I seem to remember reading something about that but I'm sure it was people who hadn't used the right one for their animal or something, like interchanging treatments for cats/dogs? I could be wrong - might do a search and remind myself. Personally I only ever use Frontline as its the only thing that really seems to work. H x
They do both a cat and dog spot on. But the reviews clearly state that the correct one for the animal was used, and in some cases that they had been used previously with no ill effects.