The number of patients with chronic diseases, such as diabetes and cardiovascular disease, is increasing in developed countries . Increasing severity of chronic diseases causes greater medical expenses  and poorer quality of life . Patients with long-term conditions also need support to prevent problems from developing and avoid having to manage complications .
Medication is very important for the treatment of chronic diseases. However, many patients have challenges with maintaining constant medication regimens according to instructions [4, 5], which results in low medication compliance rates [6,7,8]. The average level of medication adherence among patients with chronic diseases in developed countries is only 50% [7, 9, 10].
A report on patients with chronic diseases, such as diabetes and hypertension, showed that medication was only effective in one-third of patients because it was incorrectly administered [11, 12]. Other reasons why medication may be ineffective include unnecessary preventive medication [5, 13] and incorrect prescribing. For example, patients are given short-term treatment when in fact long-term medication is needed .
According to Morisky, correct ongoing use of medication requires patients to understand the necessity of their medication [6, 14] as well as the risks of their disease and why medication is important [5, 14]. According to Hulka and Svensson, good patient–healthcare provider relationships [15, 16] are important for medication adherence. Haynes stated improving medication adherence requires adequate social support . The World Health Organization (WHO) highlights the need for patient consent to and participation in their treatment ; this is similar to Kamishima, who pointed out the importance of patient agreement with their treatment . These reports suggest that it is crucial to clarify the various psychosocial factors related to self-management of medication in patients with chronic diseases. This is considered to be beneficial for supporting the ongoing use of medication by people living with chronic diseases [5, 6].
The WHO has suggested that discussion between patients and healthcare providers is an important psychosocial aspect of medication support and treatment decision-making . The WHO guidelines note that promoting patient participation depends on valid and reliable measurement of the adherence construct. Patients’ participation in decisions about medication requires good patient–healthcare provider communication. Strong emphasis is placed on the need to differentiate adherence from compliance. The main difference is that adherence requires the patient’s agreement with recommendations. The adherence concept has adopted the definition of adherence to long-term therapy as “the extent to which a person’s behavior corresponds with agreed recommendations from a healthcare provider” .
In Japan, Kamishima defined medication adherence as “the extent to which patients understand their diseases and treatment thoroughly, participate positively, and accomplish their medication behavior in line with agreed recommendations”. This definition attempts to combine behavioral and psychological aspects . Kamishima also stated that ideal medication use, based on her concept of adherence, is when patients continue to take their medication because they understand that it is necessary, in consultation and collaboration with their healthcare provider about managing their physical condition .
Medication self-management in daily life requires not only patient’s understanding of the need for medication but also good partnerships between the patient and health care provider, choice of the medication that fits into the patient’s lifestyle, and their willingness to take the medication. The presence of all these factors is part of the concept of adherence .
A recent systematic review of adherence scales covered 43 validated scales . Some tools shed light on barriers to adherence such as patient–healthcare provider relationships, self-efficacy, patient’s lifestyle and commitment. The focus of these past scales was not only on compliance but also knowledge, and psychological factors that affected expectations and rejection of medication [6, 14, 21]. However, there are no validated tools to measure the comprehensive concept of medication adherence, including psychosocial factors related to medication behavior, and particularly patient–caregiver relationships and lifestyle factors together.
Therefore, existing scales are insufficient to capture psychosocial aspects including daily life situations and lifestyles, as well as the relationship with the medical provider, to foster effective and continuing support. Considering self-management of medication adherence in daily life by focusing on social aspects, like daily life situation and lifestyle in addition to the psychological side, measurement of the patient’s medication situation can be made from a more multifaceted viewpoint. We therefore developed a 14-item Medication Adherence Scale  (see Additional files 1 and 2), to include not only compliance but also psychosocial factors related to medication behavior, and particularly patient–caregiver collaboration and relationship as well as patient lifestyle. Data collection and a survey were performed in 2009, and the results were published in the Journal of Japan in 2014. This 14-item scale was recognized by various hospital officials and researchers as a measure of medication adherence in Japan. Positive feedback of this measure has been received on an informal basis. Based on the results of analysis when considering the reliability and validity of the 14-item scale, it was thought that some items required modification, to improve the clarity and convenience of the scale. From the results of testing, some items implied a double negative and were difficult for target patients to understand. The inclusion of such items lowered the subscale alpha coefficient. We realized there was a need to further clarify the wording. Because double loading was found in factor analysis, items that were similar to other items were deleted. In addition, some question assessed concepts of another subscale. Therefore, we revised the scale to improve ease of use and accuracy of measurement; we modified the 14-item scale to create the 12-item Medication Adherence Scale. This scale included “medication compliance” and added the two psychosocial factors related to medication behavior: patient–caregiver collaboration, and daily lifestyle. The 12 items were expected to fit into the original four categories, “medication compliance”, “collaboration with healthcare providers”, “willingness to access and use information about medication”, and “acceptance to take medication and how taking medication fits patient’s lifestyle”.
In this study, we modified our original 14-item scale to a 12-item Medication Adherence Scale, with some items more clearly worded, to more accurately measure medication adherence in patients with chronic diseases, and we examined the reliability and validity of the 12-item scale.