The XIXth century witnessed the appearance of the first pharmacopoeias in the american continent; the United States Pharmacopoeia (1820) and the Farmacopea Mexicana (1846). These texts are the only ones that are reviewed and republished regularly in this continent. In this paper some of the circumstances surrounding their publication are discussed and a comparison between their last editions is laid out. Medicinal plants and the monographs dedicated to them in both texts are carefully considered because it is here that the mayor differences between these two codes arise. Their similarities and differences are discussed as well as their perspectives for the future. The study of the different editions of the mexican pharmacopoeia clearly show that after the publication of the 1930 text, a progressive exclusion of medicinal plants and their drugs takes place. These products were substituted in the official therapy by synthetic drugs. This process continued during the whole century until the arrival of the new millennium and a new edition of the text which included a new volume called Farmacopea Herbolaria de los Estados Unidos Mexicanos and implies a step in a different direction. This text includes a number of monographs of medicinal plants and their products and tries to rescue these materials form oblivion and give them a place in official therapy again. This tendency is in accordance with the American and European texts which have always included them in their pages.