ABSTRACT
In the previous chapters we only considered a drug product with a single active ingredient. In practice, while most drug products consist of a single active ingredient, some drug products contain multiple active ingredients (see e.g., Pong and Raghavarao (2001) Chow, Pong, and Chang, 2006). For example, Premarin (conjugated estrogens, USP) is known to contain at least five active ingredients: estrone, equilin, 17α-dihydroequilin, 17α-estradiol, and 17β-dihydroequiliin. Other examples include combinational drug products, such as the traditional Chinese medicines (see, e.g., Stefan and Chantal, 2005; Chow, Pong, and Chang, 2006). For a drug product with multiple active ingredients (or components), an ingredient-by-ingredient (or component-by-component) stability analysis may not be appropriate, since these active ingredients may have some unknown interactions.
