At least 1,600 pet deaths related to certain types of over-the-counter flea and tick treatments were reported to the EPA over the last five years.
The HSUS has put up a strongly worded warning about modern flea and tick products, primarily those using recent insecticides. One particular notice resonates with all advice that we like to stick to on PETS 911: before applying any medication to your pet, always consult your veterinarian. Good communication about the overall health of your pet with your vet should help you also make sound decisions on how to care for them if they do get sick and prevent them from getting sick in the first place.
There is a tendency to apply everything out-of-the-box, especially items that have extremely simple instructions. As with everything, owning a pet is not really like programming a VCR: it doesn’t come with an instruction manual. Problematically, things like flea and tick collars and medicines do come with overly simplistic instructions–put daub on back, click collar around neck–but as a result we tend to overlook the warnings and indications. In particular the HSUS article goes over some of the more toxic insecticidal chemicals that we should be watching out for.
While parts of the article do get a bit technical, it is a good idea to give it at least a once over.
Link, via the HSUS.