Young people who smoke marijuana are at increased risk for developing psychosis, new research shows. The risk is particularly high for people who are predisposed to the reality-distorting mental illness, such as those with an affected family member.

“There is a public health message here,” senior author Dr. Jim van Os, from Maastricht University in the Netherlands, told a news conference. “While it would be very difficult to expect young people to stop using (marijuana) altogether, they could be made aware of the possible negative consequences of taking the drug if they had a family history of psychosis.”

Although a link between marijuana use and psychosis is well established, the direction of the association is unclear. That is to say, while cannabis use may cause psychosis, it is also possible that people predisposed to psychosis are simply more likely to use cannabis - a theory called the self-medication hypothesis.

“To our knowledge, no (forward-looking) study to date has tested this hypothesis using information on predisposition for psychosis in relation to later” marijuana use, the researchers note.

As reported in the British Medical Journal, the researchers analyzed data from 2437 subjects, between 14 and 24 years, who were followed for 4 years. Standardized personal interviews were performed at the start of the study and at follow-up to assess predisposition to psychosis, marijuana use, and psychotic symptoms.

Marijuana use when the study began raised the risk of psychotic symptoms at follow-up by 67 percent, the researchers point out. As noted, the risk was most pronounced in subjects with a predisposition to psychosis.

In contrast, a predisposition to psychosis did not increase the likelihood of marijuana use at follow-up, which refutes the self-mediation hypothesis, the team says.

Marijuana use “moderately increases the risk of psychotic symptoms in young people but has a much stronger effect in those with evidence of predisposition for psychosis,” the researchers conclude.

SOURCE: British Medical Journal, December 1st online issue, 2004.