physicochemical factors that continue to
challenge the development of drug products 25 years later. Over
the years, it has been recognized that water sorption can affect
powder bulk density, blending, flow and compaction,3–5 capsule
mechanical properties,6 tablet hardness,7–9 disintegration7,10–12
and dissolution,10–14 excipient compatibility,15–17 and chemical
stability.18–20 The moisture properties of pharmaceutical solids
are an important part of package selection.21–23 Many commercial
drug products contain a storage statement on the label
around moisture protection, and may contain a desiccant or
special packaging, underscoring water’s relevance to detrimental
changes that it can produce on storage.
Most drugs are crystalline solids that generally sorb only
a small amount of water from the atmosphere, for example,
0.1%–0.2% water at a relative humidity (RH) as high as 90%.
The reason for the low sorption level is that water molecules