That's the conclusion of a study of more than 2,400 German teens and young adults aged 14-24.
Participants' substance use and psychosis symptoms were tracked for about four years. Psychologists interviewed participants at the study's beginning and end.
The research was conducted by experts from Maastricht University in the Netherlands, including Jim van Os, a professor in the university's psychiatry and neuropsychology department. Their study appears in today's edition of BMJ Online First.
At the study's start, 13% said they had smoked marijuana at least five times. Four years later, about 17% of all participants had had at least one psychotic symptom.
Psychotic symptoms include hallucinations, such as seeing or hearing things that aren't really there, and delusions, which are false beliefs that do not go away with logical or accurate information. Other possible psychotic symptoms are incoherent speech, confused thinking, and strange behavior. The most common psychotic disorder is schizophrenia.
Pot smokers were more likely to have psychotic symptoms than those who didn't smoke pot. The more pot that participants smoked, the greater their chance of having at least one psychotic symptom. The risk held after screening out other influences including alcohol and other drugs.
Pot had "a much stronger effect" on psychotically predisposed participants, say the researchers. People who have a family member with psychotic symptoms are more likely to suffer similar symptoms themselves.
It's not the first time that marijuana has been linked to psychosis. But until now, no one knew which came first -- the psychosis or the pot use. Were participants using pot to soothe their psychological problems?
Probably not, say the researchers. Psychotic predisposition wasn't a good predictor of future pot use, they note.
Youth may be a particularly risky time for pot use.
Puberty is "a vulnerable period" for pot's negative effects, say the researchers, citing studies of lab rats. Pot's active ingredients may interact with brain chemicals to create negative psychological side effects, they say.
In 2002, a study published in the British Medical Journal linked frequent marijuana use at a young age - more than 50 times -- to an increase in schizophrenia later in life. Similar to the current study, this previous research showed that the more pot people smoked, the more likely they were to suffer psychosis.
Another study published in the same issue showed that daily pot smoking as a teen increased the risk of depression as an adult. When that study was released in 2004, researcher Louise Arseneault, PhD, told WebMD that their research suggested that there is a direct causal link between pot smoking and psychological problems that cannot be explained by tendency toward mental illness.