognitive deficits were not found in other hypothesized domains in this community-based sample. Moreover, occasional cannabis users displayed equivalent or even slightly better executive control, social-cognitive, and memory abilities compared with nonusers, suggesting complex relationships between cannabis use and cognition in youth.
JC Scott et al.: "Cognitive functioning of adolescent and young adult cannabis users in the Philadelphia Neurodevelopmental Cohort.", Psychol Addict Behav. 2017 Jun;31(4):423-434. doi: 10.1037/adb0000268.

And truly, the issue is as complex as the thousands of substances that are produced in a puff. But for single substances the picture gets progressively a little clearer again now. For example, THC is a mixed bag of effects but CBD is often there to the rescue. Just one example:

Cannabinoids such as tetrahydrocannabinol stimulate the removal of intraneuronal Aβ, block the inflammatory response, and are protective. Altogether these data show that there is a complex and likely autocatalytic inflammatory response within nerve cells caused by the accumulation of intracellular Aβ, and that this early form of proteotoxicity can be blocked by the activation of cannabinoid receptors.
Antonio Currais et al.: "Amyloid proteotoxicity initiates an inflammatory response blocked by cannabinoids", npj Aging and Mechanisms of Disease volume 2, Article number: 16012 (2016)

Then we have to look at what the baseline is – and look at what really damages a brain: Staci Gruber: "The Grass Might Be Greener: Medical Marijuana Patients Exhibit Altered Brain Activity and Improved Executive Function after 3 Months of Treatment", Front. Pharmacol., 17 January 2018 (DOI)

Following 3 months of treatment, MMJ patients demonstrated improved task performance accompanied by changes in brain activation patterns within the cingulate cortex and frontal regions.

What were these confounders?

The difference may reflect (1) innate differences between groups in cognitive ability, antedating first cannabis use; (2) an actual neurotoxic effect of cannabis on the developing brain; or (3) poorer learning of conventional cognitive skills by young cannabis users who have eschewed academics and diverged from the mainstream culture.
Early-onset cannabis use and cognitive deficits: what is the nature of the association?

Another is "users seeking treatment" as most users seeking rreatment are those ordered to do so by a judge in a trial. There is a cognitive reserve hypothesis, explaining deviant behaviour and vulnerability. The victims of judicial treatment are also well known to be of lower socioeconomic status. And that closes the circle to the original article causing this question's answer in the first place:

Ole Rogeberg: "Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status", PNAS March 12, 2013 110 (11) 4251-4254; (DOI):

Meier et al.’s (1) estimated effect of adolescent-onset cannabis use on IQ is likely biased, and the true effect could be zero. It would be too strong to say that the results have been discredited, but fair to say that the methodology is flawed and the causal inference drawn from the results premature. Furthermore, should a direct effect of adolescent-onset cannabis use remain after controlling for confounders, the Flynn–Dickens model suggests an alternative causal path through which this may occur. This model, too, would predict reduced IQ in so far as heavy, persistent, adolescent-onset cannabis use involves a culture and norms that raise the risk of dropping out of school, getting entangled with crime, and other such behaviors. Unlike a neurotoxic effect, however, this effect would be nonpermanent and mediated by the cognitive demands of different environments. Because the effect in this case would be a result of culture rather than pharmacology, it would also have different policy implications.

There are more studies and reviews like this. They are summarised below, with a slightly older but not outdated conclusion:

In respect to the alleged mental effects of the drugs, the Commission have come to the conclusion that the moderate use of hemp drugs produces no injurious effects on the mind. It may indeed be accepted that in the case of specially marked neurotic diathesis, even the moderate use may produce mental injury. For the slightest mental stimulation or excitement may have that effect in such cases. But putting aside these quite exceptional cases, the moderate use of these drugs produces no mental injury.

