This book addresses a formulation problem with a moisture sensitive drug, flucloxacillin sodium, in capsule dosage forms by proposing a means of formulating a stable product. It contains invaluable information for pharmaceutical companies that produce capsules of flucloxacillin sodium especially in areas and countries around the globe with high humidity which can serve as a threat to the stability of the drug. The method described here can be extended to other moisture sensitive pharmaceutical ingredients that are formulated with the excipients used in the study into capsules and possibly other dosage forms. Furthermore, it provides a simple and cheap validated RP-HPLC method for the assay of flucloxacillin sodium in capsules.