Paranoia is a mental disorder that can make people falsely believe that others are lying, cheating, using them, or trying to harm them. Or it can give them an overly large sense of self-importance. People with paranoia can have either or both of these traits.
Paranoia may occur with several other mental disorders (as in paranoid schizophrenia) or Alzheimer's disease. It rarely occurs as a mental illness on its own. It is sometimes caused by using illegal drugs such as marijuana.
Treatment usually includes professional counseling and may include medicine.