Individuals who drink excessively or abuse marijuana and other substances may be “messing with their heads” in ways they don’t at all intend. The question of whether drug abuse increases the risk of developing schizophrenia and other mental illnesses has been a hotly debated topic for decades now. But a new, large-scale research study by Denmark scientists examining data on more than 3 million individuals sheds considerable new light on the subject. The research, presented this past month at the International Early Psychosis Association (IEPA) meeting in Italy, suggests that there is a pronounced link between substance abuse and psychotic disorders, specifically schizophrenia, and that the symptoms of that mental instability can be observed up to 15 years after people started abusing their substance of choice.
For years now, experts have been sounding the alarm about a possible link between substance use – especially marijuana – and psychosis. One of the best-known studies followed nearly 50,000 young Swedish soldiers for 15 years. Those who had smoked marijuana at least once were more than twice as likely to develop schizophrenia as those who had never smoked pot. The heaviest users (who said they used marijuana more than 50 times) were six times as likely to develop schizophrenia as the nonsmokers. Another recent study that followed nearly 2,000 teenagers as they became young adults observed that young people who smoked marijuana at least five times were twice as likely to have developed psychosis over the next 10 years as those who didn’t smoke pot.
However, the connection between mental health and substance abuse has historically been a difficult problem to study – or at least untangle – and previous research has been controversial, sometimes contradictory. Many studies that have targeted the relationship between drug use and schizophrenia specifically have been either limited in the samples they examined, or could not take into account things like co-abuse – people who abuse multiple substances at once.
This latest study is far more conclusive in its results because it controls for many of these potentially confounding factors. The Danish research was led by a team at Copenhagen University Hospital and was based on a database of more than 3.1 million Danes – 204,505 of whom had a history of substance abuse and 21,305 of whom had schizophrenia. Data was analyzed using a range of statistical measures, and the methodology controlled for a number of factors including gender, urbanity, other psychiatric diagnoses, co-abuse of substances, parents’ immigration to Denmark, parents’ economic status, and psychiatric history.
On the whole, the researchers found that any kind of substance abuse increased the risk of developing schizophrenia by six times. When connections between specific drugs and schizophrenic conditions were examined, it was discovered that cannabis use increased risks for schizophrenia by 5.2 times, followed closely by alcohol at 3.4 times, hallucinogenic drugs at 1.9 times, sedatives at 1.7 times, and amphetamines at 1.24 times.
“We present a large scaled population-based cohort study analyzing a wide variety of substances. Our results illustrate a robust association between almost any type of substance abuse and an increased risk of developing schizophrenia later in life,” the study’s lead authors said.
Although the research findings on the relationship between drug abuse and schizophrenia are the most compelling and clear-cut to date, an age-old problem remains: it is still difficult to prove whether the abuse caused the schizophrenia or vice versa. It is possible that someone who is predisposed to schizophrenia is more likely to abuse drugs; or individuals in whom both conditions were observed could simply be more susceptible to both problems.
Connecting all of the dots between substances like marijuana and mental disorders is undoubtedly challenging. But we do know that THC, one of the active compounds in marijuana, stimulates the brain and triggers other chemical reactions that contribute to the drug’s psychological and physical effects. And some scientists believe that marijuana use may interfere with normal brain development during the teenage years and young adulthood. Between the teen years and the mid-20s, areas of the brain responsible for judgment and problem solving are still making connections with the emotional centers of the brain. It is thought that smoking marijuana may derail this process and so increase a young person’s vulnerability to psychotic thinking.
While research on substance abuse and psychosis have not yet teased apart cause and effect definitively, this latest study provides one more reason to caution people against excessive substance use —especially if they have a family member affected by schizophrenia or some other psychotic disorder. Although it may be a difficult concept for some to swallow, the reward of a short-time high isn’t worth the long-term risk of developing a disabling mental disorder.