This new edition consolidates the texts of the five separate volumes of the third edition. In preparing this consolidated edition the opportunity has been taken to review the General notices of the Pharmacopoeia. Certain additions and amendments have been made to the notices in order to clarify the interpretation of the Pharmacopoeia and to facilitate application of the requirements by the user. The opportunity has also been taken to improve certain aspects of the layout and format of the publication. In this Fourth edition all the monograph texts are brought together in one section and the method texts in another. Each of these major sections are divided into appropriate sub-sections and the method texts are numbered for ease of cross-reference. New monographs for the following antiretroviral substances are published in the Fourth edition: didanosine, indinavir sulfate, nelfinavir mesilate, nevirapine, ritonavir, saquinovir, and saquinovir mesilate. Revision of the current monograph for Oral rehydration salts has been carried out to conform to the modified formula published in the 13th Model List of Essential Medicines (WHO Technical Report Series No. 920 2003) and in the WHO Model Formulary 2004. The revised formula has a reduced sodium chloride and glucose content providing a solution with a reduced osmolarity of 245 mOsm/l. Due to the improved effectiveness of the reduced osmolarity ORS solution, especially for children with acute non-cholera diarrhea, WHO and UNICEF now recommend that countries use and manufacture this formulation in place of the previously recommended ORS, i.e. the one published in the third edition of The International Pharmacopoeia, which has a total osmolarity of 311 mOsm/l. Method texts that have been updated include, for example, the text on high performance liquid chromatography HPLC]. This has been revised to clarify certain technical terms and to add advice on adjustment of chromatographic conditions, as recommended by the WHO Expert Committee on Specifications for Pharmaceutical Preparations in October 2004. CD-ROM version: The International Pharmacopoeia CD-ROM