Marijuana Brain Damage Less Than Thought - and Reversible

(updated with the actual study abstract, which is more informative than the Wired article)

The immediate (within a few days) detrimental effects of marijuana on working memory are pretty strong. I find working memory pretty important for the thinking and especially the computer programming I do.

But it’s not all bad - mood improves, and so does performance on some (useless-seeming to me) remote association of words and whatever “divergent thinking” is. Also, there’s weak evidence that the harmful effects will disappear completely after a few weeks of abstinence.

Heavy drinking is definitely worse for you than marijuana (aside from some increased risk of mouth/throat/lung cancer if you’re smoking it).

This is not a randomized intervention study; it’s just cognitive measurements vs. a few hundred survey responses on marijuana use. The fact that you’re thus seeing correlations and not necessarily causes is unfortunate. But it is over 8 years, which is nice:

From the abstract of the actual study in Addiction:

California Verbal Learning Test (CVLT) (immediate and delayed), Spot-the-Word test (STW), Symbol Digit Modality test (SDMT) and Digit Backwards (DB). Groups of cannabis users were defined from self-reports across three waves as: ‘never’ (n= 420) ‘remain light’ (n= 71), ‘former light’ (n= 231), ‘remain heavy’ (n= 60), ‘former heavy’ (n= 60) and ‘always former’ (since start of study) (n= 657).
So they measured 2000 people 3 times over 8 years, while asking them how much marijuana they were use. They started with 2400. Too bad we’ll never know if the dropouts were especially heavy pot users :) They performed an analysis that’s intended to show whether voluntary non-blind (obviously) changes in marijuana usage helped or hurt cognitive performance in the same individual - which would be more telling than observing some leftover unexplained correlation between pot and stupidity (after controlling for age/education/socioeconomic/etc.) at a single point in time.

 baseline there were significant differences between cannabis groups on CVLT (immediate and delayed) and SDMT. However, after controlling for education, gender, gender × group and gender × wave, there were no significant between-group differences and only CVLT immediate recall reached adjusted statistically significant longitudinal change associated with changed cannabis use (group × wave P= 0.007). Specifically, former heavy users improved their performance relative to remaining heavy users (estimated marginal means: former heavy 6.1–7.5: remain heavy 6.4–6.6
Definitely switching from “heavy use” to “former heavy use” (maybe not even completely abstaining) gives about an 8% improvement in the ability to remember things recently read (short term verbal memory) (that there is no improvement at all is less than 0.7% probable).

It’s also possible that by “controlling for education” (missing: socioeconomic class, but this is Australia) they’re hiding an effect of marijuana that makes you simultaneously stupid and less likely to finish school. This would be my major complaint with the study (unless they consider only level of education at the study start, and show expected changes per individual over time in response to changed marijuana usage).

That [marijuana makes you stupid], at least, has been the collective stereotype for decades. There’s even been some science to back it up, especially when the marijuana use begins at an early age. But now a different answer is beginning to emerge, thanks to an authoritative new study led by Robert Tait at the Australian National University. The scientists looked at the long-term cognitive effects of marijuana use in nearly 2,000 subjects between the ages of 20 and 24. The subjects were divided (based on self-reports) into several different categories, from total abstainers (n = 420) to “current light users” (n = 71) to “former heavy users” (n = 60). Over the course of eight years, the scientists gave the subjects a battery of standard cognitive tests, most of which focused on working memory, verbal memory and intelligence. One of the important advantages of this study is that the scientists controlled for a number of relevant variables, such as education and gender. In TimeMaia Szalavitz explains why this statistical adjustment is necessary:

The lower education levels of the pot smokers — and their greater likelihood of being male — had made it look like marijuana had significantly affected their intelligence. In fact, men simply tend to do worse than women on tests of verbal intelligence, while women generally underperform on math tests. The relative weighting of the tests made the impact of pot look worse than it was.

Once these population differences were corrected for, the long-term effects of marijuana use disappeared: The scientists found that “there were no significant between group differences.” In other words, the amount of pot consumed had no measurable impact on cognitive performance. The sole exception was performance on a test of short-term verbal memory, in which “current heavy users” performed slightly worse than former users. The researchers conclude that, contrary to earlier findings, the mind altering properties of marijuana are ephemeral and fleeting:

The adverse impacts of cannabis use on cognitive functions either appear to be related to pre-existing factors or are reversible in this community cohort even after potentially extended periods of use. These findings may be useful in motivating individuals to lower cannabis use, even after an extensive history of heavy intake.

This study builds on previous work by Harvard researchers demonstrating that the learning and memory impairments of heavy marijuana users typically vanish within 28 days of “smoking cessation.” (The slight impairments still existed, however, one week after smoking.) While several days might sound like a long hippocampal hangover, heavy alcohol users typically experience deficits that persist for several months, if not years. In other words, heavy marijuana use appears to be a lot less damaging than alcoholism.

Taken together, these studies demonstrate that popular stereotypes of marijuana users are unfair and untrue. While it’s definitely not a good idea to perform a cognitively demanding task (such as driving!) while stoned, smoking a joint probably also won’t lead to any measurable long-term deficits. The Dude, in other words, wasn’t dumb because he inhaled. He was dumb because he was The Dude. (All those White Russians probably didn’t help, either.)

Furthermore, there’s some intriguing evidence that marijuana can actually improve performance on some mental tests. A recent paper by scientists at University College, London looked at a phenomenon called semantic priming. This occurs when the activation of one word allows us to react more quickly to related words. For instance, the word “dog” might lead to decreased reaction times for “cat,” “pet” and “Lassie,” but won’t alter how quickly we react to “chair.”

Interestingly, the scientists found that marijuana seems to induce a state of hyper-priming, in which the reach of semantic priming extends to distantly related concepts. As a result, we hear “dog” and think of nouns that, in more sober circumstances, would seem rather disconnected, such as “leash” or “hair.” This state of hyper-priming helps explain why cannabis has been so often used as a creative fuel, as it seems to make the brain better at detecting those remote associations that lead to radically new ideas.

Why does marijuana increase access to far reaching intellectual connections? One possibility is that the beneficial effect of the drug is mediated by mood. Marijuana, after all, has long been used to quiet anxious nerves — big pharma is currently exploring targeted versions of THC as a next generation anxiolytic — as only a few puffs seem to dramatically increase feelings of relaxation and euphoria. (The technical term for this, of course, is getting stoned.) Furthermore, recent research has suggested that performance on various tests of remote associations and divergent thinking — a hallmark of creativity — are dramatically enhanced by such positive moods.