Background and aims Extended‐release formulations of naltrexone have emerged as effective treatment options for opioid use disorder. This post‐hoc analysis examined the temporal relationship between episodes of opioid use and subsequent dropout in a placebo‐controlled trial of extended‐release injection naltrexone (XR‐NTX) to draw inferences about the mechanism by which extended blockade of opioid receptors translates into clinical effectiveness.

Design This was a 24‐week multiple‐site, double‐blind, randomized trial of monthly XR‐NTX versus placebo injections. We analyzed time to dropout from treatment using survival analysis with an extended Cox model as a function of treatment (XR‐NTX versus placebo) and with weekly urine drug test (UDT) results for opioids at each week as a time‐dependent covariate.

Setting Thirteen addiction treatment programs in Russia, 2008–09.

Participants A total of 250 adults with opioid use disorder who had completed in‐patient detoxification.

Intervention XR‐NTX injection or placebo injection every 4 weeks with weekly clinic visits and biweekly counseling.

Measurements Urine toxicology for opioids measured weekly and week of dropout from treatment.

Findings The Cox model yielded a significant interaction of time‐dependent urine toxicology by treatment (P = 0.024). Among patients receiving placebo, a positive UDT in a given week increased the risk for dropout from treatment in the subsequent week [hazard ratio (HR) = 6.25; 95% confidence interval (CI) = 3.6–10.0], whereas among patients receiving XR‐NTX, a positive UDT result showed no significant effect on risk for dropout (HR = 1.67; 95% CI = 0.6–4.5). The proportion of patients who completed all 24 weeks without any positive UDT result was 31% on XR‐NTX compared with 20% on placebo (P = 0.051).