When I‌ apply Frontline Combo on my cat's neck, for a day or so he becomes slow and sort of affectionate! As if a human on opium.

This drug is supposed to deter lice and ticks for a month (though my cat still got lice!).

The ingredients as indicated on the box:

Fipronil 50mg
(S)-Methoprene 60mg
Butylhydroxyanisole(E320) 0.1mg
Excipient QSP 1 pipette 0.5 ml

I'm wondering what are biological effects of this drug combo?

How does it work in the cat's body and brain?

  • Do you mean fleas? – Rebecca RVT Aug 26 '17 at 14:31

I contacted Merial with your question and this is what the response was.

Thank you for your email regarding FRONTLINE© Combo. In answer to your question, following application as directed, FRONTLINE® Combo rapidly dissolves in the lipid(fat)-rich structures of the skin. It then spreads to all areas of the body surface by diffusion along a concentration gradient, away from the application site, through surface lipid (fat) within 24 hours. This process is aided by the normal body movement of animals.

Fipronil (the active ingredient in FRONTLINE® Combo) concentrates in the sebaceous glands in the skin. The sebaceous glands constantly secrete sebum (the oils found on skin and hair), and with this, Fipronil is secreted to coat the skin and hair. This ongoing secretion following application explains the long term activity against fleas and ticks that FRONTLINE Combo provides.

FRONTLINE® Combo has a non-systemic mode of action, which means that it kills parasites on contact rather than via the pets’ blood, and so they do not need to bite or feed on pets in order to be killed. I’m not sure how this would make your cats behaviour change. May I suggest you discuss your concerns with your veterinary surgeon who may be able to assist you further.

To put this into layman terms, this medication should not penetrate the blood-brain barrier to cause any personality changes or psychedelic effects as it does not go into the pets blood stream - it stays in the layers of the skin.

This drug is supposed to deter lice and ticks for a month (though my cat still got lice!).

If you have an infestation in your house or high volumes of fleas outside ( I know southern USA has a major problem with fleas) you will still see the occasional one on your pet, the medication will kill it over time.

  • Definitely helpful. Thanks for your enquiry. Though I don't believe that it does not go deeper than the skin, as the answer suggests. I'm still concerned about the long term effect of these anti-flea drugs. About the negative effects, if any, the sales persons are not the best people to consult. – tactac Aug 26 '17 at 18:10
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    @tactac Bayers Advantix does the same thing (the product we use in Canada), it does not go into the bloodstream so I would believe the answer I was given. It is why we were able to sell it over the counter without a dr. prescription because it did not go into the blood stream - it's considered a pesticide (think of mosquito spray for people) and not a drug. – Rebecca RVT Aug 26 '17 at 18:38

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