I used one of these in Mazatlan on my first trip south while I recovered in a cheap motel for a week. The one there was a Farmacia Similares where the doctor had a small office off to the side of "his" farmacy. His consulting fee is only 25-40 pesos and then you go next door and spend 80-100+ in the pharmacy.
Lorena went to this one on Ramon Corona the other day and was told she needed a blood analysis. We checked with the laboratory near Dentista Woo and the cost was $350 pesos. With her Seguro Popular ID she goes next door to Centro de Salud and is referred to a free laboratory near the "Happy Cow" on Benito Juarez. Today she'll pick up the lab results and go back to the doctor above. I assume he will prescribe something from his pharmacy.