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A lot of us use the term "paranoia" quite loosely. However, clinical paranoia involves persistent feelings of persecution and an inflated sense of self-importance. Unlike everyday suspicion, paranoia does not have a rational basis. There are several medical or mental health conditions that can cause paranoia, but they are not common. You cannot and should not attempt to diagnose yourself with any of these conditions. If you display any of their symptoms, see your physician or a mental health professional such as a psychiatrist or clinical psychologist. Only a trained medical professional can diagnose mental illnesses. PPD affects somewhere between 0.5% and 2.5% of the population. People with PPD are so suspicious of others that it causes dysfunction in their daily lives, such as extreme social withdrawal. Its symptoms include:  Suspicion, without grounds, of others, especially that you may be harmed, exploited, or deceived by them Suspicion of others’ trustworthiness, even friends and family Difficulty confiding in or working with others Reading hidden or threatening meanings into harmless remarks or events Grudge-bearing Social withdrawal or hostility Rapid anger reactions People with paranoid schizophrenia are usually convinced that others are out to harm them or their loved ones. They may also believe that they are extremely important (delusions of grandeur). Only about 1% of people have schizophrenia. Other common signs of paranoid schizophrenia include:  Social isolation or withdrawal Suspicion of others Guarded or reserved behavior Delusional jealousy Auditory hallucinations (“hearing things”) Delusional Disorder is the belief in one or more very specific paranoias (e.g., "The FBI is in my TV watching my every move"). It's pinpointed and not necessarily global, and the person is otherwise able to function without any obviously bizarre behavior. This disorder is extremely rare; only about 0.02% of people have delusional disorder. Common symptoms of Delusional Disorder include:  High levels of self-reference. This means that the person sees references to themselves in everything, even when this could not possibly be true (e.g., believing that an actor in a movie is talking directly to them). Irritability Depressive mood Aggressiveness Paranoia may accompany post-traumatic stress disorder (PTSD), a mental health condition that may develop after a person has experienced a trauma. Traumatic experiences can even cause hallucinations as well as paranoia. If you have experienced a trauma in the past, such as abuse, it is possible that you have developed what is known as “persecutory ideation,” or the belief that others are out to harm you. This belief can cause you to suspect others or worry about being harmed, even in situations that most people would not see as suspicious or harmful. Unlike most other paranoias, this type of fear has a basis in that it is a reaction to the trauma. Working with a mental health professional who is experienced in handling trauma can help you overcome PTSD and this type of paranoia.  The most common treatment for PTSD is cognitive behavioral therapy (CBT), which focuses on learning how your trauma has affected your thinking and behavior. You can learn new ways to think about yourself and the world that will help reduce your symptoms.  Other treatments include exposure therapy and EMDR (eye movement desensitization and reprocessing). Without help, it may be hard to figure out why you feel paranoid and determine the best way to deal with those feelings. A licensed mental health professional can help you begin to understand these feelings and help you work through them.  Keep in mind that feeling paranoid may be part of an underlying mental health condition that requires treatment. Talking to a therapist can help you understand what is going on and decide on the best course of action. Seeing a therapist is very common. People do it all the time to get better and improve their lives. You can feel good about your decision to seek help: it’s brave and shows that you care about yourself. Feel free to switch therapists! Many people feel stuck with whichever one they start with. If you're not jiving, find a new one. Find one that makes you feel comfortable and that you can see yourself trusting. It'll be the quickest path to progress. Know that your therapist is required by law to keep the information you share confidential. People with paranoia tend to be afraid to share their problems, but therapists are legally and ethically bound to keep your secrets safe. The only exceptions to this rule is if you share plans to harm yourself or others, abuse or neglect is involved in your situation, or if a court orders your therapist to reveal information because you are on trial.
Define “paranoia” properly. Look for characteristic symptoms of Paranoid Personality Disorder (PPD). Watch for signs of paranoid schizophrenia. Look for signs of Delusional Disorder. Consider whether you may have PTSD. Consider talking to a therapist about how you have been feeling.