To determine the extent to which other opioids are prescribed to patients receiving methadone in Ontario, Canada. Retrospective cohort study. Ontario, Canada from 1 April 2003 to 31 March 2010. We studied patients aged 15-64 years with publically funded drug coverage who received at least 30 days of continuous methadone maintenance therapy (MMT). The proportion of patients who received more than 7 days of a non-methadone opioid during MMT. A secondary analysis examined the extent to which non-methadone opioids were prescribed by physicians or dispensed by pharmacies not involved in a patient's MMT. Among 18,759 patients treated with methadone, 3456 (18.4%) received at least one prescription for non-methadone opioids of more than 7 days' duration. In this group, the median number of non-methadone opioid prescriptions dispensed per year was 11.9 (interquartile range 4.1-25.0). The most frequently prescribed opioids were codeine and oxycodone. Of the 73,520 non-methadone opioid prescriptions of more than 7 days' duration, nearly half (45.8%) originated from non-MMT prescribers and pharmacies. Many patients receiving methadone maintenance therapy in Ontario receive overlapping prescriptions for other opioids, often for extended periods. The associated prescribing patterns suggest that many such prescriptions may be duplicitous. The prescribing and dispensing of non-methadone opioids to patients receiving methadone maintenance therapy is likely to be observed in jurisdictions outside Ontario, Canada. © 2011 The Authors, Addiction © 2011 Society for the Study of Addiction.