Two different labelling conventions for the contents of colistin methanesulfonate (i.e. colistin base activity [CBA] and international unit [IU]) are used in different parts of the world, and have caused prescribing errors and patient safety issues. This chapter discusses the key issues on the conversion between CBA and IU, and highlights that in pharmacokinetic analyses only the absolute mass of the chemical colistin methanesulfonate should be employed, but not the CBA or IU values. The scientific evidence is unknown for the limits specified for the pharmacopeial standards of the major components of colistin methanesulfonate and polymyxin B. The package information of parenteral colistin methanesulfonate in Europe has now been significantly improved by incorporating the latest pharmacokinetic/pharmacodynamic data. However, the current package information of almost all different brands of parenteral polymyxin B products is substantially out of date without solid pharmacological data. Updating the package information of different products of both polymyxins requires the coordination between major regulatory authorities and will significantly facilitate the optimisation of their use in patients.