Major depressive disorder (MDD) is a common mental illness characterized by persistent and significant low mood resulting from various factors. It is estimated that more than 300 million individuals worldwide suffer from MDD (World Health Organization, 2023). The COVID-19 pandemic has further contributed to the global burden of MDD, with a 28 % increase in its incidence observed in 2020 (Herrman et al., 2022; Xiong et al., 2020). In China, the incidence of depression has risen rapidly, with 3.4 % of the population affected. Approximately 95 million individuals are currently suffering from depression, representing a substantial disease burden (Huang et al., 2019). MDD has a poor prognosis, with a high recurrence rate of 50–85 % (Dell'Osso et al., 2020). Antidepressant therapy is the primary treatment for MDD; however, the efficacy of antidepressants is greatly compromised by poor adherence to the prescribed medication regimen. Previous studies have indicated that non-adherence to medication regimen is a significant contributor to the poor prognosis for MDD (Marasine et al., 2021).

Evidence suggests that early, adequate antidepressant dosing and good adherence are both associated with recovery from MDD and may improve long-term outcomes in depressed patients (Sirey et al., 2017). Non-adherence could make the patient's condition worse, reduce the effectiveness of treatment, or make them less responsive to subsequent treatment. Other consequences of medication non-adherence include hospital readmission, poor quality of life or psychosocial outcomes, symptom recurrence, increased comorbidities, wasted medical resources, and increased incidence of suicide (Iseselo et al., 2016; Moritz et al., 2013a; Semahegn et al., 2020a; Tesfay et al., 2013a). Taking these factors together, it is clear that adherence to medication is an essential component in full recovery from MDD and in reducing negative consequences. Societal attitudes and misconceptions surrounding mental health, however, can lead to negative attitudes and discrimination against individuals with mental health conditions (Thornicroft et al., 2016). Attitudinal biases may contribute to a fear of judgment and rejection by others, making it difficult for individuals with MDD to seek help or disclose their condition. It is further noted that the lack of awareness surrounding mental illness leads to inadequate prescribing or poor medication adherence (Griffiths et al., 2014).

Stigma is the consequence of a number of processes including people conceptualizing what mental illness means (Elzamzamy et al., 2021; Kao et al., 2016; Krill et al., 2019), with a distinction to be made between internalized stigma (IS) and perceived stigma (PS) (Brohan et al., 2010; Vauth et al., 2007). Perceived stigma is the awareness of societal negative beliefs and attitudes towards mental illness, and IS reflects a more severe level of stigma, where individuals agree with PS, which is consequently reflected in their self-image (Firmin et al., 2016a). Stigma related to MDD may engender barriers for patients, including, for example, believing that they do not require medication treatment, fearing medication dependence, and being embarrassed to take medications. These may result in non-adherence to medication, as patients may perceive the effects of the medication as unnatural and report feeling better after discontinuing its use (Semahegn et al., 2020b).

Stigma among individuals with MDD constitutes a significant challenge for medication adherence, highlighting the need to investigate the factors that contribute to the development and perpetuation of stigma (Gerlinger et al., 2013). According to a recent American report, negative attitudes towards mental illness and its perceived association with violence or danger increased between 1996 and 2018 (Pescosolido et al., 2019). Moreover, according to the findings of Dubreucq et al. (Dubreucq et al., 2021), regions with collectivist cultures, such as China, reported relatively higher levels of internalized stigma than Western countries (Xu et al., 2017). Previous studies have primarily focused on schizophrenia and have been conducted in Western countries. Extant studies have explored factors associated with IS and PS in the general population, such as cultural background and demographic variables, including age, gender, education, et cetera. Research on factors associated with IS and PS related to depression in persons with MDD in China is relatively scarce. Consequently, it is a research priority to characterize the extent of stigma, along with its associated factors and influence on health behaviors and outcomes in individuals living with MDD in China (Papadopoulos et al., 2013).

Herein, we had two interrelated aims: to evaluate the extent of IS and PS in persons living with MDD; and to assess the association of IS and PS with medication adherence based on a longitudinal cohort of persons living with MDD in China.