Editors' note, Feb. 1, 2016: On Jan. 20, we reported on a statement from the American Psychological Association that a research paper, "Chronic Adolescent Marijuana Use as a Risk Factor for Physical and Mental Health Problems in Young Adult Men," had a statistical error. The APA now says that statement, which was titled "APA Corrects Article Regarding Teen Marijuana Use," should not have said there was an error in the paper. Jim Sliwa, a spokesman for the APA, told NPR: "There was no error. The original release was labeled a correction but it should have been a clarification."
According to the APA, another researcher had asked the APA for a supplemental analysis of the data, which the study researchers supplied. That supplemental analysis did find a slightly higher probability of meeting criteria for a psychotic diagnosis in frequent marijuana users, but it did not significantly change the conclusions of the study data. Earlier versions of this article, including the headline, referred to errors in the paper's statistical analysis. We have removed those references to make it clear that the paper was not in error.
Updated Jan. 22. 3:49 pm ET: The American Psychological Association emailed us to say that the correction it issued on Thursday was incorrect. The email says in part:
I am afraid that the correction we put out regarding the teen marijuana study was not accurate. We are in the process of placing this more accurate editor's note on the release on our website.
The original correction issued on Thursday by the APA said:
The study, published online Aug. 3 in Psychology of Addictive Behaviors, has been corrected because of a statistical error that affected some of the findings.
An excerpt from the updated correction posted on the APA website now reads:
Although these supplemental analyses indicated that teens who engaged in frequent marijuana use had a higher probability of meeting criteria for a psychotic disorder than infrequent/nonusers by their 30s, this difference did not reach statistical significance using a two-tailed test (p=.09).
We contacted the lead author of the study, Jordan Bechtold at the University of Pittsburgh Medical Center. In response, she writes to Shots in an email:
"There were no errors in the original analysis. The analysis in the supplemental paper was slightly different than the original analysis. The goals in producing the commentary were to supplement the original paper with a new set of analyses, and to provide a word of caution regarding the interpretation of the results, particularly the psychosis findings."
The analysis used in the original study is called a two-tailed test. This looks for an effect in two directions. For example, it could test whether marijuana increases the risk for psychosis and if marijuana reduces the risk. A one-tailed test would look at only one of those possibilities.
Here's the updated original post:
A study published last August found no connection between male teens' marijuana use and increased risk of psychotic disorders provided a new analysis of the data.
When the results were published last August in a journal of the American Psychological Association, even the researchers said their findings were surprising. Over the past couple of decades, there have been at least 10 different epidemiological studies that found smoking marijuana increases risk for developing psychosis later in life.
The APA released a statement saying that the "results from this study generated considerable controversy, including requests for supplemental analyses" because most previous studies have found that chronic teen cannabis users might be more prone to health problems, including mental illnesses and physical illnesses like respiratory problems.
This study, published in Psychology of Addictive Behaviors, originally found no connection between physical and mental problems in adults and smoking marijuana in their teen years.
Once the researchers reanalyzed their data, they found that the three groups of men in the study who smoked weed more frequently as youngsters were more likely to have psychotic disorders, compared with the one group that never smoked or smoked infrequently. The additional analysis found that 5 percent of the more frequent users met lifetime criteria for psychotic disorders compared with 2 percent of infrequent or nonusers.
That difference was close to being labeled "significant" using commonly accepted scientific standards.
Most of the studies on mental illness and weed, including this one, don't perfectly tease apart whether young adult cannabis use causes things like bipolar disorder or schizophrenia or if it's the other way around, says Krista Lisdahl, a clinical neuropsychologist at the University of Wisconsin, Milwaukee who was not involved in the study.
As we've written in Shots before, people with emerging psychotic disorders could be more likely to use weed than others. "Still, controlling for risk factors, marijuana does seem to increase the risk for psychotic disorders," Lisdahl says.
It is curious that the researchers still didn't find a link between depression and weed smoking. This, too, has been connected in previous studies. If people with emerging psychotic disorders are more likely to use weed because they're trying to self-medicate, then you might imagine people with depression might do the same thing.
But Lisdahl says it's really important to remember this study had about 400 men and no women. "It's really important to not generalize this to female," she says. "In my studies, the female marijuana users are the ones that really show increased depression and anxiety symptoms."
So there are a few things that this study didn't examine, as the authors noted both in the original study and the clarification:
"The sample included young men who were using marijuana in the late 1990s and early 2000s and THC content has risen recently; data were obtained from self-reports; [statistical] power was low."
The clarification also says:
"The group difference on psychotic disorder approached statistical significance and would have been significant if a more liberal test (i.e., one-tailed) was utilized."