The study cohort consisted of individuals who initiated repeated use of opioid analgesic prescriptions for a pain problem. Specific inclusion criteria were (i) adults aged 18 or over initiating a new episode of opioid use (no opioid prescriptions filled in the previous 6 months) between 1997 and 2005 (inclusive), (ii) 3 or more prescriptions for opioid analgesics in the first 90 days of the episode, and (iii) a diagnosis of a non-cancer pain problem from the prescribing physician in the two weeks prior to the initial opioid prescription. Eligible pain diagnoses were back or neck pain, osteoarthritis, headache, extremity pain, abdominal pain / hernia, menstrual pain, temporomandibular disorder pain and fractures / contusions / injuries. Individuals entered the study cohort on the 90th day of their episode, once eligibility was established, and continued to be included in the cohort whether or not they continued to receive prescriptions for opioid analgesics.
Exclusion criteria were (i) individuals with a cancer diagnosis (except non-melanoma skin cancer) in the Cancer Surveillance and End Results Registry up to the end of 2006, (ii) two or more cancer diagnoses (excluding non-melanoma skin cancer) from visit or hospital data between the episode start date and the date of censoring, (iii) individuals not enrolled for at least 270 days in the one-year period prior to study cohort entry. Persons who disenrolled from GHC after baseline were censored on their date of disenrollment, otherwise they were censored on December 31, 2006, the end of the study observation period.