Given the high importance of historical documental sources to better understand the dynamics of current pharmacopoeias, this work investigated the prescription book of the hospital of São Bento Monastery in Olinda (Northeastern Brazil) written by the physician Joaquim Jerônimo Serpa for the years of 1823 and 1824. The main aims of the work were to identify the medicinal plants used in formulations, identify the similarity among prescribed species within these years, access the proportion of plant-based medicines in the prescriptions and to access the contribution of species from the Americas in the formulations. A total of 63.8% of prescriptions had one or more plant species. We found 41 plant species in the prescriptions, most of them being exotic although native species were getting importance in the period. The most cited species were Papaver somniferum L., Rheum officinale Baill., Psychotria ipecacuanha (Brot.) Stokes, Cinchona sp. and Guaiacum officinale L. The similarity between the years was slow (46.5%), because of an increase in the number of prescribed species for the year of 1824. The importance of plants for the medicine at that time was elucidated, but, regarding native species, it is necessary to find out which factors influenced the incorporation of American plants on the official medicine.