Challenges to antagonist blockade during sustained‐release naltrexone treatment

Aims Naltrexone is a competitive opioid antagonist that effectively blocks the action of heroin and other opioid agonists. Sustained‐release naltrexone formulations are now available that provide long‐acting opioid blockade. This study investigates the use of heroin and other opioids among opioid‐dependent patients receiving treatment with long‐acting naltrexone implants, their subjective experience of drug ‘high’ after opioid use, and factors associated with opioid use. Methods Participants (n = 60) were opioid‐dependent patients receiving treatment with naltrexone implants. Outcome data on substance use, drug ‘high’, depression and criminal activity were collected over a 6‐month period. Blood samples were taken to monitor naltrexone plasma levels, and hair samples to verify self‐reported opioid use. Findings More than half [n = 34 or 56%; 95% confidence interval (CI) 44–68%)] the patients challenged the blockade with illicit opioids during the 6‐month treatment period; 44% (n = 26; 95% CI 32–56%) were abstinent from opioids. Mean opioid use was reduced from 18 [standard deviation (SD)13] days during the month preceding treatment to 6 days (SD 11) after 6 months. Of the respondents questioned on opioid ‘high’ (n = 31), nine patients (30%; 95% CI 16–47%) reported partial drug ‘high’ following illicit opioid use, and three (12%; 95% CI 3–26%) reported full ‘high’. Opioid use was associated with use of non‐opioid drugs and criminal behaviour. Conclusions Challenging naltrexone blockade with heroin on at least one occasion is common among sustained‐release naltrexone patients, but only a minority of patients use opioids regularly. Challenges represent a warning sign for poor outcomes and often occur in the context of polydrug use and social adjustment problems.