Presence of Fleas
This sounds like a no-brainer – your flea medication isn’t working if there are still fleas around – but the reality is a little more complicated than that.
“By the time you see fleas on your pet, they have generally been laying thousands and thousands of eggs in your house,” Verbrugge said. She adds that it takes between six and eight weeks for those eggs to hatch, so using flea medication as prescribed doesn’t necessarily mean they’ll be gone at the end of the prescription. “It may be working great, but the last few fleas are still hatching.”
This means flea medication requires patience beyond what you might be prepared for or are expecting, says Verbrugge, but at a certain point, proper application of a flea medication should make the problem go away.
If it doesn’t, the fleas may have found another unwilling but vulnerable host in the house. “All animals in the house need to be treated,” Verbrugge says. “If you treat your dog and not your cat, the fleas will keep on breeding, keep on laying eggs, and you will continue to see them in the house.”