Not only does marijuana harm your sperm, as a previous study suggested, but it also puts adolescents who are genetically predisposed to schizophrenia at greater risk of developing the brain disorder, researchers are now claiming.

Using a sophisticated brain imaging technique called diffusion tensor imaging (DTI), researchers at Zucker Hillside Hospital in Glen Oaks, New York, studied the brains of groups of adolescents: healthy, non-drug users; heavy marijuana smokers (daily use for at least one year); and schizophrenic patients.

Unlike magnetic resonance imaging (MRI), which provides a static picture of brain structures, DTI detects and measures the motion of water molecules in the brain, which can reveal microscopic abnormalities.

Manzar Ashtari, Ph.D., Sanjiv Kumra, M.D. and colleagues used DTI to examine the 'arcuate fasciculus', a bundle of fibers connecting the Broca's area in the left frontal lobe and the Wernicke's area in the left temporal lobe of the brain. The investigators found that repeated exposure to marijuana was related to abnormalities in the development of this fiber pathway, which is associated with the higher aspects of language and auditory functions.

"Because this language/auditory pathway continues to develop during adolescence, it is most susceptible to the neurotoxins introduced into the body through marijuana use," explained Dr. Ashtari.

The scans revealed no abnormal developmental changes in the language pathway in the healthy adolescents, but showed abnormalities in both the marijuana users and schizophrenic patients.

"These findings suggest that in addition to interfering with normal brain development, heavy marijuana use in adolescents may also lead to an earlier onset of schizophrenia in individuals who are genetically predisposed to the disorder," said co-principal-investigator Sanjiv Kumra.