The results show that people with this genetic marker using cannabis are twice as likely to develop psychosis compared to those who use the drug, but they have such a marker.

Among those who use the drug every day, the risk of psychosis increases sevenfold in those with the genetic marker.

   Previous studies have linked marijuana smoking with increased risk of psychiatric disorders such as schizophrenia, but only a small number of cannabis users presented a psychotic episode. According to the researchers, the new finding could help identify cannabis users who may be at risk for this side effect.

"Our findings help explain why a consumer of cannabis developed psychosis while her friends continue to smoke without problems," said lead researcher Dr. Marta Di Forti, Institute of Psychiatry, Kings College London.

The study involved 489 people in London, who had experienced a psychotic episode, and 278 healthy people with no history of psychiatric disorders.

The genetic marker in question is a variation of a gene called AKT1. The new finding confirms previous research that this marker also link with the risk of psychosis after marijuana use.

It was already known that the AKT1 gene is involved in dopamine signaling in the brain and is abnormal in patients with psychosis, Di Forti said.

Finding the genetic basis of psychosis induced by cannabis can lead to the development of therapies for the disease, said Dr. John Krystal.