For more than a decade, scientists have been concerned about the potential impact of marijuana on the developing, adolescent brain. Research in this area is vitally important because, according to the National Survey on Drug Use and Health, about 60 percent of all new marijuana users every year are under the age of 18.
Research in animals shows that exposure to tetrahydrocannabinol, or THC, the active ingredient in marijuana, causes permanent deficits in attention, memory, motivation and social anxiety. It’s unethical to repeat these studies in humans. However, a recent study published in the prestigious medical journal Proceedings of the National Academy of Sciences sheds light on the subject.
In short, this study shows that adolescents exposed to marijuana have a decrease in overall intelligence, perceptual reasoning, processing speed and verbal comprehension. It also shows that the more adolescents were exposed to the drug, the more deficits they experienced.
Here’s how the researchers made these findings:
The data-gathering took place in New Zealand, where researchers recruited babies from 1,037 consecutive births from 1972 to 1973. (The reason consecutive selection is important is because it reduces sample bias.) Researchers followed these children until age 38 and evaluated them at ages 7, 9, 11, 13, 18, 21, 26, 32 and 38. A full 97 percent of the original sample that was still living was studied at age 38. Immediately, we notice some real strengths of this study. These include a large sample size, recruitment strategy of consecutive births, low dropout rate among participants and the study’s length of follow-up. All these variables reduce the possible confounds of the study.
Researchers found that youth who had a diagnosis of cannabis dependence at ages 18, 21, 26, 32 and 38 years had a significant drop in IQ compared to those without cannabis dependence. For those who met criteria for cannabis dependence at ages at three or more time points, the average drop in IQ was 8 points. The decline in IQ was dose-dependent, meaning that the more time points someone had cannabis dependence, the greater their drop in IQ. The decline in IQ included all four indices of IQ: working memory, processing speed, perceptual reasoning and verbal comprehension.
The study found that only those with adolescent, not adult, onset marijuana use predicted a decline in IQ. The study also showed that the decline in IQ was not associated with possible confounding variables, such as other educational achievement, other substance use or psychosis.
In summary, this study has tremendous strengths. It is a large sample of consecutive births that had a 38-year follow-up with a very low dropout rate. The study used validated outcome measures and found a significant drop in IQ among adolescent-onset marijuana users.
The results of this study strongly support efforts to reduce adolescent marijuana use.