The dangerous ingredient is permethrin, which is an insecticide that attacks the central nervous system. Problem is that it cannot distinguish between the CNS of a pest and the CNS of an animal, and can cause nerve damage, seizures, etc etc.

Any product with this ingredient should NOT be used, especially around cats (who seem to be more sensitive) and little kids. Personally, I would never buy a flea and tick product of ANY kind over the counter. Go to the vet - you may pay more initially for the bottle of shampoo or treatment but in the long run you will save money and heartache.