The results showed that higher levels of neuroticism and lower levels of extraversion were significantly associated with polypharmacy in men and women, respectively. The odds ratio per 0.5 SD for each personality trait score provided similar results.
This study showed that higher neuroticism was associated with polypharmacy in men. Neuroticism is characterized by emotional anxiety and vulnerability to stress. Neuroticism may influence lifestyle and lead to susceptibility to disease as a result. Previous studies have shown that higher levels of neuroticism are associated with smoking  and physical inactivity . Further, consumption of more grains and dairy products, fish, vegetables, and fruits, has been associated with lower levels of neuroticism . Thus, personality traits affect the lifestyles, resulting in unhealthy lifestyles that can lead to increased disability and increased mortality. Furthermore, personality traits may affect particular illnesses that require more medications, such as depression and diabetes mellitus. According to the studies on personality traits and diseases, higher levels of neuroticism are associated with depression , diabetes mellitus , and mild cognitive impairments . Therefore, people with higher levels of neuroticism may be more prone to polypharmacy. Moreover, personality traits may affect attitude or thinking and behavior during illness. A previous study showed that regardless of the severity of anxiety and depression, patients with higher levels of neuroticism were more likely to have a perceived need for care . ten Have et al.  have reported that people with high levels of neuroticism were more likely to receive care in the specialized mental health sector and visit healthcare services. Thus, people with higher levels of neuroticism may be more likely to report many symptoms, and this may prompt their doctors to prescribe many medications.
This study also showed that lower levels of extraversion were associated with polypharmacy in women. Extraversion is characterized by gregariousness, active and positive emotions. There have been studies on the relationship between extraversion and lifestyle. Previous studies have shown that lower levels of extraversion were negatively associated with healthy aware diet . Previous studies have also shown that lower levels of extraversion were associated with physical inactivity [40, 44]. Moreover, extraverted people are likely to have healthier behaviors and better health, which may result in more prolonged survival . Thus, personality traits can affect lifestyles, resulting in unhealthy lifestyles that can lead to increased disability and mortality. Furthermore, it is possible that personality traits with lower levels of extraversion affect a particular illness requiring many medications. Previous studies have shown that lower levels of extraversion are related to depression [42, 46] and infectious disease . In addition, higher levels of extraversion have a protective effect against death from respiratory disease . Although no data were shown in this study, there was a negative correlation between extraversion and lung disease in women, consistent with the findings of previous studies . Moreover, personality traits with lower levels of extraversion may affect attitude or thinking and behavior during illness. As the characteristics of low levels of extraversion include the tendency to be reserved, retiring, and withdrawn , individuals with low levels of extraversion may not communicate their health problems to health care professionals. Lack of communication between healthcare professionals and patients can interfere with an accurate diagnosis by healthcare professionals, leading to multidrug prescriptions and multiple medical treatments. Additionally, in previous studies, extraversion was associated with social relationships  and positively correlated with affectionate support . Therefore, people with lower levels of extraversion are poorly supported, and this can make their medical conditions more serious, leading to multiple drug use.
The results of this study suggest that personality traits may affect the development of illness and medication usage (polypharmacy). However, the causal pathway by which personality traits are related to polypharmacy is unclear. There are several possible mechanisms by which personality traits may lead to polypharmacy; these include: (1) personality traits may influence lifestyle, and an unhealthy lifestyle makes individuals more prone to disease; (2) personality traits may influence the manifestation and severity of a specific illness, thus influencing the amount of medication that is required; and (3) personality traits may affect thinking and behavior patterns during illness, which may affect the use of medical services, treatment needs, and communication with doctors.
This study identified a sex difference, in that different personality traits were associated with polypharmacy. Furthermore, the background between personality and polypharmacy may have different pathways depending on sex. Because, in addition to body composition, organ, and metabolism, there are sex differences in lifestyles, such as drinking, smoking, physical activity , dietary intake , and factors leading to long-term care . In addition to the occurrence of sex-specific morbidities due to the above factors, personality-specific medical treatment behaviour may have affected the number of medications taken. Sex differences need to be examined in detail in the future.
In addition to personality traits, not having a drinking habit and having a high number of chronic diseases were independently associated with polypharmacy. Similar to the present study, a previous study reported that having a drinking habit was inversely associated with polypharmacy; that is, an increase in the frequency of alcohol consumption led to a decrease in polypharmacy . Although a causal relationship cannot be claimed, this being a cross-sectional study, it was estimated that people with chronic diseases may be more likely to quit drinking. It has also been reported that a high number of chronic diseases was associated with polypharmacy , which is consistent with the present study.
