The first great English work on the subject, widely published and used in England and America. Pereira was Professor of Materia Medica at the School of Pharmacy set up by the Pharmaceutical Society of Great Britain.  Digital facsimile of the first edition from  the Hathi Trust at this link.

Pereira revised and expanded this work through four editions, the last of which appeared the year after his death. By its fourth edition the work was expanded to two volumes in three and the title was also expanded, somewhat grandiosely, to read "Fourth edition, enlarged and improved, including notices of most of the medicinal substances in use in the civilized world, and forming an encyclopedia of materia medica."  Digital facsimile of the fourth edition from the Hathi Trust at this link.