Findings show that illicit street-drug markets are involved in promoting increased levels of nonmedical PO use among street-drug users in Canada; however, there is an indication of a separation between PO markets and markets for traditional illicit street drugs. This market separation may be due to the sources for different drug groups. Illicit drugs come mainly from illegal drug importation and distribution networks; whereas POs originate from their diversion from the medical system, either by dealers directly receiving POs through prescriptions or by runners who offload POs on behalf of others. The regular PO users in this sample, especially in Toronto, did not prefer to obtain POs directly from the medical system. They were more interested in using regular and reliable indirect sources (e.g., dealers). This may be due to the effects of intensifying safeguards against PO diversion from the medical system. The study was conducted between June and August 2007. Persons were eligible for the study if they were regular users of POs and were at least 18 years old. The interview consisted of a uniform interviewer-administered questionnaire that consisted primarily of closed- and some open-ended items that pertained to sociodemographic, drug-related income and expenditures, past and current drug use, details on opioid- and non-opioid-drug sources, and drug selling/trading activities. 2 tables, 1 note and 49 references