This study has some strengths. First, personality traits were based on a self-reported questionnaire. In this study, the NEO-FFI was used to evaluate personality traits. The NEO-FFI was developed by Costa et al. , and the reliability and validity of the Japanese version of the NEO-FFI have been verified by Shimonaka et al. . The NEO-FFI is a stable and standard measure and is often used in gerontological studies. In addition, data on medication status were also collected through interviews. To minimize the inaccuracy of the medication status due to self-reports, we asked participants to bring their medication records. Based on data in this record, the physician confirmed the medication status in face-to-face interviews. Therefore, the number of prescriptions was accurate. Second, a limited number of studies have examined the relationship between personality traits and polypharmacy in older adults. Some studies on personality traits and polypharmacy have focused on patients with impaired mental health. A previous study on older adults with mental illness found that neuroticism was associated with polypharmacy . In addition, low levels of openness, extraversion, and conscientiousness have been reported to be associated with increased psychotropic medication use in patients with bipolar disorder . This study provides new findings regarding the relationship between personality traits and polypharmacy in community-dwelling older adults.
However, this study has some limitations. First, there is a possibility of residual confounding. The results showed that personality traits were associated with polypharmacy. The variables adjusted in the analysis of the relationship between personality traits and polypharmacy included economic status as socio-economic factors; drinking and smoking status as lifestyle-related factors; and BMI, IADL, mental health, and the number of chronic diseases as health status factors. Even after adjusting for various aspects of health status, the association remained significant, so poor health status did not completely explain the number of medications prescribed to older adults. In addition, since mental illness measures were not included in the medical history, the WHO-5-J was used as an adjusting variable in the model. Therefore, the effects of mental illness were considered adjustable. Furthermore, although the data are not presented in this study, neuroticism in men and extroversion in women remain significantly associated with polypharmacy after adjusting for psychiatric drug use. However, although several factors were controlled for in the analyses, residual confounders, such as social support and social network, might be present. It has been reported that social support is associated with better self-care compliance among older patients . In addition, inclusion in social networks is associated with healthy behavior  and a reduced risk of depressive symptoms . Therefore, if the analysis is adjusted for social factors, the strength of association between personality traits and polypharmacy may be weakened.
Second, the representativeness may be limited. The participation rate of this study was 23.2%, which is a low rate. The participants had at least a degree of independence as they could attend the study venue independently. Most participants were physically and cognitively healthy; therefore, people with poor health status may not have participated in this study. Conversely, most individuals in their 70 s are independent and active. It is possible that the low participation rate in the survey was because potential participants were engaged in work or leisure activities.
Third, there were cultural differences. The participants in this study were older adults living in Japan. Therefore, attention should be paid when generalizing the study results to other countries with different medical systems and cultural backgrounds. It is necessary to consider whether personality traits are commonly associated with polypharmacy, even in older adults with diverse backgrounds.
Fourth, a causal relationship was not described. Because the study was a cross-sectional study, our findings could not identify causal relationships. However, personality traits influence lifestyle [24, 40, 44] and behavior [30, 43]. A previous study of depressed patients found that the use of medication decreased neuroticism, increased extroversion and reduced depression . However, these results are inconsistent with the results of our study. Approximately 50% of personality traits are attributable to additive genetic effects , and personality is stable after 30 years of age . Therefore, the effect of polypharmacy on personality is likely to be small. Longitudinal research is required to clarify causal relationships in the future.
Nevertheless, our results suggest that personality traits may be involved in the process of disease outbreaks or behaviors toward treatment. The relationship between personality traits and various emotional reactions and behaviors is a common phenomenon. Therefore, understanding the personality traits of individuals will be helpful in effective treatment and health interventions in independent community-dwelling older adults. In particular, personality traits may be beneficial in the prevention and management of unhealthy lifestyles, such as unbalanced dietary habits, physical inactivity, smoking, and lifestyle-related diseases caused by unhealthy lifestyles. More specifically, information on the patient’s personality traits may contribute to (1) diagnosis that considers unhealthy lifestyles and illness trends, (2) treatment plan that considers the patient’s request and attitude toward treatment, communication style, and support status, and (3) intervention design suitable for each personality trait, such as group and individual interventions.
The study results showed that higher levels of neuroticism and lower levels of extraversion in men and women, respectively, are associated with polypharmacy. Therefore, to avoid polypharmacy, it is necessary to prevent and improve unhealthy habits and mental health deterioration in men with higher levels of neuroticism or women with lower levels of extraversion. In addition, since patients with higher levels of neuroticism tend to require more medications and treatments, it is necessary to control anxiety and prevent excessive medical care in patients. Furthermore, since lower levels of extraversion involve poor communication or support, it is necessary to understand patients' symptoms by creating an environment that enhances communication and provides care considering the support environment of the patients.