There are four major disorders that are associated with significant paranoia. They include:

• Paranoid personality disorder. Condition in which a person regularly becomes suspicious without cause. To be diagnosed with this disorder, paranoid thoughts must become so severe that they cause the patient significant distress and impair their ability to function. However, like with many mental illnesses, paranoid personality disorder often goes undiagnosed because people with this condition are unlikely to seek treatment. People with this disorder are often able to function in society, generally remaining grounded in reality, and may seek out others who accept their moralistic or punitive approach to life.
  
  
• Delusional (paranoid) disorder (persecutory type). Involves a persistent, plausible delusion that does not have the features of other mental disorders. Delusions are beliefs that an individual holds that are not true, not shared by others in a culture and cannot be easily modified.
  
  The most common delusion in this disorder is the belief that one is being persecuted. Patients may believe that they are the target of a plot, or that they are in danger of being poisoned, drugged or spied upon.
  
  
• Paranoid schizophrenia. Marked by extremely bizarre delusions or hallucinations that are nearly always based on a specific theme. Patients may hear voices or believe that their thoughts are being controlled. The delusions of paranoid schizophrenia tend to be more prominent and more impossible or bizarre than those associated with other types of paranoid disorders. People with this condition are intellectually confused and may not be able to work or socialize. They are also more likely to experience disturbances in social and family life than those with other paranoid disorders.
  
  
• Substance abuse and dependence.  Paranoia is associated with the use of a variety of drugs.  Alcohol, cocaine, steroids, and amphetamines are among the most commonly used substances frequently associated with paranoia. 

In addition, paranoia may be present in individuals with certain cognitive disorders, such as Alzheimer’s disease.  Therapeutic use of certain medications, such as steroids, can also cause paranoia in some individuals.