Researchers think schizophrenia is caused by a combination of genetic and environmental factors, but there is a lot left to learn about the specific genes that make a person at risk for schizophrenia and the environmental factors that trigger these genes. There is increasing evidence that illicit drug use, particularly marijuana, may be one of those environmental factors.
Schizophrenia and Marijuana
Evidence has been around for some time linking marijuana use to psychoses; the debate concerns the nature of the relationship. Here are some theories:
- The gene that puts a person at risk for schizophrenia may also predispose him to use marijuana, sometimes called cannabis. A study of Swedish military enlistees found that cannabis use might increase the risk of schizophrenia by as much as 30 percent in people who are genetically susceptible.
- People with psychotic symptoms may use cannabis to self-medicate.
- Cannabis use triggers schizophrenia and other psychoses. Evidence is mounting to support this theory. For example, a recent review of studies that examined schizophrenia risk for cannabis users found that people who used marijuana by age 18 were more likely to develop schizophrenia than those who did not. Other studies report that people who have used marijuana more than 50 times were six times more likely to receive a schizophrenia diagnosis.
Schizophrenia and Other Drugs
Studies that have examined the relationship between schizophrenia and other drugs are less common. Alcohol is the substance most often abused by people with schizophrenia. While alcohol can cause a relapse of symptoms, there is no evidence to suggest that alcohol use causes schizophrenia. And stimulants, like cocaine and amphetamines, are linked to types of psychosis, while heroin users were actually at a lower risk for developing psychoses.
Can a Link Between Schizophrenia and Drug Use Be Proved?
Ken Duckworth, MD, medical director of the National Alliance on Mental Illness and an associate professor at Harvard Medical School in Boston, says that proving the link between schizophrenia and illicit drug use is tough.
To prove conclusively that marijuana use causes schizophrenia, a researcher would have to provide marijuana to a person at risk for schizophrenia. This is not legal, and it is unethical to knowingly expose a person to a hazardous substance. The researcher must also carefully control the person's environment, eliminating exposure to other variables that are thought to cause schizophrenia. This is neither practical nor ethical.
Instead, researchers must rely on longitudinal or "cohort" studies that track a large group of people over the course of many years. Researchers periodically take medical histories and ask questions about drug use and life experiences. The data are subject to participants' memory and willingness to disclose personal information, and the studies can take years. Despite limitations, evidence gleaned from these studies sheds light on new schizophrenia prevention strategies.
Based on the study involving the Swedish military, the researchers speculate that as many as 13 percent of schizophrenia cases could be prevented if no one used marijuana. Other studies suggest that the greatest risk is to adolescents. Because the average age of schizophrenia onset is 20 years old, delaying drug use until early adulthood may also effectively minimize risk.
There are other environmental triggers for schizophrenia; avoiding drug use will not eliminate risk entirely. Besides, says Dr. Duckworth, not all people with schizophrenia have used illegal drugs.
But a definite risk exists. A person with a family history of schizophrenia should seriously consider the risk of mental illness when making decisions about recreational drug use.