The study was published in the November issue of the American Journal of Psychiatry.
The researchers analyzed data from 109 hospitalized patients who were experiencing their first psychotic episode. The results showed that patients who had a history of using marijuana, or cannabis, and increased to daily pot smoking experienced both psychotic and pre-psychotic symptoms at earlier ages.
“We were surprised that it wasn’t just whether or not they used cannabis in adolescence that predicted the age of onset, rather it was how quickly they progressed to becoming a daily cannabis user that was the stronger predictor,” said Michael Compton, lead author and assistant professor of psychiatry in the Emory School of Medicine.
The study also found a gender difference: The female subjects who progressed to daily pot smoking had a greater increased risk for the onset of psychosis than the males.
Marijuana is the most abused illicit substance among people with schizophrenia, the most extreme form of psychosis, and previous research has shown that smoking pot is likely a risk factor for the disease.
The Emory study also focused on what is known as the prodromal period, when a person has symptoms such as unusual sensory experiences, which are often precursors to frank hallucinations and delusions. Prodromal symptoms can occur months, or years, before a diagnosis of psychosis. About 30 to 40 percent of prodomal teenagers will eventually develop schizophrenia or another psychotic disorder.
“The prodromal period is especially important because it’s considered to be a critical time for preventive intervention,” says Elaine Walker, a co-investigator of the study and professor of psychology and neuroscience at Emory.
The study also involved researchers from Emory’s Rollins School of Public Health and Georgia State University. It was funded by the National Institute of Mental Health.