and after a patient’s first drug-related poisoning (index date), thus limiting participants to two-year periods of observation to reduce heterogeneity in observation time per person. We included periods both before and after the index poisoning so as to define the poisoning as a repeatable event, allowing for the inclusion of time as a covariate.(25, 26) Thus, the unit of analysis was days, stratified within persons, and the comparison of interest was drug exposure status concurrent with drug-related poisoning (case period) versus exposure status during referent periods when poisoning events did not occur (control period).