The Indian Hemp Drugs Commission Report, completed in 1894, by order of the House of Commons of the United Kingdom (archive)

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    While you quote good studies your answer is not exactly written in an unbiased manner. Stating that something is "clearly not" true before you present any evidence that it's not true. I'd argue that an answer would me more persuasive if not written in such a manner as to make one question the bias, and thus credibility, of the answer. – dsollen Jan 15 at 17:33
  • @dsollen Thx. That's somehow conflicting advice. Most often people complain to not get a tl;dr at the start of an answer – as SE-readers are often not in the mood to read anything longer than two paragraphs. One of the points to make here was that a giant boat-load of biased studies (sometimes intentionally biased) poisoned the well of knowledge, that was better off in the 19th century before moral crusaders took action. – LangLangC Jan 15 at 17:44
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    It's not uncommon to start an answer on these boards with the ultimate conclusion, then offer the explanation and supporting information. I don't see that as a sign of bias, per se. – PoloHoleSet Jan 16 at 18:34
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    I feel there is a different between stating your conclusion is apparently not, and simply saying obviously not. Rather the evidence you provide equates to the fact being 'obvious' depends on the viewer, I would argue it's hard for any evidence to be so definitive as to be 'obvious' when doing these sort of analysis. I also recalled there being some other minor indicators of bias within the answer, though a quick scan doesn't show them now, I don't know if the question was edited or I'm just mis-remembering. I still upvoted, just wanted to point out it's good to keep neutral tone. – dsollen Jan 18 at 15:36
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Since the original article wasn't mentioned in the question, here's a link with abstract:

MEIER MH, et. al. Persistent cannabis users show neuropsychological decline from childhood to midlife. Proc Natl Acad Sci U S A. 2012 Oct 2;109(40):E2657-64. doi: 10.1073/pnas.1206820109. Epub 2012 Aug 27.

The purpose of the study, published in Proceedings of the National Academy of Sciences of the United States of America, was to

test the association between persistent cannabis use and neuropsychological decline and determine whether decline is concentrated among adolescent-onset cannabis users.

The study followed 1,037 individuals from birth (1972/1973) to age 38. Each person was interviewed at age 18, 21, 26, 32, and 38 and took IQ tests at 13 and 38.

The results are in line with what the article you linked mentions:

Persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education. Informants also reported noticing more cognitive problems for persistent cannabis users. Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline. Further, cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users.

Findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain. As DrugAbuse points out, this loss seems to also be consistent with findings in animal studies. The length of the study, publication, number of participants and variables considered show good methodology.

The difference between the two studies mentioned in the question is that the second had a large enough sample to account for socioeconomical differences, and found no direct correlation them and IQ loss associated with cannabis use.

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    In other words. Those who smoke ganja before teen were already "dumber" in the first place. They didn't get dumber by smoking ganja. They were dumber and that's why they smoke ganja. – user4951 Sep 11 '18 at 12:08
  • @user4951 The last sentence contains a serious error. The answer above relies on a study that talks about causality by indicating that the correlational results they got would indicate that. But there is no "why" in either direction, meaning: also not in the way you phrase it. – LangLangC Jan 18 at 20:08
  • There is a why in correlation. Care to explain? If A correlate with B, either A causes B, or B causes A, or C causes A and B. Other possibilities are slim. – user4951 Jan 19 at 6:42
  • @user4951 -- ... or D cause A, and E causes B, and D and E are unrelated – Martin F Jan 23 at 1:14
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To complement Yisela's answer:

There is a more recent study (from 2016) that contradicts the results of the 2012 study by Meier MH et. al:

Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies, January 2016, Proceedings of the National Academy of Sciences of the United States of America

From the abstract:

Some studies suggest that marijuana use in adolescence is linked to declines in intellectual functioning. [...] Among twin pairs discordant for marijuana use, we assessed intelligence quotient (IQ) score changes while adjusting for the effects of genetic influences and other factors shared by members of the same twin pair. Results suggest that familial confounds underlie the association between adolescent marijuana use and declining IQ scores.

So the study confirms that there is a correlation between Marijuana use and lower IQ, however the decline in IQ is not caused by the Marijuana use, but by a confounding factor:

Evidence from these two samples suggests that observed declines in measured IQ may not be a direct result of marijuana exposure but rather attributable to familial factors that underlie both marijuana initiation and low intellectual attainment.

Article about the study: Twins study finds no evidence that marijuana lowers IQ in teens

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