Psychosis is a mental and behavioral disorder in which a person has trouble telling the difference between what is real and what is not real. It may cause bizarre thoughts and behavior.

A person with psychosis may have:

• Delusions, which are firmly held but false beliefs. Irrational suspicion of others (paranoia) is a common delusion. The person may think others are watching or trying to hurt or even kill him or her.
• Hallucinations, which are false perceptions. Hallucinations can include seeing, hearing, feeling, smelling, or tasting things that are not really there.
• Personality changes.

Treatment for psychosis depends on the cause and may include care in a hospital. Medicines that diminish hallucinations and delusions and stabilize thinking and behavior are helpful. Group or individual therapy also may be helpful.