LONG-TERM, heavy cannabis use causes significant damage to parts of the brain that regulate memory and emotion, Australian researchers have found in a world-first study. Marijuana shrinks parts of the brain known as the hippocampus and amygdala. "The hippocampus helps turn an experience into a memory that can last longer than a few minutes. It has also been linked to navigational abilities...The amygdala also helps lay down permanent memories and cues our emotional response to the world. Damage to this part of the brain can lead to emotional problems such as irrational fear or phobias."
(Source) Comment: This is very interesting because for a while now, we could only speculate about the harmfulness or the benefits of marijuana use without really having any hard data. Scientific studies such as these help provide us with more reliable, testable and measurable data.