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Dr. Wenk discusses the effects cannabis use has on the youthful, developing brain and why something that is not harmful to adults can harm teens. This is a good companion piece to the study recently released that showed that regular teen use of marijuana can lower IQ points later in life. In my chapter "Who Should Not Use Marijuana" I suggest that teens not use cannabis. People should wait until they are over 18 y.o. to seek the protective benefits that marijuana offers.  

How chemicals control your thoughts and feelings.

by Gary Wenk, Ph.D. Does it matter when you start smoking marijuana?

Does it matters whether you're a boy or a girl?

Multiple studies during the past few years have shown that adolescence is a period of particular vulnerability for the brain to the effects of many different chemicals and nutrients.  The question that parents often ask is "when is the most vulnerable period?" A recently published study measured the effects of marijuana use upon the cognitive performance of a very large group children who started smoking marijuana daily during their early adolescence, i.e. prior to age 15, as compared to those adolescents who started smoking marijuana after age 15.

Sadly, the researchers were able to recruit an impressive number of young children (104!) who fell into one of these categories. About half of these children had started using marijuana prior to age 15.  They were all tested using a sophisticated set of behavioral tasks that can determine executive functioning abilities.  Executive functioning includes those abilities that we tend to associate most often with complex thought processes. We use executive functioning to perform activities such as organizing our behaviors, planning our next action and controlling our current action (some might call this impulse control), forming strategies, paying attention to important cues in the environment, and remembering details related to what's going in our current environment. The performance of these subjects was compared with the performance of children who had never used marijuana.

The scientists carefully designed their study.  For example, subjects who were diagnosed with a significant mental disorder, were currently using psychoactive medications or had a history of head trauma with loss of consciousness, or were somehow intellectually disabled, were excluded from the study.  The patients in the three groups were carefully matched for the age they started daily marijuana use, their pattern of use, the number of years of daily use and their estimation of lifetime consumption. Finally, they were also matched for their age, years of education and IQ at the time of the testing.

The results were quite interesting.  Those children who started using marijuana prior to age 15 performed more poorly on all aspects of executive function tasks as compared to people who starting using marijuana after age 15 or those who had never used marijuana. Thus, chronic marijuana use prior to age 15 will have far more deleterious effects on complex brain functioning than if one begins using marijuana after age 15. In addition, both males and females who started regular marijuana use prior to age 15 were physically smaller in height and weight; this effect was most dramatic in the males. The effects of marijuana use prior to age 15 were still evident in people who were age 55 at the time of behavioral testing!  Why?

Prior to age 15 the brain is still developing in many substantial ways.  Indeed, that's why these scientists chose this cut-off point for their investigation. Humans have complicated brains that continue to develop and make new connections long after birth. Humans finish developing the most recently evolved brain regions last: that would include the most complex regions of our cortex, particularly the frontal lobes where executive functioning and impulse control is managed.  Overall, these few brain regions are far more vulnerable to any toxin, drug or chemical in our diet. Some regions of the frontal cortex do not finish wiring themselves for service until the early twenties for females and late twenties for males. If you ever wondered why females are charged much lower car insurance rates than males at comparable ages, this is it: the frontal lobes of females are simply "on line" many years before they are in males.

Anything that impairs the normal development of these vulnerable brain regions will impair impulse control, verbal fluency, and complex problem solving.  Furthermore, any impairment in the maturation of these abilities might continue to play a central role in the development of addictive behaviors later in life.  Taken together, these findings suggest that exposure to marijuana during critical periods of brain development will have a long term effect upon higher cognitive functions.

What was also quite surprising was that the scientists did not find differences in executive functioning abilities of people who started smoking marijuana after age 15 as compared to people who had never smoke marijuana!  Therefore, starting marijuana use after age 15 is apparently not as offensive to normal cognitive development.

© Gary L. Wenk, Ph.D. Author of Your Brain on Food (Oxford University Press, 2010)