This new focus on Aw not only means a change in beyond-use dating guidelines, but also a new focus on preservatives and how to select one for a particular compounded preparation. The frequently asked questions page on USP’s website clarifies that these new guidelines do not require compounders to measure Aw for particular compounds and that these markers are intended to be used as a guide.6 It refers to General Chapter 1112 (Application of Water Activity Determination to Nonsterile Pharmaceutical Products) which provides a list of approximated Aw for various products.6,7 If Aw is not tested for a water containing preparation such as a cream or suspension, the Aw should be assumed to be greater than 0.6 and, in the absence of an appropriate study, the beyond-use date should be consistent with the maximum beyond-use date listed in table 3 of new USP 795 guidelines.1 In the absence of explicit data on a particular compound, General Chapter 1112 provides a good resource for justifying an assigned beyond-use date.