Disclosed herein are methods for treating or preventing ophthalmic and dermatologic conditions in a patient, including ocular surface conditions such as blepharitis. The methods can include topically administering directly to an ocular surface of one or more eyes of a patient in need of treatment thereof an effective amount of an isoxazoline parasiticide, formamidine parasiticide, or other active ingredient, formulated into an ophthalmic composition, the ophthalmic composition further comprising a pharmaceutically acceptable vehicle. Compositions are also disclosed.