Addiction literally mans being bound to, and such a condition is not just limited to physical disease or ailment, it takes over the entire body and mind of the affected person, who cannot function without appropriate help and support.
Any and all kinds of substances that enslave a person work their way through the brain. Drugs or alcohol or any other abusive substances are chemicals, they tap into the brain's communication system and interfere with the way nerve cells normally send, receive and process information.