There is perhaps no fact that exposes the extreme need for research on the effect of cannabis on adolescents then that: “The brain continues to undergo important development up until the age of 25. (Giedd 2004, CSAM 2009). The CSAM (California Society of Addiction Medicine) article goes on to point out that other important factors include: adolescents are at greater risk of dependency that develops far more quickly than for an adult; even before dependency, adolescents are “more significantly” affected by marijuana; structural brain damage has been found in the brains of young marijuana users; and evidence is accumulating that there are subtle effects on emotions and reasoning in all marijuana users. The incidence of psychosis and depression is increased by 30% for users. (Rey)
“The eCB (endocannabinoid) system has a major role in neurodevelopmental and maturational processes including synaptic “pruning and white-matter development, and these processes are especially prevalent during adolescence. . . . Thus the human brain may be more vulnerable to drugs at the time when use of cannabis begins.” (Curran) The eCB engages in pruning but when it is disrupted by marijuana over-stimulation, it affects various organs within the brain. The hippocampus is well-known as a center of learning and memory; for users, the size of the hippocampus increases asymmetrically. (Scalett, Uemura 1987, Medina, Schweinburg 2007) Similarly, the frontal cortex is thinner in adolescent users and this structural change is associated with impulsivity. (CSAM) “The amygdalae,
important for emotionality, have been found to be asymmetrically enlarged in female marijuana users.” (CSAM)
There is accumulating evidence that both brain function and architecture is more disrupted by cannabis when use begins early although there are no studies with a direct comparison with adult users.
I discussed the relationship between psychosis and marijuana use earlier in the Science posts, but for youth who start early, addiction is a greater problem. Curran estimated that users of cannabis are ninefold more likely to become addicted than experiencing a psychotic episode. Admitting that the term addiction is a “terminological quagmire,” Curran defines it as “an acquired, chronic, relapsing disorder that is characterized by a powerful motivation to continue to engage in an activity despite persistent negative consequences.” Dependence, on the other hand, is a lesser disorder but more prevalent.
Epidemiological studies indicate that for users who begin after age 18, there is a 9% chance of becoming dependent on marijuana. But for those who begin prior to 18, the chance of dependence is 27% – triple. CSAM states that the more serious problem of addiction is 4%, within two years, for those who begin after 18 but for those who begin at age 13, 17.4 % are addicted within two years. Even if the definitions are loose, and studies make clear that they are not maintaining that there is a causal link between the early use of marijuana and addiction, the epidemiological evidence demonstrates the danger for adolescents.
Still, some studies appear to strongly suggest some causal relationship. Marijuana in sufficient quantity reduces the number of CB1 receptors in the amygdala. The number of receptors is inversely related to inhibition, that is, the reaction to a novelty occurrence. Moreover, the amygdala is responsive to threat. Marijuana users have less response to threat related to the amount of marijuana use over a period of time.
I side with Kristi Lisdahl, PhD and Director of the Brain Imaging and the Neuropsychology Laboratory at the University of Wisconsin, who, after dutifully noting the lack of causality and confounding factors, stated: “I think legalization is moving ahead prematurely without considering the lessons we’ve learned from nicotine and alcohol prevention policy research.” Susan Weiss, PhD and director of extramural research at the National Institute on Drug Abuse, after noting that starting young and using frequently may disrupt brain development, has stated, “[t]here are a lot of open questions,” about the long term effects of marijuana. Indeed. Staci Gruber, PhD, a neuroscientist and director of the Cognitive and Clinical Neuroimaging Core and MIND at Harvard Medical School concludes: “As we’re on the precipice of all this legislation, the take home message is, there is a lot that we know, but a lot more we don’t.” (Weir)
Weir makes a percipient comment: “Unfortunately, marijuana producers have a strong incentive to hook young users. While about nine percent of adults who use cannabis become addicted, the rate is 17 percent for people who start smoking in their teens, according to NIDA figures. And as the tobacco and alcohol industries have demonstrated, such companies make the majority of their profits on a relatively small proportion of chronic users,” quoting Lisdahl.
In other words, damn the torpedoes – full speed ahead, we’re the progressive generation.