Summary of the article by Claudia Wallis, “Marijuana and the Teen Brain: How much should we worry?” (Scientific American, Nov 2017, in The Science of Health).
Adults have been warning their children about the dangers of marijuana not since the 1960s — which would have been my guess — but for well over a century. Some of those parental warnings may have been exaggerated (which does not help), yet there is still good evidence the marijuana use impairs learning, attention, and memory. Worse, use of a stronger product can make you physically ill, or even delusional. But what about brain damage?
When it comes to adults, nonusers beat chronic smokers in tests on memory, motor skills, and verbal function. Scientists are less certain about long-term effects (as when a user has stopped for a period of time). “Various lines of research suggest that cannabis use could disrupt [executive functions like emotional self-control].” Brain-imaging studies on habitual users reveal inefficient cognitive processing, altered connectivity between the hemispheres, a smaller amygdala (emotional control—“fright or flight”), and smaller hippocampi (related mainly to memory). In animal studies, exposure to marijuana during puberty causes greater cognitive damage than during adulthood. Yet all these results may not be as conclusive, or applicable to human adolescents, as we might think.
What is needed is a study tracking kids from childhood to early adulthood. Just such a study will be completed by the National Institutes of Health. The 10-year NIH study, “Adolescent Brain Cognitive Development Study,” will follow 10,000 children, from about age 10 to 20.
Even if it turns out that marijuana poses no direct threat to teens, that hardly means it is benign. Showing up high in class causes students to miss the intellectual and social stimulation that should be converging to facilitate healthy wiring of the teenage brain. The teen years are a time when we learn to navigate complex situations. To interfere with the vital neural wiring process doesn't only dumb us down. Heavy adolescent users achieve less in life. And they are less happy. As the author notes, these negative outcomes will be motivating to most teens.