Google "marijuana induced psychosis", for openers. Also, "children, acute marijuana intoxication."
Any drug consumed regularly will become a part of your body's chemistry. Hence, an addiction. True for cigarettes. True for heroin. True for smokeless tobacco. True for marijuana. Etc.
There's tons of research to indicate marijuana is not "harmless." But addicts don't care about the facts. Just the high.
Sugar, caffeine, and nicotine are WAY more addictive than the devil's lettuce.