There have been many research studies suggesting that marijuana may increase risk of schizophrenia (see here). But there is also a recent study at Harvard Medical School, led by Ashley C. Proal, that has suggested this may not be the case. This recent comparison between families with a history of schizophrenia and those without, found no connection to marijuana as a cause for the disorder.
There are over 400 different chemicals in the marijuana plant, so the issue is more complex than it might seem at first and it appears that there may be some chemicals from the plant that may actually be helpful for people who have schizophrenia.
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