Why does marijuana make some people mellow, while inducing psychotic episodes in others? According to a new article in Biological Psychiatry, scientists have pinpointed a genetic marker associated with an increased risk of cannabis-induced psychosis in certain individuals. Led by Dr. Marta Di Forti, a team of psychiatrists studied a variation in the AKT1 gene, which is involved in dopamine signaling and known to be abnormal in instances of psychosis. The team performed genotyping and assessed cannabis use for 489 patients who had experienced an episode of psychosis and 278 who had not. Their results show that people with the genetic marker who use cannabis are twice as likely to experience psychosis compared with those who use the drug but don’t have the genetic marker; and for people who light up every day, the risk for psychosis increasessevenfold for those who have the genetic marker. "Our findings help to explain why one cannabis user develops psychosis while his friends continue smoking without problems," says Di Forti. Dr. John Krystal, editor of Biological Psychiatry, hopes that research into the genetic underpinnings of “cannabis psychosis” may lead to the development of therapies for the condition, as well as biological tests that could be used to inform marijuana-prescribing physicians of their patients' risks. The drug's potential to induce psychosis is considered a greater public health concern now that the drug is legal for recreational use in Washington and Colorado, and for medical use in a growing number of states.