Some of the topical drops applied to pets for insect control, often on the back of the neck contain a chemical called "perethrin". I used to work at a pet store, and read the labels more carefully than most customers. I don't know what the difference, if any, there is between Permethrin and "perethrin". I used to consider it a typo, but it may not be. Anyway, whatever it is, it gets absorbed into your pet's blood and lymphatic system and is carried throughout the body to repel insects and ticks for a prescribed period. I'm not inclined to put potentially harmful chemicals on my own body. On pets, with a much shorter lifespan, it has probably been taken into consideration that, if it shortens their lifespan due to the toxic nature of the medication, it probably prevents a much earlier (and painful) death due to contracting Lyme and other diseases, and prevents pets from bringing the little hitchikers into your home and exposing you to those same diseases.