It has long been known that marijuana use is often linked to mental health problems such as advanced form of depression, schizophrenia, anxiety, as well as suicide. Now, new research shows that long term marijuana use may actually shrink certain parts of the brain and have lasting effects on mental health.

Australian researchers used magnetic resonance imaging (MRI) equipment to test 15 volunteers who admitted to smoking more than five joints a day for over 10 years. Then they compared the results with the scans of 16 individuals who did not use the drug.

The scans revealed structural differences in at least two different regions of the brain, the hippocampus an amygdala.

To be more specific, the smokers’ hippocampus, which is thought to govern emotion and memory, was 12 percent smaller than in non-smokers. Also, amygdala, which controls fear and aggression, was 7 percent smaller in the drug users.

Dr. Murat Yucel of the University of Melbourne who conducted the study along with colleagues at the University of Wollongong noted that “although modest use my not lead to significant neurotoxic effects, these results suggest that heavy daily use might indeed be toxic to human brain tissue.”

Marijuana is one of the world’s most commonly used illegal drugs. There are approximately 300 million users worldwide and 28 million users in the United States alone. With so many people using the drug,  and so many others fighting to make it legal, Dr. Yucel urged for “robust investigations,” to find out the long term “sequelae” of long-term cannabis use.

The findings of the study were published in the American Medical Association’s journal Archives of General Psychiatry.