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 stated, 鈥淭een Marijuana Use Not Linked to Later Depression, Lung Cancer, or Other Health Problems,鈥� while the ever-hopeful was more positive, asserting 鈥淪urprising News About Teens, Marijuana, and Health Issues. It鈥檚 All Good."
While of interest, these results are not particularly surprising, nor particularly convincing, considering the limitations of the study (moreover, they鈥檙e not really 鈥渁ll 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 鈥渆arly, chronic users鈥� 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鈥攁 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鈥檛 buy them, you should ask how many Powerball buyers did I examine? If I said, 鈥渇ewer than one hundred,鈥� 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 鈥渆arly, chronic users鈥� 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鈥檚. 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鈥攖his study focuses on marijuana potency far below that available to teens today.
Third, 鈥渃hronic use鈥� 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鈥檛 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 鈥渆arly, chronic use鈥� in this study is not fully comparable to the risk we see today.
That鈥檚 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 鈥渁ll good鈥� is all hogwash.