Deer fly repellent is a product used to protect humans and animals from deer fly bites. Two common repellents used by humans are Deet and Off. There are also permethrin-based repellents that are safe for use for both animals and humans.
Deer fly repellents generally work for several hours, although repeated use sometimes causes people to develop an allergy to the spray. Permethrin-based repellents are designed for use on clothing as opposed to skin application, and the protection the repellent provides typically lasts longer. Permethrin-based sprays that are designed for livestock and horses are applied directly to the animal's skin. The repellent typically causes deer flies to fly off of the animal as soon as they land, although they still swarm around any areas of the body that are not covered by the spray.