Background: Peru is what Peruvian anthropologist Lupe Camino calls the “health axis” of the old Central Andean culture area stretching from Ecuador to Bolivia. In particular in the North of the country the traditional use of medicinal dates back as far as the first millennium B.C. Both healers, and the wider population, often buy their medicinal plants in local markets, but there is very little comparative information available about which plants are sold under which vernacular name at any given time, for which indication, and which dosage information and information about side effects is given by vendors. For this study we used two traditionally used species groups “Hercampuri” Gentianella spec. (Gentianaceae) and “Pasuchaca” Geranium spec. (Geraniaceae.), found in the Mercado Aviación in Lima, as small, clearly circumscribed plant group frequently used to treat symptoms of diabetes as a test case to study the taxonomy, indications, dosage, indicated side effects, and additional species used as admixtures and hypothesized that: 1. A wide variety of different species is sold under the same common name, and often several common names exist for one species. 2. There is no consistency in the dosage, or a relationship between dosage and species marketed under one name. 3. However, there is consistency in the knowledge about usage and side effects. Methods: Surveys focusing on medicinal plants sold and their properties were conducted at the Mercado Aviación in Lima in December 2012. Vouchers of all specimens were deposited at the National Herbarium of Peru. Results and conclusions: Our surveys in Mercado Aviación in Lima yielded four species of Gentianella, two of Geranium, and three additional species from three genera used as common additives that were sold as anti-diabetic. These results indicate that even in case of only a few plant species, used for a very clearly circumscribed application, patients run a considerable risk when purchasing their remedies in the market. The possible side effects in this case are the more serious because diabetes has to be treated long term, and as such the patients are ingesting possible toxic remedies over a long period of time. Much more control, and a much more stringent identification of the material sold in public markets, and entering the global supply chain via internet sales, would be needed.