Association of injury after prescription opioid initiation with risk for opioid-related adverse events among older Medicare beneficiaries in the United States: A nested case-control study

Background
Injury, prevalent and potentially associated with prescription opioid use among older adults, has been implicated as a warning sign of serious opioid-related adverse events (ORAEs) including opioid misuse, dependence, and poisoning, but this association has not been empirically tested. The study aims to examine the association between incident injury after prescription opioid initiation and subsequent risk of ORAEs and to assess whether the association differs by recency of injury among older patients.

Methods and findings
This nested case-control study was conducted within a cohort of 126,752 individuals aged 65 years or older selected from a 5% sample of Medicare beneficiaries in the United States between 2011 and 2018. Cohort participants were newly prescribed opioid users with chronic noncancer pain who had no injury or ORAEs in the year before opioid initiation, had 30 days or more of observation, and had at least 1 additional opioid prescription dispensed during follow-up. We identified ORAE cases as patients who had an inpatient or outpatient encounter with diagnosis codes for opioid misuse, dependence, or poisoning. During a mean follow-up of 1.8 years, we identified 2,734 patients who were newly diagnosed with ORAEs and 10,936 controls matched on the year of cohort entry date and a disease risk score (DRS), a summary score derived from the probability of an ORAE outcome based on covariates measured prior to cohort entry and in the absence of injury. Multivariate conditional logistic regression was used to estimate ORAE risk associated with any and recency of injury, defined based on the primary diagnosis code of inpatient and outpatient encounters. Among the cases and controls, 68.0% (n = 1,859 for cases and n = 7,436 for controls) were women and the mean (SD) age was 74.5 (6.9) years. Overall, 54.0% (n = 1,475) of cases and 46.0% (n = 1,259) of controls experienced incident injury after opioid initiation. Patients with (versus without) injury after opioid therapy had higher risk of ORAEs after adjustment for time-varying confounders, including diagnosis of tobacco or alcohol use disorder, drug use disorder, chronic pain diagnosis, mental health disorder, pain-related comorbidities, frailty index, emergency department visit, skilled nursing facility stay, anticonvulsant use, and patterns of prescription opioid use (adjusted odds ratio [aOR] = 1.4; 95% confidence interval (CI) 1.2 to 1.5; P < 0.001). Increased risk of ORAEs was associated with current (≤30 days) injury (aOR = 2.8; 95% CI 2.3 to 3.4; P < 0.001), whereas risk of ORAEs was not significantly associated with recent (31 to 90 days; aOR = 0.93; 95% CI 0.73 to 1.17; P = 0.48), past (91 to 180 days; aOR = 1.08; 95% CI 0.88 to 1.33; P = 0.51), and remote (181 to 365 days; aOR = 0.88; 95% CI 0.73 to 1.1; P = 0.18) injury preceding the incident diagnosis of ORAE or matched date. Patients with injury and prescription opioid use versus those with neither in the month before the ORAE or matched date were at greater risk of ORAEs (aOR = 5.0; 95% CI 4.1 to 6.1; P < 0.001). Major limitations are that the study findings can only be generalized to older Medicare fee-for-service beneficiaries and that unknown or unmeasured confounders have the potential to bias the observed association toward or away from the null.

Conclusions
In this study, we observed that incident diagnosis of injury following opioid initiation was associated with subsequent increased risk of ORAEs, and the risk was only significant among patients with injury in the month before the index date. Regular monitoring for injury may help identify older opioid users at high risk for ORAEs.