Marijuana is not chemically addictive. It does not contain nicotine like people seem to think.
Marijuana contains a substance called THC. When consumed, it induces a psychoactive state during which short-term effects are episodic memory loss and other minor psychological effects.
Long-term effects are less clear. Some suggest that consistent use leads to short-term memory loss and may be more likely to develop psychosis. Other information suggest no significant negative long-term side-effects.