like dvs said, when u become addicted to drugs such as heroin, ur body needs the drug to function correctly. With weed, the "addiction" is 100% mental, as well as alcohol. Ur body doesnt believe it needs them to function, ur mind jus wants to feel the high. U don get withdrawl when u don hav alcohol or weed for an extended amount of time, u jus continue to live ur life. That is y i don believe addiction is right word, its more like craving it. U want it, but if u don get it, its not the end of the world. u move on, and in time, u will feel better.