Marijuana is currently regulated by the United States government as a Schedule I drug, placing it in the same category as heroin, MDMA and LSD. This is largely due to the first condition of Schedule I drugs, which is that the substance "has a high potential for abuse." The language in that clause is deliberately vague. Does abuse equal addiction? Probably not, since marijuana is not addictive like other Schedule I drugs. Rats don't self-administer the compound in a lab, it's virtually impossible to fatally overdose on the drug, and the physiological effects of marijuana withdrawal, if they occur, are far milder than those experienced by chronic amphetamine, alcohol, nicotine or opiate users. Put another way, if "abuse" means "addiction" then cigarettes should be Schedule I, not marijuana.

Rather, the case for marijuana "abuse" has always stemmed from its cognitive effects. While cigarettes are like caffeinated smoke – they increase attention and productivity, marijuana is the drug of choice for slackers, hippies and Seth Rogen characters. In popular culture, all it takes is one hit from a bong before people become ridiculously dumb, unable to solve the simplest problems or utter a coherent sentence. Potheads eat a lot and laugh at stupid jokes. The larger worry, of course, is that such damage is enduring and that "smoking dope" permanently impedes learning and memory.

That, 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 Time, Maia 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. Look, for instance, at a 2003 study by German researchers that investigated performance on a classic remote associate test (RAT), in which subjects have to find a fourth word that is associated with the three following words:

cottage Swiss cake

This answer is pretty obvious: cheese. But what about this problem?

dream ball book

That was a trick question: There is no shared association. Here's the remarkable thing about these remote associate problems: People can recognize the possibility of a solution before they've solved the problem. The German scientists demonstrated this by asking people to quickly press the spacebar whenever they were presented with a triad that had an answer. If people had no intuitions about creative associations, their guesses should have been roughly random. But that's not what the scientists found. Instead, subjects were able to efficiently sort "coherent" word problems – those with an actual answer – from incoherent problems, which are a waste of time. Before we find the solution, we can feel its presence.

And this returns us to marijuana: Putting people in a positive mood roughly doubled their accuracy at the task. All of a sudden, they were twice as good at identifying problems with possible solutions. This suggests that anything that makes us happier, reducing vigilance and anxiety, might also make us more creative. We can detect more remote associations, of course, but we also know which associations are worth pursuing, which is probably even more important. It doesn't matter if it's pot, chocolate or a stand-up comic – those substances or experiences that put a smile on our face can also increase the powers of the imagination, at least when solving particular creative problems.

So here's the very un-D.A.R.E. takeaway: Heavy marijuana use doesn't seem to cause any sort of lasting brain damage. All the negative side-effects are relatively temporary. (But those side-effects are real.) Furthermore, the sort of anxiolytic giddiness triggered by THC comes with its own unexpected benefits, which is probably why humans have been self-medicating with cannabis for thousands of years.

Photo: by zztopblue via Flickr

Note: This post originally stated that marijuana was a Schedule II drug. That was a mistake - my apologies. Although the American Medical Association has called for a review of marijuana's classification, the drug is still considered to be Schedule I.