Sherika Hill, Ph.D. — an adjunct faculty associate at the Duke University School of Medicine — is the lead author of the study, and the findings were published in the Journal of the American Academy of Child and Adolescent Psychiatry.
The researchers aimed to shift the focus from teenage marijuana use to that of adults, investigating the risk factors behind problematic marijuana use in early adulthood.
Dr. Hill explains the motivation behind the new study, saying, "A lot of current interventions and policies in the [United States] are aimed at early adolescent users. We have to start thinking about how we are going to address problematic use that may arise in a growing population of older users."
"Given that more states may be moving towards legalization of cannabis," continues Dr. Hill, "for medicinal and recreational purposes, this study raises attention about what we anticipate will be the fastest growing demographic of users — adults."
Studying cannabis use patterns
To identify risk factors among this population, Dr. Hill and her team analyzed the data available on 1,229 people taking part in the Great Smoky Mountains Study — a prospective cohort study of western North Carolina residents.
They were followed-up between 1993 and 2015, and they were assessed every year between age 9 and 16, as well as at ages 19, 21, 26, and 30.
As part of the study, the participants' mental health, education, work, and drug and alcohol consumption were monitored.
The researchers split the participants' marijuana use into distinct patterns based on the Diagnostic and Statistical Manual of Mental Disorders 5 (DSM-5) criteria for cannabis use disorder.
Habit drops off with age
The research found that three quarters of the participants (76.3 percent) did not develop a cannabis use problem during their college years or in early adulthood.
However, one quarter of them did — and the scientists divided these users into three categories: "persistent, limited, and delayed problematic cannabis use."
Specifically, 13.3 percent of all participants deemed limited users had problems with cannabis before the age of 16, in their late teenage years, or in their early 20s. But these users broke the habit with age.
Surprisingly, this group reported "more childhood family instability and dysfunction," which are factors that are usually linked with a higher level of drug use.
Dr. Hill says of these participants, "When [these] children left home, they seemed to do better [...] they didn't have as many children at a young age, and they went further in their education when they were 19 to 21 compared to those with persistent and delayed profiles."
Persistent users have a history of anxiety
In the so-called persistent users group, participants had a problematic use of marijuana at as early as 9 years of age. People in this group continued to use marijuana habitually into early adulthood — that is, in their late 20s and early 30s.
Significantly, 27 percent of these participants reported anxiety disorders as children, and 23 percent did when they were in their late teenage years, or between 19 and 21 years old. They also had the highest levels of psychiatric disorders.
"This suggests," says Dr. Hill, "that a focus on mental health and well-being could go a long way to prevent the most problematic use."
Finally, a small group of delayed users had a cannabis-free adolescence and early adulthood, but they started using marijuana between the ages 26 and 30. Most of these users experienced bullying and mistreatment as children.
"What we don't yet understand," Dr. Hill says, "is how childhood maltreatment didn't prompt earlier problematic use of cannabis between ages 19 and 21 — how individuals could be resilient to that kind of adverse experience for so long."
"One theory," she continues, "is that they were somewhat protected by having fewer peers in late adolescence who were substance users, but this is one of the questions we will continue to seek answers for."
Dr. Hill and her colleagues conclude:
"Problematic cannabis use patterns during early adulthood have distinctive risk profiles, which may be useful in tailoring targeted interventions."
This video from the Duke University Medical Center summarizes the findings.