Smoking pot may be linked to worsening schizophrenia, according to a new study.

Researchers say the results also suggest that among those likely to develop the disease, those who use marijuana may get the disease earlier in life than those who don’t.

The findings don’t prove that smoking marijuana causes schizophrenia, and the study only looked at people who already had the disease. But, “smoking marijuana may have hastened whatever process was going to happen anyway,” Daniel Foti, a PhD student at Stony Brook University on New York’s Long Island and the lead author on the study, told Reuters Health.

Patients suffering from schizophrenia - about one percent of the population - often hear or see things that don’t exist, or are convinced others are out to get them. Previous research has shown that people who smoke marijuana may be more likely to develop schizophrenia than people who don’t use drugs.

This study suggests that doctors treating schizophrenia should make marijuana use one focus of their treatment, the authors write.

The researchers followed 229 patients with schizophrenia for ten years after they were first admitted to a psychiatric hospital. They compared the patients’ use of marijuana - both recently and over their lifetimes - with how old they were when their symptoms started and how severe those symptoms currently were. The study was published online May 17 in the American Journal of Psychiatry.

Close to two-thirds of the patients had smoked marijuana at some time in their lives. Patients who had used marijuana before being hospitalized for schizophrenia had worse symptoms of psychosis than patients who hadn’t at the time they were admitted. They were also admitted at a younger age.

Researchers also found that changes in how much marijuana a patient smoked were linked with how bad that patient’s psychotic symptoms were. The link worked in both directions: patients whose symptoms had recently gotten worse reported smoking more marijuana the next time they were interviewed, and patients who started smoking more then began having worse symptoms.

Patients whose symptoms had recently gotten better generally reported smoking less marijuana at their next interview.

“There is a large and accumulating body of evidence that cannabis use and abuse can trigger the onset of psychosis,” Dr. Dolores Malaspina, a psychiatrist at New York University who was not involved in the research, told Reuters Health in an email. This may be particularly true in teenagers, whose brains are still maturing, she said.

“This study and other research shows that cannabis abuse in the early illness can also have a lasting effect on the course of the psychiatric disorder, even when people become drug-free.”

It’s hard to know what might cause the link between marijuana use and schizophrenia symptoms, Dr. Stephen Eggan, a neuroscientist at the University of Pittsburgh, told Reuters Health. “At this point it’s too early to say exactly what’s happening,” he said.

Still, Foti and his colleagues report that the link could nonetheless help certain people with schizophrenia - that by encouraging them to stop smoking marijuana, doctors might be able to help patients improve their symptoms.

The patients with the most severe symptoms, Foti said, “may be the ones that might highly benefit from intervention with drug use specifically.” If the aim is limiting psychotic symptoms, he said, “it seems that reducing or stopping marijuana use may be one avenue towards that.”

SOURCE:  American Journal of Psychiatry, online May 17, 2010.