This study was conducted to identify the alcohol consumption among hepatitis C-positive people receiving opioid maintenance therapy using self-report and biomarkers. A total of 49 people (28 male, 21 female) were hepatitis C virus (HCV) positive and were included. The alcohol use disorder identification test (AUDIT) and self-reported ethanol intake in the last 28 days were assessed. In addition to gamma-glutamyl-transferase (GGT) and mean corpuscular volume (MCV), ethyl glucuronide (EtG) and ethyl sulphate (EtS) were determined in serum and urine (UEtG, UEtS, SEtG) using liquid chromatography/tandem mass-spectroscopy (LC/MS-MS) with deuterated internal standards. Abstinence from alcohol was reported for the last 28 days by 13 participants and for the last 7 days by 22. AUDIT was > 8 in 27 cases. The maximum values were 34.8 mg/l for UEtG, 5.3 mg/l for UEtS and 0.15 for SEtG. Among the 19 UEtG positives, 8 had not reported any ethanol intake in the 7 days prior to the study. Six participants reported intake of up to 320 g of ethanol in the last 7 days, but were negative for SEtG, UEtG and UEtS. Self-reported ethanol intake in the last 28 days correlated with AUDIT score (r = 0.733, P < 0.001), with the direct ethanol metabolites and MCV. In this population, abstinence and episodic heavy drinking are more common than in the general population. Episodic heavy drinking is a significant cause of acute risk in this population. Results from biomarker testing could indicate cases of under- as well as over-reporting of alcohol consumption. Further research on the diagnostic accuracy of direct ethanol metabolites, including the use of phosphatidylethanol (PEth), in this setting is needed.