Patent medicine vendors (PMVs) supply a large portion of the drugs used by the public in African countries to treat their illnesses. Little has been reported about what actually transpires between PMVs and their customers, but nevertheless, concerns have been raised about the potential for abuse of their position. This study conducted 720 observations of PMV-customer interaction in 444 medicine shops in both the metropolis of Ibadan and the rural town of Igbo-Ora in Oyo State, Nigeria. Each interaction lasted 2 minutes on average. A quarter of the customers shared their illness problems with the shop attendant, 9% presented a prescription and the majority simply requested items for purchase. Most customers (73%) were buying drugs for themselves, while the remainder had been sent to purchase for another person. The former were more likely to be adults, while the latter were more often children and adolescents. The most common PMV behaviours are: selling the requested medicine (69%), giving their own suggestions to the customer (30%), asking questions about the illness (19%) and providing instructions on how to take the medicine (21%). Only three referrals were observed. The large number of specific drug requests was evidence of a public that was actively involved in self-care, and thus the major role of the PMV appeared to be one of salesperson meeting that need. A second role became evident when the customer actually complained about his/her illness, a practice associated with the more active PMVs who asked questions, gave suggestions and provided information. These PMV roles can be enhanced through consumer education, PMV training and policy changes to standardize and legitimize PMV contributions to primary health care.