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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.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.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.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.