Last week the Colorado Department of Public Health and Environment unveiled a new ad campaign aimed at scaring teenagers away from marijuana by warning them that it might damage their brains. Then again, it might not. Larry Wolk, the health department's executive director, concedes "more research is necessary" because "much still needs to be learned about the effect marijuana has on the brain." As the campaign itself intermittently acknowledges, claims that marijuana causes lasting neurological impairment remain controversial. But why take a chance? That is the state's message in a nutshell. Hence the slogan: "Don't be a lab rat."

As anti-drug propaganda goes, Colorado's campaign, which was prompted by fears that teenagers will be more inclined to smoke pot now that it's legal for adults (fears that so far seem to be unfounded), is relatively subtle. In one TV spot, the camera circles a dark, smoky car full of teenagers passing a pot pipe. The captions read: "Scientists at Duke University discovered that marijuana permanently decreased the IQs of teens. Some dispute that study. But what if, years from now, you learn those scientists were right?"

Colorado's approach is certainly a step up from the 1987 Partnership for a Drug-Free America ad that featured a man dropping an egg into a sizzling frying pan. "This is your brain on drugs," he declared. "Any questions?" It turned out that people had a lot of questions. Among them: Why did anyone think this kind of absurd hyperbole, which has been widely mocked ever since, would dissuade curious adolescents from trying drugs?

By comparison, Colorado's message is restrained: This might be your brain on drugs. The campaign nevertheless exaggerates the strength of the evidence linking adolescent pot smoking to brain damage as well as the level of risk facing the typical adolescent pot smoker.

The TV ad claims scientists have "discovered that marijuana permanently decreased the IQs of teens." That suggests the effect is well-established; you don't "discover" something unless it's true. A viewer unfamiliar with the research is therefore apt to conclude that the "some" who "dispute that study" are a bunch of cranks. The campaign's website likewise acknowledges doubts while implicitly dismissing them:

Is Mother Nature's miracle plant as harmless as most teens think? Maybe not. In fact, many early studies have shown the exact opposite. Scientists from Duke to Cambridge have uncovered a laundry list of troubling side effects. Schizophrenia. Permanent IQ loss. Stunted brain growth. Still, some people question this research. Claiming the studies need to go deeper. Look further. But who will be their guinea pigs? Who's going to risk their brains to find out once and for all what marijuana really does? Don't be a lab rat.

Again, if you "show" or "uncover" something, that means it's real. Yet all of these alleged effects are a matter of scientific dispute, as even Nora Volkow, head of the National Institute on Drug Abuse and no fan of marijuana, concedes. "Although multiple studies have reported detrimental effects," Volkow and three co-authors write in a recent New England Journal of Medicine article, "others have not, and the question of whether marijuana is harmful remains the subject of heated debate." There are three major reasons for that debate: 1) The evidence is mainly correlational, meaning it does not establish cause and effect, 2) observed differences between pot smokers and abstainers do not necessarily have practical significance, and 3) results based on studies of heavy users do not necessarily apply to people who consume cannabis occasionally or moderately.

Consider the 2012 study mentioned in the "Don't Be a Lab Rat" spot, which was reported in the Proceedings of the National Academy of Sciences. Based on information about 1,000 New Zealanders followed from birth to age 38, a team led by Duke University researcher Madeline Meier (who is now an assistant professor of psychology at Arizona State University) found that "persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education."

That sounds bad, but what does it actually mean? The subjects were interviewed about their cannabis use at ages 18, 21, 26, 32, and 38. They took IQ tests at ages 13 and 38. Among the heaviest users—those who were diagnosed as "cannabis dependent" three or more times—IQ scores fell about six points on average. By comparison, the IQ scores of the subjects who never smoked pot rose slightly, by less than a point.

Meier et al. did find some evidence that the IQ declines had functional significance. Informants designated by the subjects as people who knew them well were more likely to report attention and memory problems for subjects identified as dependent on cannabis.

But did marijuana cause the IQ drops? "Our data cannot definitively attest to whether this association is causal," Meier et al. concede. "For example, 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."

In a 2013 critique published by the same journal, Norwegian economist Ole Rogeberg suggests that the "third variable" could be socioeconomic status, which is related both to declines in IQ scores and to early and heavy cannabis consumption. "Meier et al.'s estimated effect of adolescent-onset cannabis use on IQ is likely biased," he writes, "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." Meier and her colleagues say socioeconomic status does not fully account for the association they found, while Rogeberg argues that their analysis does not adequately take that variable into account.

