Health & nutrition
"We have focused mainly on temporal order; is it the chicken or the egg? As the study shows, it is a bidirectional relationship," wrote the study's lead author Merel Griffith-Lendering, a doctoral candidate at Leiden University in The Netherlands.
Previous research established links between marijuana and psychosis, but scientists questioned whether pot use increased the risk of mental illness, or whether people were using pot to ease their psychotic symptoms, such as hallucinations and delusions.
"What is interesting in this study is that both processes are going on at the same time," said Dr. Gregory Seeger, medical director for addiction services at Rochester General Hospital in upstate New York.
He told Reuters Health that researchers have been especially concerned about what tetrahydrocannabinol (THC), the active property in pot, could do to a teenager's growing brain.
"That's a very vulnerable period of time for brain development," and individuals with a family history of schizophrenia and psychosis seem to be more sensitive to the toxic effects of THC, he said.
A 2010 study of 3,800 Australian teenagers found that those who used marijuana were twice as likely to develop psychosis compared to teens who never smoked pot (see Reuters Health article of March 1, 2010 here:).
But that study also found that those who suffered from hallucinations and delusions when they were younger were also more likely to use pot early on.
Chicken versus egg
For the new study, published in the journal Addiction, the researchers wanted to see which came first: pot or psychosis.
Griffith-Lendering and her colleagues used information on 2,120 Dutch teenagers, who were surveyed about their pot use when they were about 14, 16 and 19 years old.
The teens also took psychosis vulnerability tests that asked - among other things - about their ability to concentrate, their feelings of loneliness and whether they see things other people don't.
Overall, the researchers found 940 teens, or about 44 percent, reported smoking pot, and there was a bidirectional link between pot use and psychosis.
For example, using pot at 16 years old was linked to psychotic symptoms three years later, and psychotic symptoms at age 16 were linked to pot use at age 19.
This was true even when the researchers accounted for mental illness in the kids' families, alcohol use and tobacco use.
Griffith-Lendering said she could not say how much more likely young pot users were to exhibit psychotic symptoms later on.
Also, the new study cannot prove one causes the other. Genetics may also explain the link between pot use and psychosis, said Griffith-Lendering.
"We can say for some people that cannabis comes first and psychosis comes second, but for some people they have some (undiagnosed) psychosis (and) perhaps cannabis makes them feel better," said Dr Marta Di Forti, of King's College, London, who was not involved with the new research.
Di Forti, who has studied the link between pot and psychosis, told Reuters Health she considers pot a risk factor for psychosis - not a cause.
Seeger, who was also not involved with the new study, said that there needs to be more public awareness of the connection.
"I think the marijuana is not a harmless substance. Especially for teenagers, there should be more of a public health message out there that marijuana has a public health risk," he said.
"Given the severity and impact of psychotic disorders, prevention programs should take this information into consideration," she said.