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