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Should We Celebrate a Study Not Finding Adverse Health Impact for Teen Marijuana Users?

David W. Murray

A study just released by the American Psychological Association (APA) failed to find what many other studies have found: an association between marijuana use by teens and later psychological or physiological problems.

The headline from the APA press release stated, “Teen Marijuana Use Not Linked to Later Depression, Lung Cancer, or Other Health Problems,” while the ever-hopeful Washington Post was more positive, asserting “Surprising News About Teens, Marijuana, and Health Issues. It’s All Good.”

While of interest, these results are not particularly surprising, nor particularly convincing, considering the limitations of the study (moreover, they’re not really “all good”).

First, there is the problem of the sample size. It is small. Though youth were followed longitudinally from age 14 to age 36, which is impressive, in the sub-sample of youth most at risk (characterized as “early, chronic users” of marijuana), there were only 86 individuals.

This subset was then compared to three other subsets, including non-users of marijuana. The results show that the groups, when measured at the last interview, did not differ significantly.

The kinds of health events measured (such as lung cancer or psychotic episodes) are simply not common, in any population. Hence, the sample size truly makes a difference for the capacity of the study to detect impact—a non-robust study may find no impact, but that result does not mean that there is no impact when seen in a larger population.

If I told you that people who bought Powerball tickets were not more likely, in my study, to win the lottery than people who didn’t buy them, you should ask how many Powerball buyers did I examine? If I said, “fewer than one hundred,” you would be correct to wonder whether my sample was sufficiently large to cover the true probability of winning the lottery, the odds of which can be measured in the tens of thousands.

Though adverse health outcomes are not a lottery in this sense, lung cancer and even psychosis are still relatively rare in any population, whether they are marijuana users or not.

A second limitation, however, lies in how the study characterized the “early, chronic users” of marijuana. Other research has shown that early exposure to marijuana in adolescence, when the brain is developmentally vulnerable, is associated with negative outcomes, primarily psychological.

Hence, it was disappointing to learn that in this study, youth entered the study at age 14, during the time period of the late 1980’s. That means the marijuana to which they were exposed was not likely to exceed 5 percent or so THC potency, and perhaps less. THC potency has been found to be associated with adverse effects—this study focuses on marijuana potency far below that available to teens today.

Third, “chronic use” turns out to mean, in this study, those smoking an average of about 20 days a year (less than once every two weeks) for their earliest measured teen years. Their exposure doesn’t escalate to even 50 days a year until about age 18, and then continues to escalate into young adulthood. Exposure finally reached an average of about 200 days a year during their early twenties.

The problem is that the definition of “early, chronic use” in this study is not fully comparable to the risk we see today.

That’s why there is still concern for teens and marijuana use. Today, Colorado youth younger than 15 are exposed to marijuana with upwards of 15 percent THC (or even substantially higher), and some smoke daily or near-daily, even in early adolescence. That is the population most at risk for adverse outcomes, including later in life.

It will take several years before a comparably longitudinal study of this youth population, measured in their later years, can be accomplished. There are very strong grounds for concern. To call the situation “all good” is all hogwash.

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