Another possible explanation is that kids who smoke pot all the time have trouble learning the information they will need to do well on IQ tests. While Meier et al. adjusted for years of education, they did not take into account how the quality of education might have been affected by heavy cannabis consumption. "Teens who get high almost every day probably miss some of the key information that appears on IQ tests because they're not absorbing it in school," says Mitch Earleywine, a professor of psychology at the State University of New York in Albany and the author of Understanding Marijuana: A New Look at the Scientific Evidence. "Those who are crafty enough to only use on weekends also seem sharp enough to master the relevant information."

It is also important to keep in mind that Meier et al.'s most dramatic results—the ones that tend to be cited by anti-pot activists, commentators, and members of Congress—relate to the heaviest cannabis consumers. Among subjects who had used marijuana but were never diagnosed as dependent, who represented three-quarters of the cannabis consumers in the study, the average IQ drop was about one point. A 2002 study reported in the Canadian Medical Association Journalfound that IQ scores dropped for heavy users between childhood and late adolescence but rose for light users. In fact, the increase for light users (about six points) was more than twice the increase for abstainers.

Based on data from the Monitoring the Future Study, something like 16 percent of high school seniors who have smoked pot in the previous year did it on a daily or nearly daily basis in the previous month. It looks like the overwhelming majority of teenagers who smoke pot are not doing it nearly as often as the subjects in Meier et al.'s study whose IQ scores dropped significantly. So even if the association is causal, its significance for the typical pot-smoking teenager is highly debatable.

The "Don't Be a Lab Rat" campaign also highlights the risk of schizophrenia. Various studies have found that people who smoke pot are more likely than those who don't to be diagnosed with schizophrenia. But that does not mean pot smoking causes schizophrenia. Psychologically troubled people may have traits (such as impulsivity) that make them more likely to try pot, or they may find that pot helps banish disturbing thoughts and emotions.

"It is inherently difficult to establish causality in these types of studies," Volkow et al. note, "because factors other than marijuana use may be directly associated with the risk of mental illness. In addition, other factors could predispose a person to both marijuana use and mental illness. This makes it difficult to confidently attribute the increased risk of mental illness to marijuana use." Notably, the prevalence of schizophrenia seems to be about the same as it was before the big increase in marijuana use that began in the 1960s.

Ashley Proal and three other researchers at Harvard Medical School took a closer look at the relationship between marijuana and schizophrenia in a recent study involving "87 non-psychotic controls with no drug use," "84 non-psychotic controls with cannabis use," "32 patients with a schizophrenia spectrum psychosis with no drug use," and "76 patients with schizophrenia spectrum psychosis with cannabis use." Here is how they summed up their results in the journalSchizophrenia Research last December: "Having an increased familial risk for schizophrenia is the underlying basis for schizophrenia in these samples and not the cannabis use….We conclude that cannabis does not cause psychosis by itself. In genetically vulnerable individuals, while cannabis may modify the illness onset, severity and outcome, there is no evidence from this study that it can cause the psychosis."

The third fear cited by Colorado's anti-pot campaign, "stunted brain growth," is based on all three of the shortcuts I've mentioned: equating correlation with causation, assuming functional impairment, and generalizing from heavy users to all cannabis consumers.

A 2012 study reported in the journal Brain, for example, found that "cannabis users with a long history of heavy use" had less axonal connectivity in a few brain regions than nonusers did. Although the researchers conclude that "long-term cannabis use is hazardous to the white matter of the brain," the study does not actually show that cannabis use caused the observed differences, that the differences had a significant impact on cognitive ability, or that any of this is something that light to moderate cannabis consumers need to worry about. On average, the subjects in this study consumed marijuana nearly every day, went through the equivalent of 147 joints a month, and had been smoking pot for 16 years.

There are similar problems with a widely publicized study reported in The Journal of Neuroscience last April. Researchers at Northwestern University and Massachusetts General Hospital used MRIs to compare the brains of 20 young adults who reported smoking pot at least once a week and 20 controls who had used marijuana no more than five times in their lives and had not consumed it at all in the previous year. The pot smokers had higher gray-matter densities in the left nucleus accumbens, and there were "significant shape differences" between subjects and controls in that area and in the right amygdala. The differences were more pronounced in subjects who reported smoking marijuana more frequently. "Because this is a cross-sectional study," the authors noted, "causation cannot be determined." In other words, it is not clear whether the brain differences were caused by marijuana. It also is not clear how long the differences last, whether they have any functional significance, and whether they would show up in scans of people who consume less marijuana than the subjects in this study, who on average went through 11 joints a week.

