The invention provides a pharmaceutical composition of duloxetine or its pharmaceutically equivalent derivatives like salts, isomers, complexes, polymorphs, hydrates or esters thereof and at least one buffering agent. The duloxetine or its pharmaceutically equivalent derivative is present from about 2 mg to approximately 200 mg; and the buffering agent is present in an amount of approximately 0.1 mEq to approximately 2.5 mEq per mg of duloxetine. Also provided is a method for treating of major depressive disorder and or diabetic peripheral neuropathic pain comprising administering to a mammal in need of such treatment a therapeutically effective amount of a composition.