Vet Blog has an explanation of why Hartz, Sargeant’s, and other cheap over the counter flea and tick prevention products are extremely dangerous. An excerpt:
Most veterinary products are designed act on nerve cells that only fleas or other arthropods possess. They therefore are very toxic to fleas and very non-toxic to mammals (although individual dogs and cats can react to the active or inert ingredients in any of the products).
Most cheap over-the-counter products contain pyrethrins or pyrethroids. These pesticides are toxic to just about every creature. Fleas and other arthropods are most sensitive. Cats are next. Then come dogs. Their margin of safety is poor. It is my opinion that these products are dangerous, period. …
The U.S. Environmental Protection Agency is pursuing a series of actions to increase the safety of spot-on pesticide products for flea and tick control for cats and dogs. Immediately, EPA will begin reviewing labels and determining which ones need stronger and clearer labeling statements. EPA will also develop more stringent testing and evaluation requirements for both existing and new products. EPA expects these steps will help prevent adverse reactions from pet spot-on products. EPA is coordinating with Health Canada and with the Food and Drug Administration’s Center for Veterinary Medicine on these actions.
EPA doesn’t name names, but I will. HARTZ and SARGEANT’S!
I wish EPA would ban these products. Sadly, that doesn’t seem likely. I urge you, in the strongest terms possible, never to use these products.
Gilgablog 
Ten years ago, our delightful little Fonzie was very nearly killed by one of those products and then very nearly killed by the infection that set in as a result of his treatment for the pesticide. His body temperature, depressed by the first cause and elevated by the second, went through a swing of ten degrees F in four days.
Not only was the pesticide at fault, the directions for application were, too. They called not for a spot application at the back of the neck, but for a swath along the spine. Like idiots, we followed those directions — which, of course, meant that he could, and did, get at the stuff with his tongue.
Never, ever buy anything with pyrethrins.
Poor Fonzie, and you must have been terrified.
These pyrethin people should be arrested for selling this stuff, marketing it as if it’s safe, marketing it to poor people who may (reasonably) think it’s a reasonable alternative to (truly expensive) vet products they may be struggling to afford.
It’s NOT a reasonable alternative, of course, but I’m with Barchas on this: it should be banned, because there will always be people who don’t know yet that it’s extremely dangerous.