OBJECTIVES: Therapeutic duplicate (TD) is one of the most frequently reported inappropriate drug use. This study was conducted to estimate the prevalence of therapeutic duplications in ambulatory setting in Korea and to determine the patient, physician and visits characteristics associated with TD of NSAIDs. METHODS: The prevalence of TD of NSIADs was examined by analyzing Korea’s National Health Insurance claims database from January to March 2011. KFDA TD classification was used as the TD criteria which includes 64 ingredients. Multivariate logistic regression was used to determine predictors of the prescribing of TD at ambulatory care visits. Independent variables in the regression model included patient characteristics (gender, age, insurance type, comorbidity), physician characteristics (type of medical institution, specialty), and visit characteristics (the number of drugs). RESULTS: Overall 59,636,222 ambulatory care visits of 21 million patients were identified in the 1st quarter of 2011. And of these visits, 13.3% were involved in TD. The most frequent duplication were occurred between diclofenac and aceclofenac (12.4% of TD), diclofenac and talniflumate (11.2%), and diclofenac and loxoprofen sodium (10.7%). The patient characteristics associated with an increased likelihood of TD were male (OR=1.070 (95% CI=1.069-1.072), older age (18-39 years OR=1.055, 95% CI=1.051-1.059; 40-64 years OR=1.257, 95% CI=1.253-1.262; 65-74 years OR=1.283, 95% CI=1.278-1.288; 75 years older OR=1.231, 95% CI=1.226-1.236), and charlson comorbidity index ≥ 1 (OR=1.075, 95% CI=1.071-1.078). Physician characteristics associated with increased likelihood of TD were orthopedics (OR=2.091, 95% CI=2.086 - 2.096) and pediatrics (OR=1.314, 95% CI=1.308-1.320). And the number of drugs per prescription was associated with an increased likelihood of TD (4-6 OR=2.054, 95% CI=2.050-2.058; ≥7 OR=2.695, 95% CI=2.687-2.704). CONCLUSIONS: This study is the first study which demonstrated the prevalence of therapeutic duplication among NSAIDs and associated factors in Korea. And these results could contribute to both preventing therapeutic duplication and improving appropriate medication use.