Covariates and matching
The baseline characteristics were age, gender, hypertension (ICD-9-CM=401-405), chronic liver disease (ICD-9-CM=571), hyperlipidemia (ICD-9-CM=272.0-272.4), diabetes (ICD-9-CM=250), chronic kidney disease (ICD-9-CM=585), chronic obstructive pulmonary disease (COPD) (ICD-9-CM =491, 492, 496), cardiovascular disease (ICD-9-CM=410-414), stroke (ICD-9-CM=430-438), cancer (ICD-9-CM=140-208), and autoimmune disease (ICD-9-CM=710.0, 720.0, 714.0). The comorbidities were defined as occurring within one year prior to the index date with outpatient visits at least three times or once hospitalization. In addition, corticosteroids and antihistamines during the study period were contained and defined as usage for at least ≧30 days.
Then, propensity score matching based on age, gender, hypertension, hyperlipidemia, chronic liver disease, chronic kidney disease, diabetes, COPD, cancer, cardiovascular disease, stroke, autoimmune disease, corticosteroids, and antihistamines was performed in order to balance the heterogeneity of the two groups. The propensity score was considered a probability that was estimated through logistic regression. The binary variable was the constipation group and non-constipation group.