Frontline/Advantage not working on my cat

Question
Yes, please--I assume this is a dermatology question--that my cat's problem isn't systematic.

I took Tweenie to the vet on Aug 21 (a Sunday); the vet said he had fleas. I applied Frontline as soon as we came home. On Sept 4th Tweenie was covered with fleas so (rightly or not)he got an application of Advantage. Now today, Sept 10th, he still has fleas. I don't believe either topical eliminated his fleas although I've never had a problem like this--I've had many cats and have used both products before.
Tweenie is a strange little guy, is overly sensitive to the biting--he twitches constantly
and of course is miserable.
Can you help, Dr. Gotthelf? Can you think of any reason these medications aren't working?

Thank you so much for your free advice.

karin moser

Answer
Neither Frontline or Advantage are flea repellants.  What you are probably seeing are the fleas on the hair coat rather than on the skin.  Once the fleas contact the insecticide, they move away from the skin.  If you see lots of fleas, then you probably have an environmental problem somewhere the cat is going.