There are no known prior systematic reviews of the non-financial barriers to medication adherence among the U.S. elderly (ie, age ≥ 65 years). The search found only 9 studies that met the inclusion criteria, and each one studied a different population and/or disease, or used a different assessment of medication adherence, making a clear synthesis of the literature extremely difficult. Studies of adherence have been burdened by heterogeneity for decades,26-28
and recent literature seems to be no different. This heterogeneity reflects the fact that medication adherence is an extremely complex behavior, and identifying a clear set of barriers that applies to all patients is not possible and probably not advised.
In light of the serious limitations of the current literature on medication adherence among the elderly, it is important to recognize what can be learned from published studies and to focus on ways to improve future research. Based on this review, several factors have been shown to be potential barriers to adherence and can be placed into 3 categories using a previously published approach2
– patient-related factors, drug-related factors, and other factors.
Common patient-related factors
previously found to be associated with medication adherence include socio-demographics, psycho-social profile, comorbidities, cognitive ability, and health beliefs.2
Although most socio-demographic characteristics (eg, age, gender, race) and comorbidities are non-modifiable, other patient-related variables are potentially modifiable. This review found 1 study reporting that patients with hypertension having less knowledge about diseases not related to hypertension (eg, arthritis) were less likely to be adherent to their antihypertensive therapy.24
Educating patients, therefore, so that they understand the true potential risks of their disease state could possibly help minimize counterproductive health beliefs and promote motivation to adhere to treatment.
Another study reported that inadequate health literacy was associated with low adherence in unadjusted analysis but not in adjusted analyses.20
While it has been posited that an adequate level of functional health literacy is necessary to achieve successful medication adherence, this study used prescription refills records to measure adherence and it could be possible that health literacy skills are more important in taking medication correctly rather than refilling a prescription; correct medication administration would not be captured using administrative data.20
Future research on health literacy and adherence should use additional measures of adherence (eg, self-report, electronic monitoring) to better describe this complex relationship.
Cognitive function was significantly associated with underadherence in 1 study using a validated instrument (MMSE).21
Four other studies in this review measured cognitive function but did not find significant results.19,22,23,25
One of those studies, however, found that reliance on self to remember to take medications was negatively associated with adherence.23
Any research assessing the relationship between cognition and adherence should likely account for the degree of assistance with medication use received by the patient. Clearly, further research is needed to better understand the relationship between cognitive function and adherence among older adults.
Common drug-related factors
previously identified as being associated with medication adherence include the number of drugs taken, adverse effects, and administration regimens.2
While the number of drugs may be expected to have a negative impact on adherence, this has not always proven to be the case.2
Similar to prior research showing mixed results for the effect of number of drugs on adherence, the current review found 4 studies19,21,24,25
that reported a negative association between taking more drugs and adherence (ie, greater number of drugs being associated with worse adherence) and 1 study20
that found a positive association between taking more drugs and adherence (ie, greater number of drugs being associated with better adherence). Not surprisingly, 1 study found that patients who experienced side effects from their medication were more likely to be nonadherent.23
Due to the uncertainty surrounding these drug-related barriers among older adults, perhaps the research focus in this area should shift towards understanding the effect of the appropriateness of drug regimen on adherence rather than the count of the total number of drugs. Although this review did not include studies focused solely on medication regimen complexity, prior literature is fairly clear on the association with more complex dosing regimens and poorer adherence.13-16
Some other factors
potentially associated with medication adherence in previous reviews include the patient-prescriber relationship, access to medication, and social support.2
The current review found 1 study reporting that patients with hypertension who gave lower priority to discussing their hypertension with the physician were significantly more likely to be nonadherent to their blood pressure medication.24
This suggests that improving the patient-prescriber relationship is an area for future research to potentially improve patients’ adherence. Furthermore, this same study identified potential logistical barriers to successful adherence with the medication use process, including cost of medication, medication not being covered by insurance, transportation to a pharmacy, and having to switch to a generic medication, to name a few.24
Older adults face multiple logistical barriers throughout the medication use process, and identifying and addressing such barriers on an individual basis is critical to successful adherence.
Future research should strive to better describe the various types of nonadherence among older adults, including primary nonadherence (or nonfulfillment) and nonpersistence. Primary nonadherence occurs when the provider makes recommendations about a course of therapy that the patient ultimately does not initiate. 29
Nonpersistence occurs when the patient decides to stop taking the medication after starting it, without being advised by a health professional to do so. 30
None of the studies included in this review assessed primary nonadherence, and only one study measured overadherence.21
Future research in these understudied areas of the medication use process could provide greater understanding of modifiable barriers to adherence.
This review is subject to several limitations inherent in systematic reviews. First, because the search only included two databases and because of the selection of search terms, it is possible some studies were missed, despite reference mining. Second, studies with null effects might be unpublished disproportionally to those with significant results in the peer reviewed journals included in the search, potentially leading to publication bias. Finally, because the inclusion criteria were fairly strict, the generalizability of this review study is limited to the specific populations targeted (ie, U.S.-based studies assessing barriers other than cost and medication regimen complexity to medication adherence). Although this systematic review does not assess access/cost as barriers, it is clear that financial concerns do affect medication use in the elderly,7
and it is promising to note that the implementation of Medicare Part D may be improving medication adherence among older adults.31,32