GTPJay, let me tell you what pot does to some people. It sends them to mental institutions because they have an underlying condition. This underlying condition might never surface under normal circumstances, but appears because of drug use. People without an underlying condition might smoke pot their whole lives and never be affected. Pot doesn't affect everyone's brain the same, it's different for different people.
Cannabis / Marijuana (and other street drugs) Have Been Linked to Significant Increases in a Person's Risk for Schizophrenia
http://www.schizophrenia.com/prevention/cannabis.marijuana.schizophrenia.html
My friend Bill had two roommates in college. They lived in a house and all smoked pot together. One roommate began displaying symptoms of schizophrenia, such as paranoia. Bill and the other roommate did not recognize this right away. The guy kept smoking pot and eventually stayed up for several days without sleeping. After this, they talked with him some and figured out something was seriously wrong. He quite literally had lost his mind in a matter of days. He was a normal guy, a good friend of theirs and he went completely insane.
Today he resides in a mental institution, quite seriously believing he is Jesus Christ's reincarnation on Earth. The doctors said that smoking pot is likely what triggered the onset of his mental illness, and that it was severely damaging to his mental condition.
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Now let me tell you about one of my neighbors who smoked pot. He began developing migraines, which was worsened by his pot smoking. Eventually he began having seizures and they had to perform brain surgery, installing a metal plate in his head. A few years later, he went out in the forest and shot himself.