Permethrin is an insecticide used in and around U.S. homes and some topical tick preventatives. I just learned that K9 Advantix is one brand that contains this toxin. Permethrin has been classified by the FDA as a possible carcinogen (cancer causing agent).
While other products do, of course, use insecticides as well, I have not yet been made aware of their possible carcinogenicity. Needless to say, I will be trying another product during our peak flea/tick season.
Learn more about Permethrin at http://www.pesticide.org/Permethrin.pdf