DCP is an insoluble inorganic diluent. The anhydrous (triclinic crystal) and dihydrate (monoclinic structure) forms of DCP are used in pharmaceutical development (Rowe et al., 2009). The dihydrate form of DCP presents good flow properties and low hygroscopicity. Based on the temperature (40–50°C) and humidity (32%–75% relative humidity: % RH), however, the dihydrate form tends to lose water of hydration, which can cause chemical instability of APIs in dosage forms (Landín et al., 1994; Miyazaki, Sivaprakasam, Tantry, & Suryanarayanan, 2009). Because of the absence of water in its crystal structure, the anhydrous form of DCP exhibits higher porosity, leading to better compressibility and faster disintegration (Doldán, Souto, Concheiro, Martínez-Pacheco, & Gómez-Amoza, 1995). The dihydrate form transforms (partially or wholly) to the anhydrous form because of the high temperature in the wet granulation process.

Comparison between the two forms of DCP showed that the anhydrous form has better intraparticular porosity and better compressibility than the dihydrate form (Doldán et al., 1995). In addition, because of the higher intraparticular porosity, disintegration of anhydrous DCP is better than the dihydrate form. Both forms of DCP, however, needed a swelling-type disintegrant in the formulation when used (Khan & Rooke, 1976). Different grades of DCP are available, with coarse grade used for direct compression and milled grade for wet granulation or roller compaction. The milled grade has an alkaline pH and cannot be used with API incompatible with basic pH.