The International Pharmacopoeia - Fourth Edition consolidates the texts of the five separate volumes of the third edition. 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. This fourth edition will include all the monographs in the third edition together with new monographs for didanosine, indinavir sulfate, nelfinavir mesilate, nevirapine, ritonavir, saquinovir, saquinovir mesilate. 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. 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.