The disclosure relates to topical formulation that inhibits the growth of microorganisms, comprising an effective amount of an insecticide dissolved in an oil phase comprising a water-miscible or water-soluble surface active agent, a suspending agent, and a non-ionic surfactant, and an aqueous phase comprising one or more preservatives, wherein the aqueous phase is buffered to a pH at which the formulation inhibits a greater amount of microorganism growth relative to the equivalent formulation in which the aqueous phase is not buffered. This formulation may be used to treat lice infestation.
