The opioid epidemic has been associated with rising hepatitis C virus (HCV) infection incidence in North America among those who inject drugs. While opioid agonist therapy may reduce HCV infection risk, the role of dosage of such therapy on this association is unclear. Given an increasing focus on global eradication of HCV infections and use of opioid agonist therapy, this prospective observational study aimed to examine the association between the prescribed dose of opioid agonist therapy and patient-perceived dosage adequacy with the risk of HCV infection among 513 adult intravenous drug users in Montreal, Canada. Participants were tested for HCV antibodies or RNA and were administered a behavioural questionnaire regarding opioid agonist therapy exposure, prescribed dosage and perceived dosage adequacy, on a 6-month basis initially, followed by a 3-month basis. Researchers found an incidence of 11.8 acquired hepatitis C infections over 100 person-years, with 168 out of 513 participants infected. The relative risk of HCV infection acquisition varied across opioid agonist treatment dosages, with those receiving high dosages (adjusted HR 0.43, 95% CI 0.23 to 0.84) and dosages perceived to be adequate (adjusted HR 0.61, 95% CI 0.25 to 1.50) having the lowest risk. Those receiving low doses perceived to be inadequate having the highest risk (adjusted HR 1.94, 95% CI 1.11 to 3.39) compared to intravenous drug users not on opioid agonist therapy. The results of this study therefore suggest that the risk of HCV infection varies considerably with dosage of opioid agonist treatment and patient-perceived adequacy of dose.
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