Medicinal plants have been used for many years and are the source of new active substances and new drugs of pharmaceutical interest. The popular knowledge contained in the open-air markets is studied through urban ethnobotany, and is a good source of information for ethnobotanical research. In this context, we surveyed the literature on works concerning open-air markets in the State of Rio de Janeiro to gather knowledge of the commercialized plants therein. A literature search resulted in ten studies with 376 listed species, distributed in 94 families and 273 genera. Asteraceae family had the greater representation, followed by Lamiaceae and Fabaceae. Solanum was the most frequent genus. Two hundred and twenty four species could be considered potentially toxic or potentially interact with other drugs/medicines. Eighteen species are referred as "not for use during pregnancy", and 3 "not for use while nursing". These results are a source of concern since in Brazil, as it is worldwide, there is the notion that plants can never be harmful. The results for the Sørensen Coefficient showed greater similarity between works performed in very close study areas. Other studies presented low similarity, mainly because of the difficulty in plant identification or a very specific focus in methodology.