Study eligibility criteria
We included full-text English-language studies
evaluating LTOT as an exposure or
outcome in observational studies using routinely collected data
published between January 2000 and July 2019.
We excluded studies according to design (e.g., systematic reviews,
clinical trials, editorials, abstracts), medicine use (e.g., studies
evaluating other analgesics), population (studies conducted exclusively
among populations <18 years old) and setting (those focusing
on opioids use for surgery) and those who did not provide any detail of
how LTOT was measured.