As cannabis legalization expands, new research has found that the mental health risks associated with cannabis addiction are highly dependent on the patient’s age. Recent research published in American Journal of Psychiatry They found that young people with cannabis use disorders were more likely to develop mental illness than young people addicted to other drugs, but the risk for adults was relatively low. These results demonstrate the need for an age-specific approach to drug education and public health policy.
Products made from the cannabis plant have a long history in the United States, dating back to at least the 1800s for recreational use. Consumption increased during Prohibition in the 1920s and the counterculture movement of the 1960s, and continued to increase as states legalized the drug. As of early 2025, recreational marijuana is legal in 24 states and the District of Columbia, and studies show daily marijuana use exceeds daily alcohol consumption.
Expanded access means that doctors are increasingly treating cannabis use disorder, a condition in which people are unable to stop consuming cannabis even though it causes health and social problems. Medical literature in the early 20th century first hinted at a link between cannabis and psychosis, a disconnection from reality accompanied by hallucinations. Subsequently, a 1987 study of more than 45,000 Swedish Army recruits demonstrated that heavy cannabis use at the time of conscription was associated with a significantly increased risk of schizophrenia later in life.
Ryan C. Nicholson, a medical resident at Johns Hopkins University School of Medicine, led a team of researchers investigating the specific psychiatric differences between marijuana and other drugs. “A lot of our interest in this comes from the recent legalization of recreational marijuana in Maryland and other states in 2023,” Nicholson said. “We wanted to understand the cannabis-related psychotic disorders that clinicians see in the context of other drug-related psychotic disorders.”
Johannes Sureur, an associate professor at the Johns Hopkins Bloomberg School of Public Health, is a co-author of the study. “Is cannabis use a unique risk factor compared to the use of other substances such as alcohol, opioids, or cocaine? That is the question we addressed in this study, and our findings suggest that the relative risk depends on the age of the user,” Thrall said.
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To answer these questions, the research team analyzed approximately 700,000 medical records from the TriNetX commercial database. They identified patients diagnosed with drug addiction who had no history of mental illness. To ensure a fair comparison, the researchers used a statistical technique called propensity score matching.
This method pairs individuals from different groups who share similar living circumstances and health histories. The team matched patients based on factors such as age, gender, ethnicity and income level. By comparing these matched groups, the researchers were able to isolate the effects of specific drug addictions on later mental health.
The team retrospectively tracked patients’ medical records, looking for new psychiatric diagnoses. The data revealed clear differences based on patient age, with patients divided into a group of 691,806 adults and a group of 49,586 young people aged 17 and under. Among pediatric patients, the median age of those addicted to cannabis was 16 years, compared to 15 years for young people addicted to other drugs.
Of note, approximately 10 percent of patients across all addiction categories were under the age of 12. The researchers noted that this is consistent with other clinical reports showing that about 10.2 percent of adult patients treated for addiction began using drugs before the age of 11. These early exposures provided the basis for exploring long-term health effects.
Health outcomes of pediatric patients showed a clear vulnerability to mood and psychotic disorders. Youth addicted to cannabis had a 52% higher relative risk of being diagnosed with schizophrenia than youth addicted to other drugs. They also faced a 30% higher risk of recurrent major depression and a 21% higher risk of anxiety disorders.
Conversely, adults who were solely dependent on cannabis had a lower risk of developing most mental health conditions compared to adults who were dependent on other drugs. The cannabis group had a 19 percent lower risk of schizophrenia, 0.34 percent compared to 0.42 percent for the other substance groups. The risk of subsequent psychosis, recurrence of major depression, and suicide attempt was also lower in the adult cannabis group.
Researchers also studied adults who were struggling with multiple drug addictions at the same time. They compared patients addicted to cannabis and another drug to patients addicted to multiple non-cannabis drugs. Similar to the single-substance adult group, patients with combined cannabis addiction had lower rates of schizophrenia, depression, and bipolar disorder.
Breaking down the adult data by specific substances revealed more subtle patterns. The cannabis group had a lower risk of depression and psychotic disorders, especially when compared to alcohol-dependent patients. Compared to cocaine-addicted patients, the cannabis group had lower rates of schizophrenia and depression, but higher rates of anxiety.
Comparisons with opioid addiction reveal another specific variation. Adults addicted to cannabis had a slightly higher risk of developing schizophrenia than adults addicted to opioids. However, these same cannabis users had lower rates of depression and anxiety than opioid users.
The study authors proposed several biological explanations for the increased risk seen in teens. The human brain contains a communication network called the endocannabinoid system that helps regulate mood, memory, and brain development. During adolescence, receptors in this system reach their highest concentration in areas of the brain responsible for complex thinking. Introducing cannabis during this critical developmental period may change the way the prefrontal cortex matures.
The results suggest that heavy cannabis use may make young people more susceptible to developing schizophrenia and other mental illnesses. Thrall points out that this accelerating effect may make people less likely to develop these diseases later in life. This phenomenon makes it appear to pose a lower risk for adults than other recreational drugs.
Or, Thrall warns, cause and effect may be pointing in the opposite direction. People who are congenitally more likely to develop certain mental illnesses may be more likely to self-medicate with cannabis. They may seek out drugs for mental health problems before they become clinically apparent.
As with all observational studies, this study has certain limitations. Because this database relies on diagnostic coding by different physicians, researchers do not know the exact patient history that led to each diagnosis. Electronic health records also lack details about the exact type and potency of cannabis consumed.
Future research will need to track exactly how much cannabis people are consuming and measure the concentrations of chemicals in those products. Scientists hope to use brain imaging techniques to observe how long-term cannabis exposure changes the physical structure of the developing brain. Pathological studies can also help precisely map the cellular changes that occur.
In the meantime, public health officials can use this data to adjust how they talk to youth about drug use. “While there are still many unknowns on this issue, I would never encourage teenagers to use cannabis, especially the stronger cannabis products currently on the market,” Thrall said.
The study, “The Association of Cannabis Use Disorder with Other Substance Use Disorders and Psychiatric Illness: A Trend-Matched Retrospective Cohort Analysis,” was authored by Ryan C. Nicholson, Una E. Choi, Ramin Mojtabai, and Johannes Thrul.
