I have just dealt with a heart broken owner. She applied a supermarket flea product and 12 hours later her puss is in intensive care suffering from acute poisoning. Puss will almost certainly not last the day.
It was the correct product applied in the correct way.
The vet concerned is BLUE from telling people NOT to use products available in supermarkets. They are Russian roulette. The next one could kill your cat/dog.
Someone somewhere PLEASE sue the manufacturers and the supermarkets from selling this evil stuff. They are killing our pets!!