If you consume cannabis recreationally, there’s a good chance you started when you were young — research indicates the median age of first consumption is around 17 to 19, and for those who started before adulthood, it’s even younger — closer to 15.  You may also have heard about recent research claiming that cannabis can negatively impact brain development in teens, and cognitive function later in life.  Like us, you might have thought “I smoked weed when I was a teen and it didn’t hurt me — what’s the issue — isn’t weed a lot less risky than alcohol?” At Home Grow Community, we’ve read a number of reports on this, and have concluded that cannabis, when used in moderation, is reasonably safe for adults, but there is reason for concern about cannabis use for teens, just as there is for teen alcohol consumption (On average, underage drinkers consume more drinks per drinking occasion than adult drinkers. Excessive drinking is responsible for more than 4,300 deaths among underage youth each year, and cost the U.S. $24 billion in economic costs in 2010. In 2013, there were approximately 119,000 emergency rooms visits by persons aged 12 to 21 for injuries and other conditions linked to alcohol.).

Studies have shown heavy cannabis use in youth may cause behavior issues among teens, and some loss of cognitive function in adulthood.  Research on rats shows persistent cognitive difficulties, if exposed to THC in puberty. These studies, though, are relatively small and not necessarily conclusive — there may be other factors at play. More research is needed, especially studies that track cognition and behavior over time.

While the research is not conclusive, as responsible cannabis consumers, we believe we should have honest conversations with teens close to us, explain the dangers of cannabis for them, and encourage them to wait until their brains have fully matured before they experience the health and wellness benefits of weed.

First Experiences With Cannabis

Studies for age of first-time cannabis use vary—one in 1999 claimed the median first time was at age 19; other sources report 17.  A study published by the National Institutes of Health conducted in waves from 1994 to 2008, found that among teen users, the median first time use, by race/ethnicity, varied from approximately 13 to 17:

Native American     13.5

Hispanic                    14.7

Multi-racial              15.1

Black                          15.5

White                         15.8

Asian                          16.8

Initial Research Findings

The American Psychology Association (APA) reported a study in New Zealand that showed a loss of cognitive function: “Duke University psychologist Terrie Moffitt, PhD, and colleagues collected data from the Dunedin Multidisciplinary Health and Development Study, longitudinal research that has followed 1,000 New Zealanders born in 1972. Participants answered questions about marijuana use at 18, 21, 26, 32 and 38. The team found that persistent marijuana use was linked to a decline in IQ, even after the researchers controlled for educational differences. The most persistent users — those who reported using the drug in three or more waves of the study — experienced a drop in neuropsychological functioning equivalent to about six IQ points.”

The APA also reported on a study in the U.S. that might explain the New Zealand findings: “Jodi Gilman, PhD, at Massachusetts General Hospital/Harvard Center for Addiction Medicine, and colleagues used MRI to look for brain changes in 18- to 25-year-olds who smoked marijuana at least once per week, but were not dependent on the drug. Compared with nonusers, the smokers had changes in the shape, volume and gray matter density of two brain regions associated with addiction: the nucleus accumbens (which plays a role in motivation, pleasure and reward processing) and the amygdala (a region involved in memory, emotion and decision-making). Participants who smoked more often had more significant differences.”

Psychology Today reports studies with similar conclusions: “In a study published in December 2013, researchers at Northwestern Medicine discovered that the developing teenage brain may be particularly vulnerable to excessive marijuana use. The researchers found that teens who smoked marijuana daily for about three years had abnormal changes in their brain structures related to working memory and performed poorly on memory tasks. In an alarming twist, the study found abnormalities in brain structure and also identified memory problems two years after the heavy marijuana users had stopped smoking pot as teenagers. The researchers found that memory-related structures in their brains appeared to shrink and collapse inward, reflecting a possible decrease in neuron volume. These findings indicate that there could be long-term detriments of chronic marijuana use as a teenager.”

In an OpEd piece in the New York Times on June 16th, physicians Kenneth L. Davis and Mary Jeanne Kreek claimed that the case for teen brain damage made by these studies, plus the prevalence of much stronger cannabis, argued that the age of legal cannabis consumption should be raised from 21 to 25 in states where recreational cannabis is permitted.

Issues With This Research

The APA reported that other studies have failed to corroborate the Mass General Harvard study: “Barbara Weiland, PhD, at the University of Colorado at Boulder, and colleagues attempted to replicate Gilman’s study in adolescents and adults who smoked marijuana daily. But Weiland’s team argued that previous studies, including Gilman’s, failed to adequately control for alcohol use by the participants. After carefully matching for alcohol intake in the control and experimental subjects, the researchers failed to find physical differences in the nucleus acumbens or the amygdala of daily marijuana smokers.”

They went on to cite other research that ran counter to expectation: “…the evidence against marijuana is frustratingly mixed. While some studies have found increased risk for mood disorders and psychotic symptoms among marijuana users, for instance, a new study by Jordan Bechtold, PhD, at the University of Pittsburgh Medical Center, and colleagues found that chronic use among teenage boys did not raise the risk of later depression, lung cancer, asthma or psychotic symptoms.”

Scientific American also has issues with anti-cannabis research findings: “But the case for permanent damage is not airtight. Studies in rats tend to use much higher doses of THC than even a committed pothead would absorb, and rodent adolescence is just a couple of weeks long—nothing like ours. With brain-imaging studies, the samples are small, and the causality is uncertain. It is particularly hard to untangle factors such as childhood poverty, abuse and neglect, which also make their mark on brain anatomy and which correlate with more substance abuse…”

Counseling Teens

If you are the parents or grandparents of teens, or you work professionally with teens as a teacher or health professional, you may want to speak with them, about the possible impact of use on their brains.

We agree with Scientific American in how not to deal with the issue when speaking with teens: “Exaggerating the perils of cannabis—the risks of brain damage, addiction, psychosis—has not helped. Any whiff of Reefer Madness  hyperbole is perfectly calibrated to trigger an adolescent’s instinctive skepticism for whatever an adult suggests.”

Instead, consider an open and honest discussion about your own use, the research, your concerns about the long term effect of cannabis use on them, and your belief in the principles of responsible cannabis use.


National Institute of Health: “Age of First Cigarette, Alcohol and Marijuana Use by U.S. Biracial/Ethnic Use: A Population-Based Study”

Centers for Disease Control and Prevention: “Fact Sheets—Underage Drinking”

American Psychology Association: “Marijuana and the developing brain”

New York Times OpEd: “Marijuana Damages Young Brains”

NBC News: “Even a little marijuana may change teen brains, study finds”

Psychology Today: “heavy Marijuana Use Alters Teenage Brain Structure”

Scientific American” “What Pot Really Does to the Teen Brain”