There is a whole lot of misconception about Cannabis and its effects. It is widely thought to be a mild medicine, not causing much harm. Such misrepresentations of the drug have resulted in making it a Class C drug.
However scientists from Karolinska Institute in Sweden have unearthed the harmful effects of cannabis, when they exposed rats to THC, a key causative ingredient in cannabis. It was found that the brain of those rats exposed to THC, got attuned to heroin, necessitating an increase in heroin intake during subsequent trials.
Cannabis is also linked to risks of cancer, schizophrenia, bipolar disorder and psychosis.