Those nuances generally were lost in press coverage of the study, which presented the MRI scans as evidence that smoking pot causes brain damage. News outlets claimed the study found that "marijuana re-shapes brains of users" (NBC News), that "even casually smoking marijuana can change your brain" (The Washington Post), that "casual pot use impacts brains of young adults" (The Oregonian), that "recreational pot use" is "harmful to young people's brains" (Time), that "casual marijuana use" is "bad for young adults" (The Times of India), and that "even 'casual' marijuana use can knacker bits of your brain" (Gizmodo UK). A Medical News Today headline quoted the researchers as saying "casual marijuana use changes the brain," although that statement does not appear in the article under the headline, in the study itself, or in press releases about the study issued by Northwestern University, Massachusetts General, and the Society for Neuroscience, which publishes The Journal of Neuroscience. Similarly, an MSN NZ headline had the study claiming that "cannabis use 'alters brain regions,'" another phrase that is absent from the study and the press releases.

Although they seem to have been misquoted in some cases, the researchers themselves are partly responsible for the misrepresentation of their findings. As John Gever pointed out in a coolheaded MedPage Today analysis, the study says "the left nucleus accumbens was consistently affected by cannabis use," even though the authors acknowledge elsewhere that their data cannot prove causation. One of the study's authors, Northwestern University psychiatrist Hans Breiter, went even further in the Society for Neuroscience press release, saying, "This study raises a strong challenge to the idea that casual marijuana use isn't associated with bad consequences." Gever responded: "Um, no, it doesn't—not without before-and-after MRI scans showing brain structure changes in users that differ from nonusers and documentation of functional impairments associated with those changes."

Yet Breiter went further still in the Northwestern press release. "People think a little recreational use shouldn't cause a problem, if someone is doing OK with work or school," he said. "Our data directly [say] this is not the case." No, they don't, as the study itself concedes. In the Massachusetts General press release, Breiter claimed the study, which was supported by the National Institute on Drug Abuse, has policy implications: "Our findings—which need to be followed up with longer-terms studies—raise serious concerns about efforts to legalize recreational marijuana use, particularly for young adults."

What is the nature of the "bad consequences" and "problem[s]" that Breiter fears? "These two brain regions have been broadly implicated in processes underlying addiction," he said, "so it's a real problem that people claiming their marijuana use does not negatively impact their lives show significant changes in these structures." Harvard psychologist Jodie Gilman, the study's lead author, offered similar comments in the Northwestern press release. "It may be that we're seeing a type of drug learning in the brain," she said. "We think when people are in the process of becoming addicted, their brains form these new connections." But everything we experience has an impact on the brain, and it is not clear what the practical significance of these particular "new connections" might be, since Gilman et al. emphasize that their subjects "were not dependent" on marijuana.

Presumably Gilman and Breiter are not suggesting that anyone who smokes pot once a week is destined for addiction, which seems inconsistent with research on patterns of cannabis consumption. Data from the National Comorbidity Survey indicate that 9 percent of cannabis consumers experience "dependence" at some point in their lives. According to the National Survey on Drug Use and Health, about 18 million Americans have used marijuana in the last month, while about 3 million qualify as "dependent" at some point during a given year.

While the researchers were not always careful in explaining the significance of their findings, the misunderstandings reflected in the press coverage got a strong boost from the publicity departments at Northwestern and Massachusetts General, which picked headlines for their press releases that encouraged reporters to conflate correlation with causation and differences with damage:

Those summaries are actually more misleading than a lot of the headlines in the general press, which is saying something.

Not surprisingly, the "Don't Be a Lab Rat" website links liberally to overheated, hyperbolic discussions like these, eschewing calmer, more skeptical treatments of the same studies by journalists such as Time science writer Maia Szalavitz. This slant belies the state's "just the facts" pose and gives the campaign a serious credibility problem. The installations featuring human-sized rat cages don't help.

"I don't particularly like the tone of this," says Earleywine, "but the task is almost impossible. With alcohol and cigarettes, they can actually point to conclusive data. For preventing cannabis use, they might as well say, 'Don't smoke pot, because something bad might happen.' I don't want teenagers going to class high because it's a waste of time. I certainly don't want someone who barely knows how to drive trying to do so impaired. But if they hit the pipe twice a month at home in front of the TV and a plate of nachos, I might as well start cautioning them about weight gain. We just don't have the evidence that disasters are imminent."

This article originally appeared at Forbes.com.