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.