A topical pharmaceutical formulation comprising 0.5 ppm to 1000 ppm of a tertiary amine oxide of general formula I ##STR00001## wherein R.sub.1 is a C.sub.8-C.sub.18alkyl, and R.sub.2 and R.sub.3 are independently selected from a C.sub.1-C.sub.4alkyl or C.sub.1-C.sub.4hydroxyalkyl. The tertiary amine oxide functions as a preservative or enhances the preservative efficacy in such formulations developed for topical administration. In particular, the topical formulation is an ophthalmic composition.