Marijuana is the most widely used illicit drug in the United States—however, it is becoming less and less illicit. Since 1996, twenty-three states have legalized marijuana for medicinal uses. Recently, states such as Colorado, Washington, Oregon, Alaska, and Washington D.C. have permitted recreational use of the drug for those over 21 years-of-age. However, this new legislation is not without controversy and a main concern seems to be that, despite age limits, legalization of marijuana might make it more accessible to the youth.
Over the last half-century there has been a profusion of research on the effects of marijuana. The majority of that research has yielded mixed evidence on the long-term effects of marijuana use. However, “there’s growing literature and it’s all pointing in the same direction: Starting young and using frequently may disrupt brain development” says Susan Weiss, PhD, director of research at the National Institute on Drug Abuse (NIDA).
Although few longitudinal studies have been conducted on the effects of adolescent marijuana use, a long-term prospective study out of New Zealand revealed some troubling findings. One thousand New Zealanders born in 1972 were given questions regarding marijuana use at the ages of 18, 21, 26, 32, and 38. They also underwent neuropsychological testing at ages 13 and 38. The researchers found that regular marijuana use was linked to a decline in IQ, with the most persistent users experiencing a drop in neuropsychological functioning equivalent to six IQ points. This is a significant drop because, according to Weiss, “that’s in the same realm as what you’d see with lead exposure”.
Researchers believe adolescents to be uniquely vulnerable to lasting damage from marijuana use primarily because their brains have not fully developed. According to Staci Gruber, PhD, a neuroscientist at Harvard Medical School and director of the Marijuana Investigations for Neuroscientific Discovery (MIND), the brain does not fully develop until the mid-20s and during this period of neurodevelopment the brain in especially sensitive to drug exposure.
A number of others studies have revealed corroborating evidence regarding adolescent brain changes due to marijuana use. When studying heavy, regular marijuana users who began using before and after age 16, Gruber revealed damage to their brain’s white matter—the part of the brain that facilitates communication between neurons. Gruber also revealed that earl-onset marijuana users made twice as many mistakes on tests of executive functioning, which included abstract thinking, planning, flexibility, and inhibition of inappropriate responses.
Although Gruber’s studies were conducted on heavy users, Jodi Gilman, PhD, found structural differences in the brains of adolescents who were less frequent users. Compared to non-users, Gilman found changes in the shape, volume, and gray matter density in the nucleus accumbens and amygdala—two brain regions associated with addiction.
But the verdict cannot be passed just yet. Other studies have revealed little evidence in terms of marijuana use resulting in brain abnormalities. Further, Barbara Weiland, PhD, replicated Gilman’s study and, after accounting for alcohol use, failed to find physical differences in the nucleus accumbens and amygdala.
Since the evidence against marijuana is at a frustrating back-and-forth, NIDA plans to launch the Adolescent Brain and Cognitive Development (ABCD) study later this year in hopes of revealing a clearer picture of marijuana’s potential risks to the youth. Furthermore, important questions are still left unanswered, such as whether there is a safe level of marijuana use, and whether the brain changes associated with marijuana are permanent. Ultimately, the common belief in the scientific and research community is that legislation is outpacing science. “As we’re on the precipice of all this legislation, the take-home message is, there’s a lot that we know, but a lot more we don’t”, says Gruber.