I am not aware of any “rule” that says that individuals diagnosed with schizophrenia cannot be diagnosed with borderline personality disorder.
As stated on Psycom’s website, “The term “borderline schizophrenia” does not refer to an established diagnosis in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition. It is sometimes used, however, to refer to an individual who meets some, but not all, of the criterion for schizophrenia or to refer to parallel symptoms of borderline personality disorder and schizophrenia. Schizophrenia and borderline personality disorder (BPD) can share symptoms. One study found that both patients with schizophrenia and patients with BPD experience hearing voices. The difference between the two being that paranoid delusions occurred in less than one-third of patients with BPD, compared with two-thirds in schizophrenia. The study also found that auditory hallucinations are common in both populations. According to the results of this study, schizophrenia and BPD frequently co-exist, however, accurate diagnosis of either or both conditions plays an important role in establishing an effective treatment plan. While BPD is characterized by a pattern of instability in interpersonal relationships; schizophrenia is characterized by a range of cognitive, behavioral, and emotional dysfunctions.”
Perhaps the person you learned this information from incorrectly believes that borderline personality disorder means that the individual is “bordering” between neurosis and psychosis. Under this theory, a person could not be schizophrenic because they have not crossed the psychosis threshold (i.e. their bordering between neurosis and psychosis), a required feature of a schizophrenia diagnosis. “Borderline” does not mean that an individual is teetering between neurosis and psychosis, although some believe this. People with borderline personality disorder display certain symptoms such as mood and self-image instability, difficulty engaging in relationships, anger and depression, “all or nothing thinking”, and so forth. A person with schizophrenia can display these aforementioned symptoms, in addition to psychosis, as well as some of the other symptoms of schizophrenia. The two disorders are not mutually exclusive meaning that if a person has schizophrenia, then he or she cannot be diagnosed with borderline personality disorder. From my understanding, it is possible for an individual to have both disorders simultaneously. It may not be common, per se, but it’s still possible. I hope this information helps. Thank you for writing.
This article has been updated from the original version, which was originally published here on August 25, 2008.