The single most significant reason why individuals with schizophrenia and bipolar disorder fail to take their medication is because of their lack of awareness of their illness (anosognosia). The single most common reason cited by 55 percent of individuals was that they did not believe they were sick. Other important reasons are concurrent alcohol or drug abuse and a poor relationship between psychiatrist and patient. Medication side effects, widely assumed to be the most important reasons for medication non adherence, are, in fact, a less important reason compared to the other factors cited.
Source: Journal of Clinical Psychiatry