Pyrethrins are iffy for cats IMO; I have personally seen 2 kitties neurologically poisoned with dog "flea drops" despite what this vet says. http://www.peteducation.com/article....articleid=1359

http://frontline.us.merial.com/produ...ducts_faqs.asp
The oil in the flea stuff is spread through the oil glands, not the blood.
2. How do FRONTLINE Top Spot and FRONTLINE Plus spread over a pet's body? How long does this take?
FRONTLINE Top Spot and Plus spread over the pet's body by a process called translocation. When applied, these products are gradually dispersed by the pet's natural oils, collecting in the oil glands in the skin. It is then "wicked" onto the hair over the next 30 days. The translocation process can take up to 24 hours to complete.

The "burns" from Frontline and Advantage seem idiosyncratic; just 2 weeks ago Fifi got a nasty, oozy, burn-type reaction from Top Spot, while Jazz gets a similar thing from Advantage (the others get no adverse reaction). Still, I would NEVER go back to where we were 20-25 years ago when fleas were rampant and there was very little to be done about them.

BTW, Advantage has been around 10 years, not 5. http://advantage.petparents.com/aboutAdvantage.cfm
Frontline came out in '96. http://frontline.us.merial.com/products/index.asp