Prescription opioid use is highly prevalent and may be replacing heroin as the predominant illicit opioid that is used. Little is known about specific prescription opioid use characteristics, or issues faced by these individuals in treatment. The major aims of the two studies comprising this thesis were: 1) to systematically and quantitatively assess different occasions of use for the prescription opioid hydromorphone; and 2) to evaluate how current substance use and psychiatric symptoms may be related in a population of prescription opioid users enrolled in a low-threshold Methadone Maintenance Treatment (MMT) program. Eighty-two participants from a low-threshold MMT program in Halifax, Nova Scotia were interviewed regarding their lifetime and current substance use, specific past occasions of hydromorphone use, and current psychiatric symptoms. A subsample of 26 participants was interviewed a second time, one day later, to assess reliability of participants’ self-report on the above-mentioned study measures. It was found that many variables were reliably reported between the two interviews by the subsample. With regards to the first major thesis aim, hydromorphone was found to be a prevalent, highly-favoured prescription opioid in the sample. Characteristics of initial, but not later, hydromorphone use varied by prescription status at initiation. Later use of hydromorphone shared many characteristics previously documented with heroin. With regards to the second major thesis aim, participants reported high rates of current substance use and psychiatric symptoms. Current substance use and psychiatric symptoms appeared to be related; notably, non-prescribed benzodiazepine use predicted depression and anxiety symptoms, and general anxiety predicted non-prescribed benzodiazepine use. In summary, while the results may not be representative of all prescription opioid users, or MMT clients, the thesis presented novel findings with a unique and vulnerable population. The findings supplement the existing literature in terms of describing how prescription opioids may be used during specific occasions, and in describing psychiatric and substance use issues faced by prescription opioid users enrolled in low-threshold MMT in Nova Scotia.