New research has found little link between marijuana use and schizophrenia, contrary to previous studies that suggested pot-smoking adolescents were at greater risk of developing the mental illness.

A recent study conducted at Harvard Medical School compared families with a history of schizophrenia and those without such a history, and it’s the first family study to examine non-psychotic cannabis users and non-cannabis users as independent sample groups.

While marijuana use seemed to have an effect on the age of onset for schizophrenia, researchers found that family risk was the crucial underlying factor.

“The results of the current study suggest that having an increased familial morbid risk for schizophrenia may be the underlying basis for schizophrenia in cannabis users and not cannabis use by itself,” the researchers found.

The study examined 282 people from the New York and Boston areas who were divided into four groups: those with no lifetime history of psychotic illness, cannabis or any other drug use; those with no lifetime history of psychotic illness, and a history of heavy cannabis use during adolescence, but no other drug use; patients with no lifetime history of cannabis use or any other drug and less than 10 years of being ill; patients with a history of heavy cannabis use and no other drug use during adolescence and prior to the onset of psychosis.

Researchers asked the subjects about their family history of mental illness and drug use, which expanded the study to include nearly 4,300 people.

The study, which was published earlier this month in Schizophrenia Research, found evidence to suggest that cannabis users were more likely to develop depression or bipolar disorders, but the scientists said more research was needed to understand the relationship.

The researchers also found that drug abuse itself appears to have an important genetic component.

“Drug abuse is present more frequently in family members of all three samples compared to those of non-cannabis abusing controls,” the researchers said. “This is in line with past research confirming a genetic predisposition for drug use.”

[Image via Agence France-Presse]