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.