Individuals with paranoid schizophrenia and persecutory delusional disorder experience what is known as persecutory delusions: an irrational, yet unshakable, belief that someone is plotting against them. Paranoid beliefs are prevalent in many individuals and are often the cause of disputes between individuals and society.
Outside of paranoia caused solely by psychiatric illnesses, the most common cause of paranoia is the use of illegal drugs such as cannabis, amphetamines and cocaine. All of these narcotics cause harm which can lead to paranoia, or in turn cause psychiatric illnesses that then lead to the sufferer experiencing paranoia. As with other mental problems, paranoia may be treated by the sufferer recognizing the problem and seeking medical assistance. Treatment can be difficult as the patient often does not trust the caretaker.
Claude Steiner wrote:
- Traditionally, psychiatry has dealt with paranoid ideation by further discounting its validity. In my experience, effective therapy encourages the expression of people's paranoid fantasies and seeks their possible, even if only partial validation by willingly searching for the grain of truth in them. Mindlessness, the alienation from our mind is best treated by replacing lies and discounts with truthfulness. 
Paranoid delusions may be shared by individuals such as within political movements, social groups and national groups. This is known as mass paranoia where the group irrationally believes it is the victim of a conspiracy aimed at its destruction.