The purpose of this study is to assess the epidemiology of Treatment Resistant Depression (TRD) in the nationally insured population in Taiwan including incidence, duration of clinical episodes, and prevalence by age and sex.

This is a retrospective study of approximately one million subjects randomly selected from an anonymized database National Health Insurance Research Database (NHIRD). Participants aged 18 years or older who have had neither a depression diagnosis nor a dispensing of an antidepressant medication in the last four months of 2004 will enter the study when they receive in 2005 a depression diagnosis and a dispensing of an antidepressant medication within 30 days of each other and will be followed up to 8 years or until they have 4 months with neither a depression diagnosis nor a dispensing of an antidepressant medication. Treatment Resistant Depression (TRD) Incidence, TRD prevalence by age and sex and duration of clinical episodes will be estimated.

Inclusion Criteria:

• Participants who have been continuously enrolled in the insurance plan since January 1, 2004 (ignoring breaks of less than 30 days) and have not received an exclusion diagnosis and during the last 4 months of 2004, a) Have not received a diagnosis of depression and b) Have not received a dispensing of an AD medication
• Participants with a diagnosis of a depressive disorder including dysthymic disorder (International Classification of Diseases [ICD]-9 codes 296 depression-related, 300.4, 311) and Major Depressive Disorder (ICD-9 296.2, 296.3) will be followed through December 31, 2013

Exclusion Criteria:

- Participants who receive following diagnosis will be excluded, a) Mania, b) Schizophrenia (ICD-9 codes 295), c) Bipolar disorder (ICD-9 codes 296 bipolar-related), D) Dementia (ICD-9 codes 290 or 294); Note: Major depressive disorder (MDD) with psychotic behavior (ICD 296.24 for single episode, ICD 296.34 for recurrent episode) is not an exclusion

Adults aged 18 and older are randomly sampled from the National Health Insurance Research Database (NHIRD).