The Impact Of Water Activity On The Chemical and Physical Stability Of Oral Solid Dosage Products

By Paula Bracco, PhD., Study Manager at Lighthouse Instruments

Generally, a variety of specific tests are necessary to evaluate the chemical and physical characteristics of an oral solid dosage (OSD) throughout the product life cycle. Water activity is a function of temperature and defines the fraction of water that is loosely bound in the drug product; or in other words “active” and available to participate in unwanted reactions. The product stability can therefore be defined in terms of water activity and adding water activity testing into stability protocols can be a useful tool.

In addition to defining product stability, water activity can also identify detrimental effects of moisture to the physical stability of the OSD, even when the chemical stability remains within specification. This article will present three case studies to show that direct water activity measurements can determine the impact of moisture on potency and dissolution.