Marijuana Use Hastens Onset of Schizophrenia

A review of 83 studies provides strong evidence that reducing marijuana use could delay or even prevent some forms of psychosis.
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Honing in on the risks of cannabis, scientists have found that marijuana use hastens the onset of schizophrenia by nearly three years for those already at risk for the disorder.

In the Archives of General Psychiatry, a team of researchers reports that the onset of psychosis occurs about 2.7 years earlier for people who use marijuana than for those who don’t. And the loss of even 2.7 symptom-free years can worsen a patient’s prognosis for life, they say.

“We’ve known for many years that people who develop schizophrenia earlier have a number of poorer outcomes,” said Michael Compton, a co-author of the report and a professor of psychiatry and behavioral science at George Washington University. “The earlier the disease starts, the greater the social disruption it causes. You’re less likely to finish high school, go off to college, get your first car and get married.”

Schizophrenia is a disease characterized by severe hallucinations and delusions. Typically, it begins between the ages of 18 and 28 — slightly younger for men and older for women. Between 80 and 85 percent of the time, genes are the cause. The findings of an earlier onset for marijuana users have obvious implications for young people who have a family history of schizophrenia or who are beginning to show signs of the disease — but doctors caution that people may be at risk and not know it. Most schizophrenics don’t have relatives who are afflicted.

“The risk is hidden,” Compton said. “We usually don’t know who is at risk until they have developed the disorder.”

National health surveys show that about 16 million Americans regularly use marijuana. It is the third most widely used addictive drug, after tobacco and alcohol. Most people begin using marijuana in their teenage years. Surveys also repeatedly have found more substance abuse, especial with marijuana, among people who have been diagnosed with a psychotic disorder. As reported previously in Miller-McCune, some studies suggest that people who are more vulnerable to psychosis are more likely to use marijuana, which in turn could contribute to an increased risk of developing mental illness.

Compton’s team, which included researchers from the Prince of Wales Hospital and the University of New South Wales in Sydney, Australia, looked at 83 studies on the onset of psychotic disorders, comparing 8,200 patients who were using alcohol and illegal drugs, including marijuana, with 14,350 patients who were not. Most of the participants had been diagnosed with schizophrenia.

Among the samples of marijuana users, the team found, the onset of psychosis was 2.7 years earlier than among patients who did not use drugs, although it varied according to gender. For the men, onset was 1.9 years earlier and for women, it was 3.4 years earlier. For substance abusers in general, the onset of psychosis was 2.0 years earlier than for people who did not drink or use drugs. The team did not find a connection between alcohol use alone and an earlier onset of psychosis.

In the context of a growing body of literature on the risks of marijuana, the team also suggested that for some people using it less or not at all might even prevent psychosis from developing in the first place. Scientists believe that marijuana may precipitate or even trigger schizophrenia by disrupting brain development during the period of neurological maturation in adolescence.

“That’s our leading hypothesis,” Compton said. “We know that adolescence is a critical time of brain development, so marijuana use during that time may have an effect on certain pathways in the brain that are involved with schizophrenia.”

Even if the onset of psychosis is genetically inevitable, Compton said, an extra two or three years without the disease could allow many patients to achieve some of the important milestones of late adolescence and early adulthood — important steps that could help them live with their disability later on.

“I don’t think that the news about marijuana as a potential risk factor for schizophrenia has been publicized,” Compton said. “The literature is accumulating, and it’s time to let the public know of this potential risk.”

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