Use extra caution with tick products. When it comes to tick prevention—or combination flea-and-tick products—the news is even grimmer. Most products designed to repel these buggers include possible carcinogens and nervous-system toxins like fipronil, permethrin, pyrethrins, or imidacloprid. “Our recommendation for ticks is to use the least toxic option available, at the lowest level, and only when you need it,” Rotkin-Ellman says. If you live in an area where ticks and Lyme disease are prevalent, you probably need protection—but talk to your vet about how much and how often. Pregnant women and young children should minimize their exposure.