This study is one of the largest to date comparing adherence to oral hypoglycemic medications among diabetes patients with and without schizophrenia. Contrary to our hypothesis and previous work that observed poorer quality of diabetes care in diabetes patients with than without mental illnesses,28
we found that adherence to oral hypoglycemic medications was better among diabetes patients with schizophrenia than in diabetes patients without this diagnosis. These findings support a growing body of research that suggests that the quality of certain processes and intermediate outcomes of care related to adequate glycemic control are not compromised in diabetes patients with serious mental illnesses who are in ongoing care compared with those without these conditions.29–34
Numerous studies in nonpsychiatric populations have demonstrated an association between adherence to oral hypoglycemic medications and enhanced control of blood glucose and other positive diabetes outcomes, including fewer hospitalizations and lower health-care costs.2–11
Although determining whether diabetes patients with schizophrenia who exhibit superior medication adherence also have enhanced glycemic control was beyond the scope of the current investigation, this important question merits continued study.
Relative to diabetes patients without schizophrenia, individuals with schizophrenia exhibited greater adherence to oral hypoglycemic medications despite having higher rates of several risk factors shown to be independently associated with medication nonadherence in this study. These characteristics, which we controlled for in multivariable analyses, included black race, homelessness, depression, and substance use disorders. Individuals with schizophrenia were also less likely to receive their prescriptions by mail, a characteristic associated with increased adherence in this study.
However, several characteristics shown to promote medication adherence also occurred more frequently in diabetes patients with schizophrenia. These patients had significantly greater contact with the health-care system overall, having had significantly more hospitalizations and a greater number of outpatient visits for diabetes-, non-diabetes medical-, and psychiatric-related reasons in the prior year. Further, those with schizophrenia had higher levels of military service–connected disability and lower out-of-pocket prescription costs as well as were prescribed more complex medication regimens, all observed to enhance medication adherence. However, in our multivariate analyses, we adjusted for all these risk and protective factors.
As such, other patient- or treatment-related attributes of those with diabetes and schizophrenia may help to explain why their adherence to oral hypoglycemic medications was clearly better than that observed in diabetes patients without schizophrenia. Somatic medical conditions such as diabetes occur significantly more frequently in people with schizophrenia, and many are treated with antipsychotic medications that induce metabolic adverse effects.35
As such, mental health clinicians, family members, and patients with schizophrenia are paying greater attention to overall medical well being as well as to the potential medical consequences of psychiatric treatments.36
This enhanced awareness may be leading to beneficial effects for patients in medication management and other aspects of diabetes treatment. In addition, schizophrenia, like diabetes, is a chronic medical condition that requires active self-care over many years to prevent short- and long-term adverse consequences. Although having a chronic psychiatric disorder is often disabling and can have deleterious effects on multiple aspects of patients’ functioning and quality of life, the day-to-day self-management requirements for a psychiatric condition including medication taking may impart benefits, such as familiarity, in the treatment and outcomes of a co-occurring medical illness. Additional research is needed to confirm this hypothesis, particularly given the large volume of empirical evidence17–19
and clinical experience attesting to schizophrenia patients’ significant difficulties adhering to antipsychotic treatments and the resultant adverse outcomes.
Despite our finding of schizophrenia patients having better adherence to diabetes medications than those without schizophrenia, our observation that nearly half of all patients, regardless of co-occurring psychiatric diagnosis, had less than 80% of needed medications over a 1-year period warrants serious attention by clinicians and researchers. The observed rates of nonadherence were consistent with the review by Cramer12
but were higher than those from recent studies of the general population with diabetes, for which rates ranged from 20% to 35%.2,3,8
Despite extensive diabetes quality improvement efforts implemented in the VA, the greater overall illness burden or life circumstances of VA patients37
may explain the higher overall rates of nonadherence to diabetes medications that we observed. Our results were similar, however, to the few studies of adherence to diabetes medications conducted in patients with serious mental illness in the VA, which reported rates of nonadherence ranging from 29%–52%.20,21
The results of our study suggest that efforts to improve adherence to diabetes medications are needed in all VA patients with diabetes and that such efforts should address several of the modifiable risk factors as well as the protective factors that we observed. For example, more research is needed to determine whether interventions that include reducing homelessness, adequately treating depression and substance use disorders, increasing contact with health-care providers, reducing out-of-pocket prescription costs, and increasing access to medications by mailing refills can have beneficial effects on medication adherence and subsequent diabetes outcomes.
While we were able to comment on several patient and treatment characteristics associated with medication nonadherence in this study, we were unable to investigate the role of other potential factors that might have influenced medication adherence among these patients, including insight into illness, the quality of the patient-provider therapeutic alliance, and patients’ attitudes toward diabetes and its complications. Further, because our study focused exclusively on the largely older, male population of patients receiving care in the VA health-care system, our results may not generalize to other diabetes patients receiving care in different treatment settings that place less (or more) emphasis on the quality of processes and outcomes of diabetes care. Our study was also limited to individuals with diagnosed diabetes, so the prevalence of the disorder is likely higher than we observed. Further, although everyone included in the study had used VA inpatient or outpatient health-care services in the past year, many VA patients also receive health services outside of the VA system,38
and we were unable to account for this with the data source we used for this study. Our data also did not permit us to account for the known deleterious effects of cognitive impairments on medication adherence. But if we presume that cognitive impairments were worse in the schizophrenia group than in the nonmentally ill group,39
the higher adherence levels among patients with schizophrenia would suggest that cognitive impairments did not in this case severely disadvantage such patients. Finally, we did not examine whether individuals in the comparison group with major mental illnesses other than schizophrenia exhibited similar patterns of adherence to oral hypoglycemic medications as those with schizophrenia. Because the prevalence of type 2 diabetes is also elevated in individuals with other psychiatric illnesses such as major depressive disorder and bipolar disorder, patterns of use of diabetes medications in these patients merit further study.
Among the strengths of our investigation were the large sample size of diabetes patients with schizophrenia and an equally large comparison sample of patients without schizophrenia receiving diabetes care in similar treatment settings. In addition, we were advantaged by the availability of VA administrative pharmacy data that enabled us to use the MPR as our measure of medication adherence as opposed to other subjective methods including patient and clinician self-reports that have been shown to overestimate adherence.40
The MPR is an objective, unobtrusive, and validated measure of adherence that has been widely used in studies characterizing adherence to both diabetes and schizophrenia medications and has been linked to disease outcomes.2,4,7,8,11,17–19
In conclusion, this large investigation revealed that diabetes patients with schizophrenia exhibited superior adherence to oral hypoglycemic medications relative to a comparable sample of diabetes patients actively receiving care in the VA health-care system. The linkage of adherence to glucose control as well as the reasons driving this advantage require further study in order to improve patient outcomes in the future.