Abstract: The present invention pertains to a stabilized pharmaceutical composition which comprises a mixture of (a) a pharmaceutical agent unstable at a pH above about 3.5; and (b) a stabilizing amount of an acidic pharmaceutically acceptable carrier to stabilize the pharmaceutical agent. The acidic pharmaceutically acceptable carrier is a dried premixture of a pharmaceutically acceptable carrier and an aqueous solution of an acid. The stabilized pharmaceutical composition has a pH value of less than about 3.5, and when stored at a temperature of about 50.degree. C. for about 4 weeks at about 27% relative humidity, retains at least about 80% of the pharmaceutical agent. This invention also pertains to sustained-release forms of the stabilized pharmaceutical compositions as well as to novel methods for preparing and using the stabilized pharmaceutical compositions and to stabilized pharmaceutical compositions made by the novel method.