I have treated many patients who have been hallucinating, and a lot of them do not have schizophrenia. About 1% of the population has schizophrenia, but 10 - 27% of the general population has experienced at least one hallucination, usually the visual type. Most times these hallucinations are not due to schizophrenia; usually they are due to the use of a substance like alcohol or cocaine or they are caused by a depressive disorder. Many people also experience mild hallucinations as they wake up or as they go to sleep: so-called hypnopompic and hypnagogic hallucinations, respectively. Sometimes these types of hallucinations suggest the presence of the sleep disorder narcolepsy.
Hallucinations are experienced in many neurologic and psychiatric disorders other than schizophrenia. One example that isn't often discussed is that about half of people who have migraines experience migrainous hallucinations. These are usually visual hallucinations of geometrical patterns. In rare cases a person may also have the experience of seeing objects which appear much larger or smaller than they really are. This phenomenon is sometimes known as Alice in Wonderland Syndrome, named after the famous author Lewis Carroll who experienced these symptoms and wrote detailed and beautiful descriptions of what he went through in his fiction. Visual hallucinations are also experienced as a side effect of certain illicit substances like mescaline and LSD. Patients who experience delirium often have hallucinations that resolve with the treatment of the cause of the delirium. Common causes of delirium include infections and reactions to prescription medications.
Schizophrenia is classically associated with auditory hallucinations. These are found in 60 - 90% of people with schizophrenia, but are also present in many other disorders. For example, 20% of patients in the manic phase of bipolar disorder and almost 10% of patients with major depressive disorder experience auditory hallucinations. These disorders are managed primarily with medications: usually antipsychotic medications in psychotic depression and schizophrenia, and mood stabilizers in bipolar disorder. Electroconvulsive therapy has also been shown to be effective in reducing or eliminating hallucinations in these disorders. Typically it is only used once several medications have been tried unsuccessfully.
As this blog has hopefully shown, there are a wide range of causes for hallucinations and it is important for patients' physicians to make sure they are not caused by something other than schizophrenia, such as a medication, infection, or metabolic abnormality.