2.4 SSRI exposure
For each patient, we calculated the average daily doses of each individual SSRI; the prescribed doses in milligrams during the follow-up period was assigned as the numerator, and the follow-up period was assigned as the denominator. The average daily dose was further divided by the milligram in a DDD form according to World Health Organization classification system to standardize the different potency of inhibitors. Cumulative DDD (cDDD) was defined as the total number of DDDs prescribed during the follow-up period and was calculated to evaluate the effects of accumulated doses[22]. To observe the associations with low doses and the potential dose-response relationships, we classified all patients into low-, medium-, and high-dose SSRI users based on the tertiles of the distribution of cDDDs in the control group. Furthermore, to assess whether the risks of UGIB differed by exposure time window, the total number of days from the last SSRI prescription to index date were counted and categorized into four levels (<14, 15–30, 30–90, and >90 days) representing current use, recent use, recent past use, and remote use, respectively.