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Marijuana has no harmful affects on the body, or they haven't been fully discovered yet.
Myth: Marijuana Can Cause Permanent Mental Illness. Among adolescents, even occasional marijuana use may cause psychological damage. During intoxication, marijuana users become irrational and often behave erratically.
This hasn't been proven.
Overall, you're just thinking like the average person with a knowitall attitude.
Fact: Moderate smoking of marijuana appears to pose minimal danger to the lungs. Like tobacco smoke, marijuana smoke contains a number of irritants and carcinogens. But marijuana users typically smoke much less often than tobacco smokers, and over time, inhale much less smoke. As a result, the risk of serious lung damage should be lower in marijuana smokers. There have been no reports of lung cancer related solely to marijuana, and in a large study presented to the American Thoracic Society in 2006, even heavy users of smoked marijuana were found not to have any increased risk of lung cancer.
Enjoy my knowledge lecture!