Dear Christopher Cat: The flea-tick product I’m using on my six barn cats isn’t working. It’s also costly, since I have to buy one vial for each cat every month. I have syringes, and I plan to divide the contents of one large dog flea-tick vial equally among six syringes, remove the needles, and then apply the product to my cats’ skin. You may want to share this tip with your readers.

Christopher Responds: Not a chance, because it’s a potentially deadly idea. A flea-tick product labeled for dogs should never be applied to a cat.

Most canine topical flea-tick products contain ingredients that are safe for dogs but toxic to cats – so toxic, in fact, that a cat can be poisoned if he rubs up against a dog whose product hasn’t dried yet.

The most common such ingredients are permethrin and other chemicals that end in “-thrin.” They are related to pyrethrins, natural plant chemicals that repel insects. Many are classified as pyrethroids, which means they have been chemically modified to enhance and prolong their effectiveness.

Pyrethrins and pyrethroids are safe for dogs, but cats are exquisitely sensitive to them. Toxic signs in cats include drooling, vomiting, weakness, loss of coordination, hyperactivity, tremors, seizures and death.

Ask your veterinarian about feline flea-tick products that are more effective than what you’re using now.