0.81 g sodium alendronate trihydrate and 1.12 g sodium pyrophosphate decahydrate (Na4P2O7.10 H2O) were dissolved in 75 mL water. 1.21 g calcium chloride dihydrate was dissolved in 75 mL water. The temperature of the alendronate-pyrophosphate solution was brought to 40 to 50° C. Calcium chloride and sodium hydroxide solutions were simultaneously added to the alendronate-pyrophosphate solution while stirring to keep the pH at 7.4 until all the calcium chloride solution was added. The mixture was cooled to room temperature. The precipitate was collected by filtration and washed with water. The solid was dried in an oven at 105° C. overnight. 1.06 g of calcium complex salt of alendronate with pyrophosphate was thus obtained with a molecular ratio of 1:1.
